TreeMap 源码解读(JDK 1.8)

概述

TreeMap 从名字中可以看出应该是跟树有关，没错，TreeMap 便是直接使用红黑树实现的，而 HashMap 则是当链表数组和某个链表同时达到一定长度时才会将链表转为红黑树。同时 TreeMap 还是有序的，这就要求 Key 必须是可以进行比较的！即实现 Comparable 接口，否则在插入或者查询时会抛出类型转换异常，接下来看源码。

源码

public class TreeMap<K,V>
    extends AbstractMap<K,V>
    implements NavigableMap<K,V>, Cloneable, java.io.Serializable
{
    /**
     * key 的比较器
     *
     * @serial
     */
    private final Comparator<? super K> comparator;

    /**
    * 红黑树的根节点
    */
    private transient Entry<K,V> root;

    /**
     * key value 的映射个数
     */
    private transient int size = 0;

    /**
     * 数据结构被修改的次数
     */
    private transient int modCount = 0;

    /**
     * 默认的构造函数
     */
    public TreeMap() {
        comparator = null;
    }

    /**
     * 指定 key 的 comparator
     */
    public TreeMap(Comparator<? super K> comparator) {
        this.comparator = comparator;
    }

    /**
     * 构造函数，将参数 map 中的数据导入到当前 map 中，不设置 comparator
     */
    public TreeMap(Map<? extends K, ? extends V> m) {
        comparator = null;
        putAll(m);
    }

    /**
     * 构造函数，传入一个 sortedMap，将参数中的数据跟 comparator 都设置到
     * 当前 map
     */
    public TreeMap(SortedMap<K, ? extends V> m) {
        comparator = m.comparator();
        try {
            buildFromSorted(m.size(), m.entrySet().iterator(), null, null);
        } catch (java.io.IOException cannotHappen) {
        } catch (ClassNotFoundException cannotHappen) {
        }
    }


    // 以下是一些查询操作

    /**
     * 返回当前 map 的大小
     */
    public int size() {
        return size;
    }

    /**
     * 判断参数 key 是否在当前 map 中存在
     */
    public boolean containsKey(Object key) {
        return getEntry(key) != null;
    }

    /**
     * 判断参数 value 是否在当前 map 中存在
     */
    public boolean containsValue(Object value) {
        // 从小到大遍历 treeMap
        for (Entry<K,V> e = getFirstEntry(); e != null; e = successor(e))
            if (valEquals(value, e.value))
                return true;
        return false;
    }

    /**
     * 通过 key 查询 value
     */
    public V get(Object key) {
        // 通过 key 查询节点
        Entry<K,V> p = getEntry(key);
        // 如果节点不为空返回节点保存的 value，否则返回空
        return (p==null ? null : p.value);
    }

    /**
    * 获取比较器
    */
    public Comparator<? super K> comparator() {
        return comparator;
    }

    /**
     * 获取第一个 key
     */
    public K firstKey() {
        return key(getFirstEntry());
    }

    /**
     * 获取最后一个 key
     */
    public K lastKey() {
        return key(getLastEntry());
    }

    /**
     * 将指定 map 中的数据复制到当前 map 中
     */
    public void putAll(Map<? extends K, ? extends V> map) {
        // 当前 map 的大小
        int mapSize = map.size();

        // 如果是第一次插入数据且传入的 map 也是有序 map
        if (size==0 && mapSize!=0 && map instanceof SortedMap) {
            // 参数 map 的 comparator
            Comparator<?> c = ((SortedMap<?,?>)map).comparator();
            // 如果 comparator 相同
            if (c == comparator || (c != null && c.equals(comparator))) {
                ++modCount;
                // build 方法
                try {
                    buildFromSorted(mapSize, map.entrySet().iterator(),
                                    null, null);
                } catch (java.io.IOException cannotHappen) {
                } catch (ClassNotFoundException cannotHappen) {
                }
                return;
            }
        }
        // 循环调用 put 方法
        super.putAll(map);
    }

    /**
     * 通过 key 查询对应的红黑树节点
     */
    final Entry<K,V> getEntry(Object key) {
        // 如果 comparator 不为空，通过 comparator 的
        // compare 方法比较查询
        if (comparator != null)
            return getEntryUsingComparator(key);
        // 参数校验
        if (key == null)
            throw new NullPointerException();
        // 将 key 转成 Comparable 类型
        @SuppressWarnings("unchecked")
        Comparable<? super K> k = (Comparable<? super K>) key; 
        Entry<K,V> p = root;
        // 从头结点开始遍历比较，注意：HashMap 中是使用 equals 判断
        // 两个 key 是否相等，而 TreeMap 是使用 compareTo 方法
        while (p != null) {
            int cmp = k.compareTo(p.key);
            if (cmp < 0)
                p = p.left;
            else if (cmp > 0)
                p = p.right;
            else
                return p;
        }
        return null;
    }

    /**
     * 使用 comparator 的 compare 方法比较两个 key 的大小
     */
    final Entry<K,V> getEntryUsingComparator(Object key) {
        @SuppressWarnings("unchecked")
        K k = (K) key;
        Comparator<? super K> cpr = comparator;
        if (cpr != null) {
            // 从根节点开始遍历，然后调用 comparator 的 compare 方法比较
            Entry<K,V> p = root;
            while (p != null) {
                int cmp = cpr.compare(k, p.key);
                if (cmp < 0)
                    p = p.left;
                else if (cmp > 0)
                    p = p.right;
                else
                    return p;
            }
        }
        return null;
    }

    /**
     * 返回指定 key 对应的节点，如果 key 不存在，则返回的节点的 key 比参数 key 大且离参数 key 最近
     * 如果 TreeMap 中最大的 key 都小于参数 key，则返回空
     */
    final Entry<K,V> getCeilingEntry(K key) {
        Entry<K,V> p = root;
        // 从根节点开始遍历
        while (p != null) {
            // 比较
            int cmp = compare(key, p.key);
            // 如果 key 比当前节点的 key 小，比较左子节点
            if (cmp < 0) {
                // 如果左子节点不为空，则更新当前节点
                if (p.left != null)
                    p = p.left;
                // 如果为空则返回当前节点
                else
                    return p;
            }
            // 如果比当前节点大，则去跟当前节点的右子节点去比较
            else if (cmp > 0) {
                // 如果右子节点不为空，更新当前节点为右子节点
                if (p.right != null) {
                    p = p.right;
                } 
                // 否则从当前节点开始遍历向上遍历，知道父节点是爷爷节点的左子节点，
                // 然后返回左子节点
                else {
                    Entry<K,V> parent = p.parent;
                    Entry<K,V> ch = p;
                    while (parent != null && ch == parent.right) {
                        ch = parent;
                        parent = parent.parent;
                    }
                    return parent;
                }
            } 
            // 如果相等则返回当前节点
            else
                return p;
        }
        return null;
    }

    /**
     * 跟 getCeilingEntry 是相反操作，如果 key 存在，则返回对应的节点
     * 否则返回的节点的 key 比参数 key 小且离参数 key 最近，如果 TreeMap
     * 中最小 key 还比参数 key 大，则返回空
     */
    final Entry<K,V> getFloorEntry(K key) {
        Entry<K,V> p = root;
        while (p != null) {
            int cmp = compare(key, p.key);
            if (cmp > 0) {
                if (p.right != null)
                    p = p.right;
                else
                    return p;
            } else if (cmp < 0) {
                if (p.left != null) {
                    p = p.left;
                } else {
                    Entry<K,V> parent = p.parent;
                    Entry<K,V> ch = p;
                    while (parent != null && ch == parent.left) {
                        ch = parent;
                        parent = parent.parent;
                    }
                    return parent;
                }
            } else
                return p;

        }
        return null;
    }

    /**
     * 返回一个节点，节点的 key 比参数 key 大且离参数 key 最近
     * 如果 TreeMap 中最大的 key 都小于参数 key，则返回空
     */
    final Entry<K,V> getHigherEntry(K key) {
        Entry<K,V> p = root;
        while (p != null) {
            int cmp = compare(key, p.key);
            if (cmp < 0) {
                if (p.left != null)
                    p = p.left;
                else
                    return p;
            } else {
                if (p.right != null) {
                    p = p.right;
                } else {
                    Entry<K,V> parent = p.parent;
                    Entry<K,V> ch = p;
                    while (parent != null && ch == parent.right) {
                        ch = parent;
                        parent = parent.parent;
                    }
                    return parent;
                }
            }
        }
        return null;
    }

    /**
     * 返回的节点否则返回的节点的 key 比参数 key 小且离参数 key 最近，
     * 如果 TreeMap中最小 key 还比参数 key 大，则返回空
     */
    final Entry<K,V> getLowerEntry(K key) {
        Entry<K,V> p = root;
        while (p != null) {
            int cmp = compare(key, p.key);
            if (cmp > 0) {
                if (p.right != null)
                    p = p.right;
                else
                    return p;
            } else {
                if (p.left != null) {
                    p = p.left;
                } else {
                    Entry<K,V> parent = p.parent;
                    Entry<K,V> ch = p;
                    while (parent != null && ch == parent.left) {
                        ch = parent;
                        parent = parent.parent;
                    }
                    return parent;
                }
            }
        }
        return null;
    }

    /**
     * 插入数据，如果 key 存在，则旧值会被替换
     */
    public V put(K key, V value) {
        Entry<K,V> t = root;
        // 如果是第一次插入数据，直接设置 root
        if (t == null) {
            // 参数校验，参数类型有可能不是 Comparable 类型，则会抛出类型转换异常
            compare(key, key);

            // 设置根节点和 map 大小
            root = new Entry<>(key, value, null);
            size = 1;
            // 修改次数 + 1
            modCount++;
            return null;
        }

        int cmp;
        Entry<K,V> parent;
        Comparator<? super K> cpr = comparator;
        // 如果 comparator 不为空，则通过 comparator.compare 比较大小，
        // 确定插入位置
        if (cpr != null) {
            // 从根节点开始遍历，比较大小
            do {
                // 更新父节点为当前节点
                parent = t;
                cmp = cpr.compare(key, t.key);
                // 如果比当前节点小，则更新当前节点为左子节点
                if (cmp < 0)
                    t = t.left;
                // 否则更新为右子节点
                else if (cmp > 0)
                    t = t.right;
                // 如果相等则替换旧值
                else
                    return t.setValue(value);
            } while (t != null);
        }
        // 如果 comparator 为空，则使用 key 自己的 compareTo 方法比较大小
        else {
            // 参数校验
            if (key == null)
                throw new NullPointerException();
            @SuppressWarnings("unchecked")
            Comparable<? super K> k = (Comparable<? super K>) key;
            // 从根部开始遍历，比较大小
            do {
                // 更新父节点为当前节点
                parent = t;
                cmp = k.compareTo(t.key);
                if (cmp < 0)
                    t = t.left;
                else if (cmp > 0)
                    t = t.right;
                else
                    return t.setValue(value);
            } while (t != null);
        }
        // 创建新节点
        Entry<K,V> e = new Entry<>(key, value, parent);
        // 根据比较结果判断插入父节点的左子树还是右子树
        if (cmp < 0)
            parent.left = e;
        else
            parent.right = e;
        // 插入数据后修正红黑树性质
        fixAfterInsertion(e);
        // map 数量 + 1
        size++;
        // 修改次数 + 1
        modCount++;
        return null;
    }

    /**
     * 如果 map 中存在参数 key，则 key 与参数 key 相等的节点
     */
    public V remove(Object key) {
        // 通过 key 查询到节点
        Entry<K,V> p = getEntry(key);
        // 如果节点不存在则直接返回
        if (p == null)
            return null;
        
        // 记录 key 对应的 value
        V oldValue = p.value;
        // 删除节点
        deleteEntry(p);
        // 返回 value
        return oldValue;
    }

    /**
     * 清空 map
     */
    public void clear() {
        // 修改次数 + 1
        modCount++;
        // 大小设置为 0
        size = 0;
        // 根节点设置为空
        root = null;
    }

    /**
     * 返回当前 map 的浅拷贝实例（key value 不是通过 clone 创建的）
     */
    public Object clone() {
        // 克隆对象
        TreeMap<?,?> clone;
        try {
            clone = (TreeMap<?,?>) super.clone();
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }

        // 将 map 设置为初始化妆台
        clone.root = null;
        clone.size = 0;
        clone.modCount = 0;
        clone.entrySet = null;
        clone.navigableKeySet = null;
        clone.descendingMap = null;

        // 赋值一份新的数据
        try {
            clone.buildFromSorted(size, entrySet().iterator(), null, null);
        } catch (java.io.IOException cannotHappen) {
        } catch (ClassNotFoundException cannotHappen) {
        }

        // 返回结果
        return clone;
    }

    // 重写 NavigableMap 的方法

    /**
    * 获取最小节点，返回一个简单的 Entry 对象，只包含 key value
     */
    public Map.Entry<K,V> firstEntry() {
        return exportEntry(getFirstEntry());
    }

    /**
     * 返回最大节点，返回一个简单的 Entry 对象，只包含 key value
     */
    public Map.Entry<K,V> lastEntry() {
        return exportEntry(getLastEntry());
    }

    /**
     * 删除最小节点，并返回
     */
    public Map.Entry<K,V> pollFirstEntry() {
        // 获取最小节点
        Entry<K,V> p = getFirstEntry();
        // 转换为简化的 Entry 对象
        Map.Entry<K,V> result = exportEntry(p);
        // 如果最小节点不为空则移除
        if (p != null)
            deleteEntry(p);
        // 返回最下节点的简化对象
        return result;
    }

    /**
     * 删除最大节点，并返回
     */
    public Map.Entry<K,V> pollLastEntry() {
        // 获取最大节点
        Entry<K,V> p = getLastEntry();
        // 转换为简化的 Entry 对象
        Map.Entry<K,V> result = exportEntry(p);
        // 如果最大节点不为空则移除
        if (p != null)
            deleteEntry(p);
        // 返回最大节点的简化对象
        return result;
    }

    /**
     * 返回一个简化节点，节点的 key 比参数 key 小且离参数 key 最近
     */
    public Map.Entry<K,V> lowerEntry(K key) {
        return exportEntry(getLowerEntry(key));
    }

    /**
     * 返回一个比 key 小且离 key 最近的 key
     */
    public K lowerKey(K key) {
        return keyOrNull(getLowerEntry(key));
    }

    /**
     * 返回一个简化节点，key 存在则是与 key 与参数相等的节点，
     * 否则返回的节点的 key 比参数 key 小且离参数 key 最近
     */
    public Map.Entry<K,V> floorEntry(K key) {
        return exportEntry(getFloorEntry(key));
    }

    /**
     * 返回一个key，如果 key 存在则返回 key，否则返回一个比 key 小
     * 且离 key 最近的 key
     */
    public K floorKey(K key) {
        return keyOrNull(getFloorEntry(key));
    }

    /**
     * 返回一个简化节点，如果 key 存在则返回指定 key 对应的简化节点，
     * 如果 key 不存在，则返回的简化节点的 key 比参数 key 大且离参数 key 最近
     * 如果 TreeMap 中最大的 key 都小于参数 key，则返回空
     */
    public Map.Entry<K,V> ceilingEntry(K key) {
        return exportEntry(getCeilingEntry(key));
    }

    /**
     * 如果 key 存在，则返回 key，否则返回一个比参数 key 大且离参数 key
     * 最近的 key
     */
    public K ceilingKey(K key) {
        return keyOrNull(getCeilingEntry(key));
    }

    /**
     * 返回一个简化节点，节点的 key 比参数 key 大且离参数 key 最近
     * 如果 TreeMap 中最大的 key 都小于参数 key，则返回空
     */
    public Map.Entry<K,V> higherEntry(K key) {
        return exportEntry(getHigherEntry(key));
    }

    /**
     * 返回一个 key，这个 key 比参数 key 大且离参数 key 最近
     */
    public K higherKey(K key) {
        return keyOrNull(getHigherEntry(key));
    }


    /**
     * key value 映射条目的 set 集合
     */
    private transient EntrySet entrySet;
    /**
    * map 中 key 的 set 集合
    */
    private transient KeySet<K> navigableKeySet;
    /**
    * 根据 key 倒序排列的 map，treeMap 中的变化也会体现到该 map 中
    */
    private transient NavigableMap<K,V> descendingMap;

    /**
     * 返回 map 包含的所有 key 的 set 集合
     */
    public Set<K> keySet() {
        return navigableKeySet();
    }

    /**
     * 返回 map 包含的所有 key 的有序 set 集合
     */
    public NavigableSet<K> navigableKeySet() {
        KeySet<K> nks = navigableKeySet;
        return (nks != null) ? nks : (navigableKeySet = new KeySet<>(this));
    }

    /**
     * 返回 map 包含的所有 key 的倒序 set 集合
     */
    public NavigableSet<K> descendingKeySet() {
        return descendingMap().navigableKeySet();
    }

    /**
     * 返回 map 包含的所有 value 的集合
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        if (vs == null) {
            vs = new Values();
            values = vs;
        }
        return vs;
    }

    /**
     * 返回 map 中所有 key value 映射的条目 set 集合
     */
    public Set<Map.Entry<K,V>> entrySet() {
        EntrySet es = entrySet;
        return (es != null) ? es : (entrySet = new EntrySet());
    }

    /**
     * 返回一个根据 key 倒序的 map
     */
    public NavigableMap<K, V> descendingMap() {
        NavigableMap<K, V> km = descendingMap;
        return (km != null) ? km :
            (descendingMap = new DescendingSubMap<>(this,
                                                    true, null, true,
                                                    true, null, true));
    }

    /**
     * 子 map ，有序
     */
    public NavigableMap<K,V> subMap(K fromKey, boolean fromInclusive,
                                    K toKey,   boolean toInclusive) {
        return new AscendingSubMap<>(this,
                                     false, fromKey, fromInclusive,
                                     false, toKey,   toInclusive);
    }

    /**
     * 返回一个 NavigableMap，里面包含的是从第一个 key 到指定 key 映射的数据
     */
    public NavigableMap<K,V> headMap(K toKey, boolean inclusive) {
        return new AscendingSubMap<>(this,
                                     true,  null,  true,
                                     false, toKey, inclusive);
    }

    /**
     * 返回一个 NavigableMap，里面包含的是从指定 key 到最后一个 key 映射的数据
     */
    public NavigableMap<K,V> tailMap(K fromKey, boolean inclusive) {
        return new AscendingSubMap<>(this,
                                     false, fromKey, inclusive,
                                     true,  null,    true);
    }

    /**
     * 指定两个 key 之间的映射数据
     */
    public SortedMap<K,V> subMap(K fromKey, K toKey) {
        return subMap(fromKey, true, toKey, false);
    }

    /**
     * 返回一个 SortedMap，里面包含的是从第一个 key 到指定 key 映射的数据
     */
    public SortedMap<K,V> headMap(K toKey) {
        return headMap(toKey, false);
    }

    /**
     * 返回一个 NavigableMap，里面包含的是从指定 key 到最后一个 key 映射的数据
     */
    public SortedMap<K,V> tailMap(K fromKey) {
        return tailMap(fromKey, true);
    }

    /**
    * 如果 key 对应的 value 为 oldValue，则替换为 newValue
    */
    @Override
    public boolean replace(K key, V oldValue, V newValue) {
        Entry<K,V> p = getEntry(key);
        if (p!=null && Objects.equals(oldValue, p.value)) {
            p.value = newValue;
            return true;
        }
        return false;
    }

    /**
    * 将 key 的 value 替换为传入的 value
    */
    @Override
    public V replace(K key, V value) {
        Entry<K,V> p = getEntry(key);
        if (p!=null) {
            V oldValue = p.value;
            p.value = value;
            return oldValue;
        }
        return null;
    }

    /**
    * 遍历，并执行 action 函数
    */
    @Override
    public void forEach(BiConsumer<? super K, ? super V> action) {
        Objects.requireNonNull(action);
        int expectedModCount = modCount;
        for (Entry<K, V> e = getFirstEntry(); e != null; e = successor(e)) {
            action.accept(e.key, e.value);

            if (expectedModCount != modCount) {
                throw new ConcurrentModificationException();
            }
        }
    }

    /**
    * 遍历并执行 function 函数替换 value
    */
    @Override
    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
        Objects.requireNonNull(function);
        int expectedModCount = modCount;

        for (Entry<K, V> e = getFirstEntry(); e != null; e = successor(e)) {
            e.value = function.apply(e.key, e.value);

            if (expectedModCount != modCount) {
                throw new ConcurrentModificationException();
            }
        }
    }

    
    /**
    * map 中的 value 集合
    */
    class Values extends AbstractCollection<V> {
        public Iterator<V> iterator() {
            return new ValueIterator(getFirstEntry());
        }

        public int size() {
            return TreeMap.this.size();
        }

        public boolean contains(Object o) {
            return TreeMap.this.containsValue(o);
        }

        public boolean remove(Object o) {
            for (Entry<K,V> e = getFirstEntry(); e != null; e = successor(e)) {
                if (valEquals(e.getValue(), o)) {
                    deleteEntry(e);
                    return true;
                }
            }
            return false;
        }

        public void clear() {
            TreeMap.this.clear();
        }

        public Spliterator<V> spliterator() {
            return new ValueSpliterator<K,V>(TreeMap.this, null, null, 0, -1, 0);
        }
    }

    /**
    * Map 中的 key value 映射条目集合
    */
    class EntrySet extends AbstractSet<Map.Entry<K,V>> {
        public Iterator<Map.Entry<K,V>> iterator() {
            return new EntryIterator(getFirstEntry());
        }

        public boolean contains(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?,?> entry = (Map.Entry<?,?>) o;
            Object value = entry.getValue();
            Entry<K,V> p = getEntry(entry.getKey());
            return p != null && valEquals(p.getValue(), value);
        }

        public boolean remove(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?,?> entry = (Map.Entry<?,?>) o;
            Object value = entry.getValue();
            Entry<K,V> p = getEntry(entry.getKey());
            if (p != null && valEquals(p.getValue(), value)) {
                deleteEntry(p);
                return true;
            }
            return false;
        }

        public int size() {
            return TreeMap.this.size();
        }

        public void clear() {
            TreeMap.this.clear();
        }

        public Spliterator<Map.Entry<K,V>> spliterator() {
            return new EntrySpliterator<K,V>(TreeMap.this, null, null, 0, -1, 0);
        }
    }

    /*
     * key 迭代器
     */

    Iterator<K> keyIterator() {
        return new KeyIterator(getFirstEntry());
    }

    /**
    * 倒序 key 迭代器
    */
    Iterator<K> descendingKeyIterator() {
        return new DescendingKeyIterator(getLastEntry());
    }

    /**
    * key 的 Set 集合
    */
    static final class KeySet<E> extends AbstractSet<E> implements NavigableSet<E> {
        private final NavigableMap<E, ?> m;
        KeySet(NavigableMap<E,?> map) { m = map; }

        public Iterator<E> iterator() {
            if (m instanceof TreeMap)
                return ((TreeMap<E,?>)m).keyIterator();
            else
                return ((TreeMap.NavigableSubMap<E,?>)m).keyIterator();
        }

        public Iterator<E> descendingIterator() {
            if (m instanceof TreeMap)
                return ((TreeMap<E,?>)m).descendingKeyIterator();
            else
                return ((TreeMap.NavigableSubMap<E,?>)m).descendingKeyIterator();
        }

        public int size() { return m.size(); }
        public boolean isEmpty() { return m.isEmpty(); }
        public boolean contains(Object o) { return m.containsKey(o); }
        public void clear() { m.clear(); }
        public E lower(E e) { return m.lowerKey(e); }
        public E floor(E e) { return m.floorKey(e); }
        public E ceiling(E e) { return m.ceilingKey(e); }
        public E higher(E e) { return m.higherKey(e); }
        public E first() { return m.firstKey(); }
        public E last() { return m.lastKey(); }
        public Comparator<? super E> comparator() { return m.comparator(); }
        public E pollFirst() {
            Map.Entry<E,?> e = m.pollFirstEntry();
            return (e == null) ? null : e.getKey();
        }
        public E pollLast() {
            Map.Entry<E,?> e = m.pollLastEntry();
            return (e == null) ? null : e.getKey();
        }
        public boolean remove(Object o) {
            int oldSize = size();
            m.remove(o);
            return size() != oldSize;
        }
        public NavigableSet<E> subSet(E fromElement, boolean fromInclusive,
                                      E toElement,   boolean toInclusive) {
            return new KeySet<>(m.subMap(fromElement, fromInclusive,
                                          toElement,   toInclusive));
        }
        public NavigableSet<E> headSet(E toElement, boolean inclusive) {
            return new KeySet<>(m.headMap(toElement, inclusive));
        }
        public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
            return new KeySet<>(m.tailMap(fromElement, inclusive));
        }
        public SortedSet<E> subSet(E fromElement, E toElement) {
            return subSet(fromElement, true, toElement, false);
        }
        public SortedSet<E> headSet(E toElement) {
            return headSet(toElement, false);
        }
        public SortedSet<E> tailSet(E fromElement) {
            return tailSet(fromElement, true);
        }
        public NavigableSet<E> descendingSet() {
            return new KeySet<>(m.descendingMap());
        }

        public Spliterator<E> spliterator() {
            return keySpliteratorFor(m);
        }
    }

    /**
     * TreeMap 迭代器的父类
     */
    abstract class PrivateEntryIterator<T> implements Iterator<T> {
        Entry<K,V> next;
        Entry<K,V> lastReturned;
        int expectedModCount;

        PrivateEntryIterator(Entry<K,V> first) {
            expectedModCount = modCount;
            lastReturned = null;
            next = first;
        }

        public final boolean hasNext() {
            return next != null;
        }

        final Entry<K,V> nextEntry() {
            Entry<K,V> e = next;
            if (e == null)
                throw new NoSuchElementException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            next = successor(e);
            lastReturned = e;
            return e;
        }

        final Entry<K,V> prevEntry() {
            Entry<K,V> e = next;
            if (e == null)
                throw new NoSuchElementException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            next = predecessor(e);
            lastReturned = e;
            return e;
        }

        public void remove() {
            if (lastReturned == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            // deleted entries are replaced by their successors
            if (lastReturned.left != null && lastReturned.right != null)
                next = lastReturned;
            deleteEntry(lastReturned);
            expectedModCount = modCount;
            lastReturned = null;
        }
    }

    /**
    * key value 映射条目迭代器
    */
    final class EntryIterator extends PrivateEntryIterator<Map.Entry<K,V>> {
        EntryIterator(Entry<K,V> first) {
            super(first);
        }
        public Map.Entry<K,V> next() {
            return nextEntry();
        }
    }

    /**
    * value 迭代器
    */
    final class ValueIterator extends PrivateEntryIterator<V> {
        ValueIterator(Entry<K,V> first) {
            super(first);
        }
        public V next() {
            return nextEntry().value;
        }
    }

    /**
    * key 迭代器
    */
    final class KeyIterator extends PrivateEntryIterator<K> {
        KeyIterator(Entry<K,V> first) {
            super(first);
        }
        public K next() {
            return nextEntry().key;
        }
    }

    /**
    * 倒序 key 迭代器
    */
    final class DescendingKeyIterator extends PrivateEntryIterator<K> {
        DescendingKeyIterator(Entry<K,V> first) {
            super(first);
        }
        public K next() {
            return prevEntry().key;
        }
        public void remove() {
            if (lastReturned == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            deleteEntry(lastReturned);
            lastReturned = null;
            expectedModCount = modCount;
        }
    }

    /**
     * 比较 k1，k2 两个参数大小
     */
    @SuppressWarnings("unchecked")
    final int compare(Object k1, Object k2) {
        return comparator==null ? ((Comparable<? super K>)k1).compareTo((K)k2)
            : comparator.compare((K)k1, (K)k2);
    }

    /**
     * 判断 o1，o2 两个参数是否相等
     */
    static final boolean valEquals(Object o1, Object o2) {
        return (o1==null ? o2==null : o1.equals(o2));
    }

    /**
     * 将 TreeMap 的 Entry 对象转换为 Map 的 Entry 对象
     */
    static <K,V> Map.Entry<K,V> exportEntry(TreeMap.Entry<K,V> e) {
        return (e == null) ? null :
            new AbstractMap.SimpleImmutableEntry<>(e);
    }

    /**
     * 返回 Entry 的 key 字段，如果 e 为空则返回 null
     */
    static <K,V> K keyOrNull(TreeMap.Entry<K,V> e) {
        return (e == null) ? null : e.key;
    }

    /**
     * 返回 Entry 的 key 字段，如果 e 为空则抛出异常
     */
    static <K> K key(Entry<K,?> e) {
        if (e==null)
            throw new NoSuchElementException();
        return e.key;
    }


    /**
     * Dummy value serving as unmatchable fence key for unbounded
     * SubMapIterators
     */
    private static final Object UNBOUNDED = new Object();

    /**
     * 子 map 的父类
     */
    abstract static class NavigableSubMap<K,V> extends AbstractMap<K,V>
        implements NavigableMap<K,V>, java.io.Serializable {
        private static final long serialVersionUID = -2102997345730753016L;
        /**
         * The backing map.
         */
        final TreeMap<K,V> m;

        /**
         * Endpoints are represented as triples (fromStart, lo,
         * loInclusive) and (toEnd, hi, hiInclusive). If fromStart is
         * true, then the low (absolute) bound is the start of the
         * backing map, and the other values are ignored. Otherwise,
         * if loInclusive is true, lo is the inclusive bound, else lo
         * is the exclusive bound. Similarly for the upper bound.
         */
        final K lo, hi;
        final boolean fromStart, toEnd;
        final boolean loInclusive, hiInclusive;

        NavigableSubMap(TreeMap<K,V> m,
                        boolean fromStart, K lo, boolean loInclusive,
                        boolean toEnd,     K hi, boolean hiInclusive) {
            if (!fromStart && !toEnd) {
                if (m.compare(lo, hi) > 0)
                    throw new IllegalArgumentException("fromKey > toKey");
            } else {
                if (!fromStart) // type check
                    m.compare(lo, lo);
                if (!toEnd)
                    m.compare(hi, hi);
            }

            this.m = m;
            this.fromStart = fromStart;
            this.lo = lo;
            this.loInclusive = loInclusive;
            this.toEnd = toEnd;
            this.hi = hi;
            this.hiInclusive = hiInclusive;
        }

        // internal utilities

        final boolean tooLow(Object key) {
            if (!fromStart) {
                int c = m.compare(key, lo);
                if (c < 0 || (c == 0 && !loInclusive))
                    return true;
            }
            return false;
        }

        final boolean tooHigh(Object key) {
            if (!toEnd) {
                int c = m.compare(key, hi);
                if (c > 0 || (c == 0 && !hiInclusive))
                    return true;
            }
            return false;
        }

        final boolean inRange(Object key) {
            return !tooLow(key) && !tooHigh(key);
        }

        final boolean inClosedRange(Object key) {
            return (fromStart || m.compare(key, lo) >= 0)
                && (toEnd || m.compare(hi, key) >= 0);
        }

        final boolean inRange(Object key, boolean inclusive) {
            return inclusive ? inRange(key) : inClosedRange(key);
        }

        /*
         * Absolute versions of relation operations.
         * Subclasses map to these using like-named "sub"
         * versions that invert senses for descending maps
         */

        final TreeMap.Entry<K,V> absLowest() {
            TreeMap.Entry<K,V> e =
                (fromStart ?  m.getFirstEntry() :
                 (loInclusive ? m.getCeilingEntry(lo) :
                                m.getHigherEntry(lo)));
            return (e == null || tooHigh(e.key)) ? null : e;
        }

        final TreeMap.Entry<K,V> absHighest() {
            TreeMap.Entry<K,V> e =
                (toEnd ?  m.getLastEntry() :
                 (hiInclusive ?  m.getFloorEntry(hi) :
                                 m.getLowerEntry(hi)));
            return (e == null || tooLow(e.key)) ? null : e;
        }

        final TreeMap.Entry<K,V> absCeiling(K key) {
            if (tooLow(key))
                return absLowest();
            TreeMap.Entry<K,V> e = m.getCeilingEntry(key);
            return (e == null || tooHigh(e.key)) ? null : e;
        }

        final TreeMap.Entry<K,V> absHigher(K key) {
            if (tooLow(key))
                return absLowest();
            TreeMap.Entry<K,V> e = m.getHigherEntry(key);
            return (e == null || tooHigh(e.key)) ? null : e;
        }

        final TreeMap.Entry<K,V> absFloor(K key) {
            if (tooHigh(key))
                return absHighest();
            TreeMap.Entry<K,V> e = m.getFloorEntry(key);
            return (e == null || tooLow(e.key)) ? null : e;
        }

        final TreeMap.Entry<K,V> absLower(K key) {
            if (tooHigh(key))
                return absHighest();
            TreeMap.Entry<K,V> e = m.getLowerEntry(key);
            return (e == null || tooLow(e.key)) ? null : e;
        }

        /** Returns the absolute high fence for ascending traversal */
        final TreeMap.Entry<K,V> absHighFence() {
            return (toEnd ? null : (hiInclusive ?
                                    m.getHigherEntry(hi) :
                                    m.getCeilingEntry(hi)));
        }

        /** Return the absolute low fence for descending traversal  */
        final TreeMap.Entry<K,V> absLowFence() {
            return (fromStart ? null : (loInclusive ?
                                        m.getLowerEntry(lo) :
                                        m.getFloorEntry(lo)));
        }

        // Abstract methods defined in ascending vs descending classes
        // These relay to the appropriate absolute versions

        abstract TreeMap.Entry<K,V> subLowest();
        abstract TreeMap.Entry<K,V> subHighest();
        abstract TreeMap.Entry<K,V> subCeiling(K key);
        abstract TreeMap.Entry<K,V> subHigher(K key);
        abstract TreeMap.Entry<K,V> subFloor(K key);
        abstract TreeMap.Entry<K,V> subLower(K key);

        /** Returns ascending iterator from the perspective of this submap */
        abstract Iterator<K> keyIterator();

        abstract Spliterator<K> keySpliterator();

        /** Returns descending iterator from the perspective of this submap */
        abstract Iterator<K> descendingKeyIterator();

        // public methods

        public boolean isEmpty() {
            return (fromStart && toEnd) ? m.isEmpty() : entrySet().isEmpty();
        }

        public int size() {
            return (fromStart && toEnd) ? m.size() : entrySet().size();
        }

        public final boolean containsKey(Object key) {
            return inRange(key) && m.containsKey(key);
        }

        public final V put(K key, V value) {
            if (!inRange(key))
                throw new IllegalArgumentException("key out of range");
            return m.put(key, value);
        }

        public final V get(Object key) {
            return !inRange(key) ? null :  m.get(key);
        }

        public final V remove(Object key) {
            return !inRange(key) ? null : m.remove(key);
        }

        public final Map.Entry<K,V> ceilingEntry(K key) {
            return exportEntry(subCeiling(key));
        }

        public final K ceilingKey(K key) {
            return keyOrNull(subCeiling(key));
        }

        public final Map.Entry<K,V> higherEntry(K key) {
            return exportEntry(subHigher(key));
        }

        public final K higherKey(K key) {
            return keyOrNull(subHigher(key));
        }

        public final Map.Entry<K,V> floorEntry(K key) {
            return exportEntry(subFloor(key));
        }

        public final K floorKey(K key) {
            return keyOrNull(subFloor(key));
        }

        public final Map.Entry<K,V> lowerEntry(K key) {
            return exportEntry(subLower(key));
        }

        public final K lowerKey(K key) {
            return keyOrNull(subLower(key));
        }

        public final K firstKey() {
            return key(subLowest());
        }

        public final K lastKey() {
            return key(subHighest());
        }

        public final Map.Entry<K,V> firstEntry() {
            return exportEntry(subLowest());
        }

        public final Map.Entry<K,V> lastEntry() {
            return exportEntry(subHighest());
        }

        public final Map.Entry<K,V> pollFirstEntry() {
            TreeMap.Entry<K,V> e = subLowest();
            Map.Entry<K,V> result = exportEntry(e);
            if (e != null)
                m.deleteEntry(e);
            return result;
        }

        public final Map.Entry<K,V> pollLastEntry() {
            TreeMap.Entry<K,V> e = subHighest();
            Map.Entry<K,V> result = exportEntry(e);
            if (e != null)
                m.deleteEntry(e);
            return result;
        }

        // Views
        transient NavigableMap<K,V> descendingMapView;
        transient EntrySetView entrySetView;
        transient KeySet<K> navigableKeySetView;

        public final NavigableSet<K> navigableKeySet() {
            KeySet<K> nksv = navigableKeySetView;
            return (nksv != null) ? nksv :
                (navigableKeySetView = new TreeMap.KeySet<>(this));
        }

        public final Set<K> keySet() {
            return navigableKeySet();
        }

        public NavigableSet<K> descendingKeySet() {
            return descendingMap().navigableKeySet();
        }

        public final SortedMap<K,V> subMap(K fromKey, K toKey) {
            return subMap(fromKey, true, toKey, false);
        }

        public final SortedMap<K,V> headMap(K toKey) {
            return headMap(toKey, false);
        }

        public final SortedMap<K,V> tailMap(K fromKey) {
            return tailMap(fromKey, true);
        }

        // View classes

        abstract class EntrySetView extends AbstractSet<Map.Entry<K,V>> {
            private transient int size = -1, sizeModCount;

            public int size() {
                if (fromStart && toEnd)
                    return m.size();
                if (size == -1 || sizeModCount != m.modCount) {
                    sizeModCount = m.modCount;
                    size = 0;
                    Iterator<?> i = iterator();
                    while (i.hasNext()) {
                        size++;
                        i.next();
                    }
                }
                return size;
            }

            public boolean isEmpty() {
                TreeMap.Entry<K,V> n = absLowest();
                return n == null || tooHigh(n.key);
            }

            public boolean contains(Object o) {
                if (!(o instanceof Map.Entry))
                    return false;
                Map.Entry<?,?> entry = (Map.Entry<?,?>) o;
                Object key = entry.getKey();
                if (!inRange(key))
                    return false;
                TreeMap.Entry<?,?> node = m.getEntry(key);
                return node != null &&
                    valEquals(node.getValue(), entry.getValue());
            }

            public boolean remove(Object o) {
                if (!(o instanceof Map.Entry))
                    return false;
                Map.Entry<?,?> entry = (Map.Entry<?,?>) o;
                Object key = entry.getKey();
                if (!inRange(key))
                    return false;
                TreeMap.Entry<K,V> node = m.getEntry(key);
                if (node!=null && valEquals(node.getValue(),
                                            entry.getValue())) {
                    m.deleteEntry(node);
                    return true;
                }
                return false;
            }
        }

        /**
         * Iterators for SubMaps
         */
        abstract class SubMapIterator<T> implements Iterator<T> {
            TreeMap.Entry<K,V> lastReturned;
            TreeMap.Entry<K,V> next;
            final Object fenceKey;
            int expectedModCount;

            SubMapIterator(TreeMap.Entry<K,V> first,
                           TreeMap.Entry<K,V> fence) {
                expectedModCount = m.modCount;
                lastReturned = null;
                next = first;
                fenceKey = fence == null ? UNBOUNDED : fence.key;
            }

            public final boolean hasNext() {
                return next != null && next.key != fenceKey;
            }

            final TreeMap.Entry<K,V> nextEntry() {
                TreeMap.Entry<K,V> e = next;
                if (e == null || e.key == fenceKey)
                    throw new NoSuchElementException();
                if (m.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                next = successor(e);
                lastReturned = e;
                return e;
            }

            final TreeMap.Entry<K,V> prevEntry() {
                TreeMap.Entry<K,V> e = next;
                if (e == null || e.key == fenceKey)
                    throw new NoSuchElementException();
                if (m.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                next = predecessor(e);
                lastReturned = e;
                return e;
            }

            final void removeAscending() {
                if (lastReturned == null)
                    throw new IllegalStateException();
                if (m.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                // deleted entries are replaced by their successors
                if (lastReturned.left != null && lastReturned.right != null)
                    next = lastReturned;
                m.deleteEntry(lastReturned);
                lastReturned = null;
                expectedModCount = m.modCount;
            }

            final void removeDescending() {
                if (lastReturned == null)
                    throw new IllegalStateException();
                if (m.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                m.deleteEntry(lastReturned);
                lastReturned = null;
                expectedModCount = m.modCount;
            }

        }

        final class SubMapEntryIterator extends SubMapIterator<Map.Entry<K,V>> {
            SubMapEntryIterator(TreeMap.Entry<K,V> first,
                                TreeMap.Entry<K,V> fence) {
                super(first, fence);
            }
            public Map.Entry<K,V> next() {
                return nextEntry();
            }
            public void remove() {
                removeAscending();
            }
        }

        final class DescendingSubMapEntryIterator extends SubMapIterator<Map.Entry<K,V>> {
            DescendingSubMapEntryIterator(TreeMap.Entry<K,V> last,
                                          TreeMap.Entry<K,V> fence) {
                super(last, fence);
            }

            public Map.Entry<K,V> next() {
                return prevEntry();
            }
            public void remove() {
                removeDescending();
            }
        }

        // Implement minimal Spliterator as KeySpliterator backup
        final class SubMapKeyIterator extends SubMapIterator<K>
            implements Spliterator<K> {
            SubMapKeyIterator(TreeMap.Entry<K,V> first,
                              TreeMap.Entry<K,V> fence) {
                super(first, fence);
            }
            public K next() {
                return nextEntry().key;
            }
            public void remove() {
                removeAscending();
            }
            public Spliterator<K> trySplit() {
                return null;
            }
            public void forEachRemaining(Consumer<? super K> action) {
                while (hasNext())
                    action.accept(next());
            }
            public boolean tryAdvance(Consumer<? super K> action) {
                if (hasNext()) {
                    action.accept(next());
                    return true;
                }
                return false;
            }
            public long estimateSize() {
                return Long.MAX_VALUE;
            }
            public int characteristics() {
                return Spliterator.DISTINCT | Spliterator.ORDERED |
                    Spliterator.SORTED;
            }
            public final Comparator<? super K>  getComparator() {
                return NavigableSubMap.this.comparator();
            }
        }

        final class DescendingSubMapKeyIterator extends SubMapIterator<K>
            implements Spliterator<K> {
            DescendingSubMapKeyIterator(TreeMap.Entry<K,V> last,
                                        TreeMap.Entry<K,V> fence) {
                super(last, fence);
            }
            public K next() {
                return prevEntry().key;
            }
            public void remove() {
                removeDescending();
            }
            public Spliterator<K> trySplit() {
                return null;
            }
            public void forEachRemaining(Consumer<? super K> action) {
                while (hasNext())
                    action.accept(next());
            }
            public boolean tryAdvance(Consumer<? super K> action) {
                if (hasNext()) {
                    action.accept(next());
                    return true;
                }
                return false;
            }
            public long estimateSize() {
                return Long.MAX_VALUE;
            }
            public int characteristics() {
                return Spliterator.DISTINCT | Spliterator.ORDERED;
            }
        }
    }

    /**
     * 正序子 map
     */
    static final class AscendingSubMap<K,V> extends NavigableSubMap<K,V> {
        private static final long serialVersionUID = 912986545866124060L;

        AscendingSubMap(TreeMap<K,V> m,
                        boolean fromStart, K lo, boolean loInclusive,
                        boolean toEnd,     K hi, boolean hiInclusive) {
            super(m, fromStart, lo, loInclusive, toEnd, hi, hiInclusive);
        }

        public Comparator<? super K> comparator() {
            return m.comparator();
        }

        public NavigableMap<K,V> subMap(K fromKey, boolean fromInclusive,
                                        K toKey,   boolean toInclusive) {
            if (!inRange(fromKey, fromInclusive))
                throw new IllegalArgumentException("fromKey out of range");
            if (!inRange(toKey, toInclusive))
                throw new IllegalArgumentException("toKey out of range");
            return new AscendingSubMap<>(m,
                                         false, fromKey, fromInclusive,
                                         false, toKey,   toInclusive);
        }

        public NavigableMap<K,V> headMap(K toKey, boolean inclusive) {
            if (!inRange(toKey, inclusive))
                throw new IllegalArgumentException("toKey out of range");
            return new AscendingSubMap<>(m,
                                         fromStart, lo,    loInclusive,
                                         false,     toKey, inclusive);
        }

        public NavigableMap<K,V> tailMap(K fromKey, boolean inclusive) {
            if (!inRange(fromKey, inclusive))
                throw new IllegalArgumentException("fromKey out of range");
            return new AscendingSubMap<>(m,
                                         false, fromKey, inclusive,
                                         toEnd, hi,      hiInclusive);
        }

        public NavigableMap<K,V> descendingMap() {
            NavigableMap<K,V> mv = descendingMapView;
            return (mv != null) ? mv :
                (descendingMapView =
                 new DescendingSubMap<>(m,
                                        fromStart, lo, loInclusive,
                                        toEnd,     hi, hiInclusive));
        }

        Iterator<K> keyIterator() {
            return new SubMapKeyIterator(absLowest(), absHighFence());
        }

        Spliterator<K> keySpliterator() {
            return new SubMapKeyIterator(absLowest(), absHighFence());
        }

        Iterator<K> descendingKeyIterator() {
            return new DescendingSubMapKeyIterator(absHighest(), absLowFence());
        }

        final class AscendingEntrySetView extends EntrySetView {
            public Iterator<Map.Entry<K,V>> iterator() {
                return new SubMapEntryIterator(absLowest(), absHighFence());
            }
        }

        public Set<Map.Entry<K,V>> entrySet() {
            EntrySetView es = entrySetView;
            return (es != null) ? es : (entrySetView = new AscendingEntrySetView());
        }

        TreeMap.Entry<K,V> subLowest()       { return absLowest(); }
        TreeMap.Entry<K,V> subHighest()      { return absHighest(); }
        TreeMap.Entry<K,V> subCeiling(K key) { return absCeiling(key); }
        TreeMap.Entry<K,V> subHigher(K key)  { return absHigher(key); }
        TreeMap.Entry<K,V> subFloor(K key)   { return absFloor(key); }
        TreeMap.Entry<K,V> subLower(K key)   { return absLower(key); }
    }

    /**
     * 倒序子 map
     */
    static final class DescendingSubMap<K,V>  extends NavigableSubMap<K,V> {
        private static final long serialVersionUID = 912986545866120460L;
        DescendingSubMap(TreeMap<K,V> m,
                        boolean fromStart, K lo, boolean loInclusive,
                        boolean toEnd,     K hi, boolean hiInclusive) {
            super(m, fromStart, lo, loInclusive, toEnd, hi, hiInclusive);
        }

        private final Comparator<? super K> reverseComparator =
            Collections.reverseOrder(m.comparator);

        public Comparator<? super K> comparator() {
            return reverseComparator;
        }

        public NavigableMap<K,V> subMap(K fromKey, boolean fromInclusive,
                                        K toKey,   boolean toInclusive) {
            if (!inRange(fromKey, fromInclusive))
                throw new IllegalArgumentException("fromKey out of range");
            if (!inRange(toKey, toInclusive))
                throw new IllegalArgumentException("toKey out of range");
            return new DescendingSubMap<>(m,
                                          false, toKey,   toInclusive,
                                          false, fromKey, fromInclusive);
        }

        public NavigableMap<K,V> headMap(K toKey, boolean inclusive) {
            if (!inRange(toKey, inclusive))
                throw new IllegalArgumentException("toKey out of range");
            return new DescendingSubMap<>(m,
                                          false, toKey, inclusive,
                                          toEnd, hi,    hiInclusive);
        }

        public NavigableMap<K,V> tailMap(K fromKey, boolean inclusive) {
            if (!inRange(fromKey, inclusive))
                throw new IllegalArgumentException("fromKey out of range");
            return new DescendingSubMap<>(m,
                                          fromStart, lo, loInclusive,
                                          false, fromKey, inclusive);
        }

        public NavigableMap<K,V> descendingMap() {
            NavigableMap<K,V> mv = descendingMapView;
            return (mv != null) ? mv :
                (descendingMapView =
                 new AscendingSubMap<>(m,
                                       fromStart, lo, loInclusive,
                                       toEnd,     hi, hiInclusive));
        }

        Iterator<K> keyIterator() {
            return new DescendingSubMapKeyIterator(absHighest(), absLowFence());
        }

        Spliterator<K> keySpliterator() {
            return new DescendingSubMapKeyIterator(absHighest(), absLowFence());
        }

        Iterator<K> descendingKeyIterator() {
            return new SubMapKeyIterator(absLowest(), absHighFence());
        }

        final class DescendingEntrySetView extends EntrySetView {
            public Iterator<Map.Entry<K,V>> iterator() {
                return new DescendingSubMapEntryIterator(absHighest(), absLowFence());
            }
        }

        public Set<Map.Entry<K,V>> entrySet() {
            EntrySetView es = entrySetView;
            return (es != null) ? es : (entrySetView = new DescendingEntrySetView());
        }

        TreeMap.Entry<K,V> subLowest()       { return absHighest(); }
        TreeMap.Entry<K,V> subHighest()      { return absLowest(); }
        TreeMap.Entry<K,V> subCeiling(K key) { return absFloor(key); }
        TreeMap.Entry<K,V> subHigher(K key)  { return absLower(key); }
        TreeMap.Entry<K,V> subFloor(K key)   { return absCeiling(key); }
        TreeMap.Entry<K,V> subLower(K key)   { return absHigher(key); }
    }

    /**
     * This class exists solely for the sake of serialization
     * compatibility with previous releases of TreeMap that did not
     * support NavigableMap.  It translates an old-version SubMap into
     * a new-version AscendingSubMap. This class is never otherwise
     * used.
     *
     * @serial include
     */
    private class SubMap extends AbstractMap<K,V>
        implements SortedMap<K,V>, java.io.Serializable {
        private static final long serialVersionUID = -6520786458950516097L;
        private boolean fromStart = false, toEnd = false;
        private K fromKey, toKey;
        private Object readResolve() {
            return new AscendingSubMap<>(TreeMap.this,
                                         fromStart, fromKey, true,
                                         toEnd, toKey, false);
        }
        public Set<Map.Entry<K,V>> entrySet() { throw new InternalError(); }
        public K lastKey() { throw new InternalError(); }
        public K firstKey() { throw new InternalError(); }
        public SortedMap<K,V> subMap(K fromKey, K toKey) { throw new InternalError(); }
        public SortedMap<K,V> headMap(K toKey) { throw new InternalError(); }
        public SortedMap<K,V> tailMap(K fromKey) { throw new InternalError(); }
        public Comparator<? super K> comparator() { throw new InternalError(); }
    }


    // Red-black mechanics

    private static final boolean RED   = false;
    private static final boolean BLACK = true;

    /**
     * Node in the Tree.  Doubles as a means to pass key-value pairs back to
     * user (see Map.Entry).
     */

    static final class Entry<K,V> implements Map.Entry<K,V> {
        K key;
        V value;
        Entry<K,V> left;
        Entry<K,V> right;
        Entry<K,V> parent;
        boolean color = BLACK;

        /**
         * Make a new cell with given key, value, and parent, and with
         * {@code null} child links, and BLACK color.
         */
        Entry(K key, V value, Entry<K,V> parent) {
            this.key = key;
            this.value = value;
            this.parent = parent;
        }

        /**
         * Returns the key.
         *
         * @return the key
         */
        public K getKey() {
            return key;
        }

        /**
         * Returns the value associated with the key.
         *
         * @return the value associated with the key
         */
        public V getValue() {
            return value;
        }

        /**
         * Replaces the value currently associated with the key with the given
         * value.
         *
         * @return the value associated with the key before this method was
         *         called
         */
        public V setValue(V value) {
            V oldValue = this.value;
            this.value = value;
            return oldValue;
        }

        public boolean equals(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?,?> e = (Map.Entry<?,?>)o;

            return valEquals(key,e.getKey()) && valEquals(value,e.getValue());
        }

        public int hashCode() {
            int keyHash = (key==null ? 0 : key.hashCode());
            int valueHash = (value==null ? 0 : value.hashCode());
            return keyHash ^ valueHash;
        }

        public String toString() {
            return key + "=" + value;
        }
    }

    /**
     * Returns the first Entry in the TreeMap (according to the TreeMap's
     * key-sort function).  Returns null if the TreeMap is empty.
     */
    final Entry<K,V> getFirstEntry() {
        Entry<K,V> p = root;
        if (p != null)
            while (p.left != null)
                p = p.left;
        return p;
    }

    /**
     * Returns the last Entry in the TreeMap (according to the TreeMap's
     * key-sort function).  Returns null if the TreeMap is empty.
     */
    final Entry<K,V> getLastEntry() {
        Entry<K,V> p = root;
        if (p != null)
            while (p.right != null)
                p = p.right;
        return p;
    }

    /**
     * Returns the successor of the specified Entry, or null if no such.
     */
    static <K,V> TreeMap.Entry<K,V> successor(Entry<K,V> t) {
        if (t == null)
            return null;
        else if (t.right != null) {
            Entry<K,V> p = t.right;
            while (p.left != null)
                p = p.left;
            return p;
        } else {
            Entry<K,V> p = t.parent;
            Entry<K,V> ch = t;
            while (p != null && ch == p.right) {
                ch = p;
                p = p.parent;
            }
            return p;
        }
    }

    /**
     * Returns the predecessor of the specified Entry, or null if no such.
     */
    static <K,V> Entry<K,V> predecessor(Entry<K,V> t) {
        if (t == null)
            return null;
        else if (t.left != null) {
            Entry<K,V> p = t.left;
            while (p.right != null)
                p = p.right;
            return p;
        } else {
            Entry<K,V> p = t.parent;
            Entry<K,V> ch = t;
            while (p != null && ch == p.left) {
                ch = p;
                p = p.parent;
            }
            return p;
        }
    }

    /**
     * Balancing operations.
     *
     * Implementations of rebalancings during insertion and deletion are
     * slightly different than the CLR version.  Rather than using dummy
     * nilnodes, we use a set of accessors that deal properly with null.  They
     * are used to avoid messiness surrounding nullness checks in the main
     * algorithms.
     */

    private static <K,V> boolean colorOf(Entry<K,V> p) {
        return (p == null ? BLACK : p.color);
    }

    private static <K,V> Entry<K,V> parentOf(Entry<K,V> p) {
        return (p == null ? null: p.parent);
    }

    private static <K,V> void setColor(Entry<K,V> p, boolean c) {
        if (p != null)
            p.color = c;
    }

    private static <K,V> Entry<K,V> leftOf(Entry<K,V> p) {
        return (p == null) ? null: p.left;
    }

    private static <K,V> Entry<K,V> rightOf(Entry<K,V> p) {
        return (p == null) ? null: p.right;
    }

    /** From CLR */
    private void rotateLeft(Entry<K,V> p) {
        if (p != null) {
            Entry<K,V> r = p.right;
            p.right = r.left;
            if (r.left != null)
                r.left.parent = p;
            r.parent = p.parent;
            if (p.parent == null)
                root = r;
            else if (p.parent.left == p)
                p.parent.left = r;
            else
                p.parent.right = r;
            r.left = p;
            p.parent = r;
        }
    }

    /** From CLR */
    private void rotateRight(Entry<K,V> p) {
        if (p != null) {
            Entry<K,V> l = p.left;
            p.left = l.right;
            if (l.right != null) l.right.parent = p;
            l.parent = p.parent;
            if (p.parent == null)
                root = l;
            else if (p.parent.right == p)
                p.parent.right = l;
            else p.parent.left = l;
            l.right = p;
            p.parent = l;
        }
    }

    /** From CLR */
    private void fixAfterInsertion(Entry<K,V> x) {
        x.color = RED;

        while (x != null && x != root && x.parent.color == RED) {
            if (parentOf(x) == leftOf(parentOf(parentOf(x)))) {
                Entry<K,V> y = rightOf(parentOf(parentOf(x)));
                if (colorOf(y) == RED) {
                    setColor(parentOf(x), BLACK);
                    setColor(y, BLACK);
                    setColor(parentOf(parentOf(x)), RED);
                    x = parentOf(parentOf(x));
                } else {
                    if (x == rightOf(parentOf(x))) {
                        x = parentOf(x);
                        rotateLeft(x);
                    }
                    setColor(parentOf(x), BLACK);
                    setColor(parentOf(parentOf(x)), RED);
                    rotateRight(parentOf(parentOf(x)));
                }
            } else {
                Entry<K,V> y = leftOf(parentOf(parentOf(x)));
                if (colorOf(y) == RED) {
                    setColor(parentOf(x), BLACK);
                    setColor(y, BLACK);
                    setColor(parentOf(parentOf(x)), RED);
                    x = parentOf(parentOf(x));
                } else {
                    if (x == leftOf(parentOf(x))) {
                        x = parentOf(x);
                        rotateRight(x);
                    }
                    setColor(parentOf(x), BLACK);
                    setColor(parentOf(parentOf(x)), RED);
                    rotateLeft(parentOf(parentOf(x)));
                }
            }
        }
        root.color = BLACK;
    }

    /**
     * Delete node p, and then rebalance the tree.
     */
    private void deleteEntry(Entry<K,V> p) {
        modCount++;
        size--;

        // If strictly internal, copy successor's element to p and then make p
        // point to successor.
        if (p.left != null && p.right != null) {
            Entry<K,V> s = successor(p);
            p.key = s.key;
            p.value = s.value;
            p = s;
        } // p has 2 children

        // Start fixup at replacement node, if it exists.
        Entry<K,V> replacement = (p.left != null ? p.left : p.right);

        if (replacement != null) {
            // Link replacement to parent
            replacement.parent = p.parent;
            if (p.parent == null)
                root = replacement;
            else if (p == p.parent.left)
                p.parent.left  = replacement;
            else
                p.parent.right = replacement;

            // Null out links so they are OK to use by fixAfterDeletion.
            p.left = p.right = p.parent = null;

            // Fix replacement
            if (p.color == BLACK)
                fixAfterDeletion(replacement);
        } else if (p.parent == null) { // return if we are the only node.
            root = null;
        } else { //  No children. Use self as phantom replacement and unlink.
            if (p.color == BLACK)
                fixAfterDeletion(p);

            if (p.parent != null) {
                if (p == p.parent.left)
                    p.parent.left = null;
                else if (p == p.parent.right)
                    p.parent.right = null;
                p.parent = null;
            }
        }
    }

    /** From CLR */
    private void fixAfterDeletion(Entry<K,V> x) {
        while (x != root && colorOf(x) == BLACK) {
            if (x == leftOf(parentOf(x))) {
                Entry<K,V> sib = rightOf(parentOf(x));

                if (colorOf(sib) == RED) {
                    setColor(sib, BLACK);
                    setColor(parentOf(x), RED);
                    rotateLeft(parentOf(x));
                    sib = rightOf(parentOf(x));
                }

                if (colorOf(leftOf(sib))  == BLACK &&
                    colorOf(rightOf(sib)) == BLACK) {
                    setColor(sib, RED);
                    x = parentOf(x);
                } else {
                    if (colorOf(rightOf(sib)) == BLACK) {
                        setColor(leftOf(sib), BLACK);
                        setColor(sib, RED);
                        rotateRight(sib);
                        sib = rightOf(parentOf(x));
                    }
                    setColor(sib, colorOf(parentOf(x)));
                    setColor(parentOf(x), BLACK);
                    setColor(rightOf(sib), BLACK);
                    rotateLeft(parentOf(x));
                    x = root;
                }
            } else { // symmetric
                Entry<K,V> sib = leftOf(parentOf(x));

                if (colorOf(sib) == RED) {
                    setColor(sib, BLACK);
                    setColor(parentOf(x), RED);
                    rotateRight(parentOf(x));
                    sib = leftOf(parentOf(x));
                }

                if (colorOf(rightOf(sib)) == BLACK &&
                    colorOf(leftOf(sib)) == BLACK) {
                    setColor(sib, RED);
                    x = parentOf(x);
                } else {
                    if (colorOf(leftOf(sib)) == BLACK) {
                        setColor(rightOf(sib), BLACK);
                        setColor(sib, RED);
                        rotateLeft(sib);
                        sib = leftOf(parentOf(x));
                    }
                    setColor(sib, colorOf(parentOf(x)));
                    setColor(parentOf(x), BLACK);
                    setColor(leftOf(sib), BLACK);
                    rotateRight(parentOf(x));
                    x = root;
                }
            }
        }

        setColor(x, BLACK);
    }

    private static final long serialVersionUID = 919286545866124006L;

    /**
     * Save the state of the {@code TreeMap} instance to a stream (i.e.,
     * serialize it).
     *
     * @serialData The <em>size</em> of the TreeMap (the number of key-value
     *             mappings) is emitted (int), followed by the key (Object)
     *             and value (Object) for each key-value mapping represented
     *             by the TreeMap. The key-value mappings are emitted in
     *             key-order (as determined by the TreeMap's Comparator,
     *             or by the keys' natural ordering if the TreeMap has no
     *             Comparator).
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // Write out the Comparator and any hidden stuff
        s.defaultWriteObject();

        // Write out size (number of Mappings)
        s.writeInt(size);

        // Write out keys and values (alternating)
        for (Iterator<Map.Entry<K,V>> i = entrySet().iterator(); i.hasNext(); ) {
            Map.Entry<K,V> e = i.next();
            s.writeObject(e.getKey());
            s.writeObject(e.getValue());
        }
    }

    /**
     * Reconstitute the {@code TreeMap} instance from a stream (i.e.,
     * deserialize it).
     */
    private void readObject(final java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in the Comparator and any hidden stuff
        s.defaultReadObject();

        // Read in size
        int size = s.readInt();

        buildFromSorted(size, null, s, null);
    }

    /** Intended to be called only from TreeSet.readObject */
    void readTreeSet(int size, java.io.ObjectInputStream s, V defaultVal)
        throws java.io.IOException, ClassNotFoundException {
        buildFromSorted(size, null, s, defaultVal);
    }

    /** Intended to be called only from TreeSet.addAll */
    void addAllForTreeSet(SortedSet<? extends K> set, V defaultVal) {
        try {
            buildFromSorted(set.size(), set.iterator(), null, defaultVal);
        } catch (java.io.IOException cannotHappen) {
        } catch (ClassNotFoundException cannotHappen) {
        }
    }


    /**
     * Linear time tree building algorithm from sorted data.  Can accept keys
     * and/or values from iterator or stream. This leads to too many
     * parameters, but seems better than alternatives.  The four formats
     * that this method accepts are:
     *
     *    1) An iterator of Map.Entries.  (it != null, defaultVal == null).
     *    2) An iterator of keys.         (it != null, defaultVal != null).
     *    3) A stream of alternating serialized keys and values.
     *                                   (it == null, defaultVal == null).
     *    4) A stream of serialized keys. (it == null, defaultVal != null).
     *
     * It is assumed that the comparator of the TreeMap is already set prior
     * to calling this method.
     *
     * @param size the number of keys (or key-value pairs) to be read from
     *        the iterator or stream
     * @param it If non-null, new entries are created from entries
     *        or keys read from this iterator.
     * @param str If non-null, new entries are created from keys and
     *        possibly values read from this stream in serialized form.
     *        Exactly one of it and str should be non-null.
     * @param defaultVal if non-null, this default value is used for
     *        each value in the map.  If null, each value is read from
     *        iterator or stream, as described above.
     * @throws java.io.IOException propagated from stream reads. This cannot
     *         occur if str is null.
     * @throws ClassNotFoundException propagated from readObject.
     *         This cannot occur if str is null.
     */
    private void buildFromSorted(int size, Iterator<?> it,
                                 java.io.ObjectInputStream str,
                                 V defaultVal)
        throws  java.io.IOException, ClassNotFoundException {
        this.size = size;
        root = buildFromSorted(0, 0, size-1, computeRedLevel(size),
                               it, str, defaultVal);
    }

    /**
     * Recursive "helper method" that does the real work of the
     * previous method.  Identically named parameters have
     * identical definitions.  Additional parameters are documented below.
     * It is assumed that the comparator and size fields of the TreeMap are
     * already set prior to calling this method.  (It ignores both fields.)
     *
     * @param level the current level of tree. Initial call should be 0.
     * @param lo the first element index of this subtree. Initial should be 0.
     * @param hi the last element index of this subtree.  Initial should be
     *        size-1.
     * @param redLevel the level at which nodes should be red.
     *        Must be equal to computeRedLevel for tree of this size.
     */
    @SuppressWarnings("unchecked")
    private final Entry<K,V> buildFromSorted(int level, int lo, int hi,
                                             int redLevel,
                                             Iterator<?> it,
                                             java.io.ObjectInputStream str,
                                             V defaultVal)
        throws  java.io.IOException, ClassNotFoundException {
        /*
         * Strategy: The root is the middlemost element. To get to it, we
         * have to first recursively construct the entire left subtree,
         * so as to grab all of its elements. We can then proceed with right
         * subtree.
         *
         * The lo and hi arguments are the minimum and maximum
         * indices to pull out of the iterator or stream for current subtree.
         * They are not actually indexed, we just proceed sequentially,
         * ensuring that items are extracted in corresponding order.
         */

        if (hi < lo) return null;

        int mid = (lo + hi) >>> 1;

        Entry<K,V> left  = null;
        if (lo < mid)
            left = buildFromSorted(level+1, lo, mid - 1, redLevel,
                                   it, str, defaultVal);

        // extract key and/or value from iterator or stream
        K key;
        V value;
        if (it != null) {
            if (defaultVal==null) {
                Map.Entry<?,?> entry = (Map.Entry<?,?>)it.next();
                key = (K)entry.getKey();
                value = (V)entry.getValue();
            } else {
                key = (K)it.next();
                value = defaultVal;
            }
        } else { // use stream
            key = (K) str.readObject();
            value = (defaultVal != null ? defaultVal : (V) str.readObject());
        }

        Entry<K,V> middle =  new Entry<>(key, value, null);

        // color nodes in non-full bottommost level red
        if (level == redLevel)
            middle.color = RED;

        if (left != null) {
            middle.left = left;
            left.parent = middle;
        }

        if (mid < hi) {
            Entry<K,V> right = buildFromSorted(level+1, mid+1, hi, redLevel,
                                               it, str, defaultVal);
            middle.right = right;
            right.parent = middle;
        }

        return middle;
    }

    /**
     * Find the level down to which to assign all nodes BLACK.  This is the
     * last `full' level of the complete binary tree produced by
     * buildTree. The remaining nodes are colored RED. (This makes a `nice'
     * set of color assignments wrt future insertions.) This level number is
     * computed by finding the number of splits needed to reach the zeroeth
     * node.  (The answer is ~lg(N), but in any case must be computed by same
     * quick O(lg(N)) loop.)
     */
    private static int computeRedLevel(int sz) {
        int level = 0;
        for (int m = sz - 1; m >= 0; m = m / 2 - 1)
            level++;
        return level;
    }

    /**
     * Currently, we support Spliterator-based versions only for the
     * full map, in either plain of descending form, otherwise relying
     * on defaults because size estimation for submaps would dominate
     * costs. The type tests needed to check these for key views are
     * not very nice but avoid disrupting existing class
     * structures. Callers must use plain default spliterators if this
     * returns null.
     */
    static <K> Spliterator<K> keySpliteratorFor(NavigableMap<K,?> m) {
        if (m instanceof TreeMap) {
            @SuppressWarnings("unchecked") TreeMap<K,Object> t =
                (TreeMap<K,Object>) m;
            return t.keySpliterator();
        }
        if (m instanceof DescendingSubMap) {
            @SuppressWarnings("unchecked") DescendingSubMap<K,?> dm =
                (DescendingSubMap<K,?>) m;
            TreeMap<K,?> tm = dm.m;
            if (dm == tm.descendingMap) {
                @SuppressWarnings("unchecked") TreeMap<K,Object> t =
                    (TreeMap<K,Object>) tm;
                return t.descendingKeySpliterator();
            }
        }
        @SuppressWarnings("unchecked") NavigableSubMap<K,?> sm =
            (NavigableSubMap<K,?>) m;
        return sm.keySpliterator();
    }

    final Spliterator<K> keySpliterator() {
        return new KeySpliterator<K,V>(this, null, null, 0, -1, 0);
    }

    final Spliterator<K> descendingKeySpliterator() {
        return new DescendingKeySpliterator<K,V>(this, null, null, 0, -2, 0);
    }

    /**
     * Base class for spliterators.  Iteration starts at a given
     * origin and continues up to but not including a given fence (or
     * null for end).  At top-level, for ascending cases, the first
     * split uses the root as left-fence/right-origin. From there,
     * right-hand splits replace the current fence with its left
     * child, also serving as origin for the split-off spliterator.
     * Left-hands are symmetric. Descending versions place the origin
     * at the end and invert ascending split rules.  This base class
     * is non-commital about directionality, or whether the top-level
     * spliterator covers the whole tree. This means that the actual
     * split mechanics are located in subclasses. Some of the subclass
     * trySplit methods are identical (except for return types), but
     * not nicely factorable.
     *
     * Currently, subclass versions exist only for the full map
     * (including descending keys via its descendingMap).  Others are
     * possible but currently not worthwhile because submaps require
     * O(n) computations to determine size, which substantially limits
     * potential speed-ups of using custom Spliterators versus default
     * mechanics.
     *
     * To boostrap initialization, external constructors use
     * negative size estimates: -1 for ascend, -2 for descend.
     */
    static class TreeMapSpliterator<K,V> {
        final TreeMap<K,V> tree;
        TreeMap.Entry<K,V> current; // traverser; initially first node in range
        TreeMap.Entry<K,V> fence;   // one past last, or null
        int side;                   // 0: top, -1: is a left split, +1: right
        int est;                    // size estimate (exact only for top-level)
        int expectedModCount;       // for CME checks

        TreeMapSpliterator(TreeMap<K,V> tree,
                           TreeMap.Entry<K,V> origin, TreeMap.Entry<K,V> fence,
                           int side, int est, int expectedModCount) {
            this.tree = tree;
            this.current = origin;
            this.fence = fence;
            this.side = side;
            this.est = est;
            this.expectedModCount = expectedModCount;
        }

        final int getEstimate() { // force initialization
            int s; TreeMap<K,V> t;
            if ((s = est) < 0) {
                if ((t = tree) != null) {
                    current = (s == -1) ? t.getFirstEntry() : t.getLastEntry();
                    s = est = t.size;
                    expectedModCount = t.modCount;
                }
                else
                    s = est = 0;
            }
            return s;
        }

        public final long estimateSize() {
            return (long)getEstimate();
        }
    }

    static final class KeySpliterator<K,V>
        extends TreeMapSpliterator<K,V>
        implements Spliterator<K> {
        KeySpliterator(TreeMap<K,V> tree,
                       TreeMap.Entry<K,V> origin, TreeMap.Entry<K,V> fence,
                       int side, int est, int expectedModCount) {
            super(tree, origin, fence, side, est, expectedModCount);
        }

        public KeySpliterator<K,V> trySplit() {
            if (est < 0)
                getEstimate(); // force initialization
            int d = side;
            TreeMap.Entry<K,V> e = current, f = fence,
                s = ((e == null || e == f) ? null :      // empty
                     (d == 0)              ? tree.root : // was top
                     (d >  0)              ? e.right :   // was right
                     (d <  0 && f != null) ? f.left :    // was left
                     null);
            if (s != null && s != e && s != f &&
                tree.compare(e.key, s.key) < 0) {        // e not already past s
                side = 1;
                return new KeySpliterator<>
                    (tree, e, current = s, -1, est >>>= 1, expectedModCount);
            }
            return null;
        }

        public void forEachRemaining(Consumer<? super K> action) {
            if (action == null)
                throw new NullPointerException();
            if (est < 0)
                getEstimate(); // force initialization
            TreeMap.Entry<K,V> f = fence, e, p, pl;
            if ((e = current) != null && e != f) {
                current = f; // exhaust
                do {
                    action.accept(e.key);
                    if ((p = e.right) != null) {
                        while ((pl = p.left) != null)
                            p = pl;
                    }
                    else {
                        while ((p = e.parent) != null && e == p.right)
                            e = p;
                    }
                } while ((e = p) != null && e != f);
                if (tree.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
            }
        }

        public boolean tryAdvance(Consumer<? super K> action) {
            TreeMap.Entry<K,V> e;
            if (action == null)
                throw new NullPointerException();
            if (est < 0)
                getEstimate(); // force initialization
            if ((e = current) == null || e == fence)
                return false;
            current = successor(e);
            action.accept(e.key);
            if (tree.modCount != expectedModCount)
                throw new ConcurrentModificationException();
            return true;
        }

        public int characteristics() {
            return (side == 0 ? Spliterator.SIZED : 0) |
                Spliterator.DISTINCT | Spliterator.SORTED | Spliterator.ORDERED;
        }

        public final Comparator<? super K>  getComparator() {
            return tree.comparator;
        }

    }

    static final class DescendingKeySpliterator<K,V>
        extends TreeMapSpliterator<K,V>
        implements Spliterator<K> {
        DescendingKeySpliterator(TreeMap<K,V> tree,
                                 TreeMap.Entry<K,V> origin, TreeMap.Entry<K,V> fence,
                                 int side, int est, int expectedModCount) {
            super(tree, origin, fence, side, est, expectedModCount);
        }

        public DescendingKeySpliterator<K,V> trySplit() {
            if (est < 0)
                getEstimate(); // force initialization
            int d = side;
            TreeMap.Entry<K,V> e = current, f = fence,
                    s = ((e == null || e == f) ? null :      // empty
                         (d == 0)              ? tree.root : // was top
                         (d <  0)              ? e.left :    // was left
                         (d >  0 && f != null) ? f.right :   // was right
                         null);
            if (s != null && s != e && s != f &&
                tree.compare(e.key, s.key) > 0) {       // e not already past s
                side = 1;
                return new DescendingKeySpliterator<>
                        (tree, e, current = s, -1, est >>>= 1, expectedModCount);
            }
            return null;
        }

        public void forEachRemaining(Consumer<? super K> action) {
            if (action == null)
                throw new NullPointerException();
            if (est < 0)
                getEstimate(); // force initialization
            TreeMap.Entry<K,V> f = fence, e, p, pr;
            if ((e = current) != null && e != f) {
                current = f; // exhaust
                do {
                    action.accept(e.key);
                    if ((p = e.left) != null) {
                        while ((pr = p.right) != null)
                            p = pr;
                    }
                    else {
                        while ((p = e.parent) != null && e == p.left)
                            e = p;
                    }
                } while ((e = p) != null && e != f);
                if (tree.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
            }
        }

        public boolean tryAdvance(Consumer<? super K> action) {
            TreeMap.Entry<K,V> e;
            if (action == null)
                throw new NullPointerException();
            if (est < 0)
                getEstimate(); // force initialization
            if ((e = current) == null || e == fence)
                return false;
            current = predecessor(e);
            action.accept(e.key);
            if (tree.modCount != expectedModCount)
                throw new ConcurrentModificationException();
            return true;
        }

        public int characteristics() {
            return (side == 0 ? Spliterator.SIZED : 0) |
                Spliterator.DISTINCT | Spliterator.ORDERED;
        }
    }

    static final class ValueSpliterator<K,V>
            extends TreeMapSpliterator<K,V>
            implements Spliterator<V> {
        ValueSpliterator(TreeMap<K,V> tree,
                         TreeMap.Entry<K,V> origin, TreeMap.Entry<K,V> fence,
                         int side, int est, int expectedModCount) {
            super(tree, origin, fence, side, est, expectedModCount);
        }

        public ValueSpliterator<K,V> trySplit() {
            if (est < 0)
                getEstimate(); // force initialization
            int d = side;
            TreeMap.Entry<K,V> e = current, f = fence,
                    s = ((e == null || e == f) ? null :      // empty
                         (d == 0)              ? tree.root : // was top
                         (d >  0)              ? e.right :   // was right
                         (d <  0 && f != null) ? f.left :    // was left
                         null);
            if (s != null && s != e && s != f &&
                tree.compare(e.key, s.key) < 0) {        // e not already past s
                side = 1;
                return new ValueSpliterator<>
                        (tree, e, current = s, -1, est >>>= 1, expectedModCount);
            }
            return null;
        }

        public void forEachRemaining(Consumer<? super V> action) {
            if (action == null)
                throw new NullPointerException();
            if (est < 0)
                getEstimate(); // force initialization
            TreeMap.Entry<K,V> f = fence, e, p, pl;
            if ((e = current) != null && e != f) {
                current = f; // exhaust
                do {
                    action.accept(e.value);
                    if ((p = e.right) != null) {
                        while ((pl = p.left) != null)
                            p = pl;
                    }
                    else {
                        while ((p = e.parent) != null && e == p.right)
                            e = p;
                    }
                } while ((e = p) != null && e != f);
                if (tree.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
            }
        }

        public boolean tryAdvance(Consumer<? super V> action) {
            TreeMap.Entry<K,V> e;
            if (action == null)
                throw new NullPointerException();
            if (est < 0)
                getEstimate(); // force initialization
            if ((e = current) == null || e == fence)
                return false;
            current = successor(e);
            action.accept(e.value);
            if (tree.modCount != expectedModCount)
                throw new ConcurrentModificationException();
            return true;
        }

        public int characteristics() {
            return (side == 0 ? Spliterator.SIZED : 0) | Spliterator.ORDERED;
        }
    }

    static final class EntrySpliterator<K,V>
        extends TreeMapSpliterator<K,V>
        implements Spliterator<Map.Entry<K,V>> {
        EntrySpliterator(TreeMap<K,V> tree,
                         TreeMap.Entry<K,V> origin, TreeMap.Entry<K,V> fence,
                         int side, int est, int expectedModCount) {
            super(tree, origin, fence, side, est, expectedModCount);
        }

        public EntrySpliterator<K,V> trySplit() {
            if (est < 0)
                getEstimate(); // force initialization
            int d = side;
            TreeMap.Entry<K,V> e = current, f = fence,
                    s = ((e == null || e == f) ? null :      // empty
                         (d == 0)              ? tree.root : // was top
                         (d >  0)              ? e.right :   // was right
                         (d <  0 && f != null) ? f.left :    // was left
                         null);
            if (s != null && s != e && s != f &&
                tree.compare(e.key, s.key) < 0) {        // e not already past s
                side = 1;
                return new EntrySpliterator<>
                        (tree, e, current = s, -1, est >>>= 1, expectedModCount);
            }
            return null;
        }

        public void forEachRemaining(Consumer<? super Map.Entry<K, V>> action) {
            if (action == null)
                throw new NullPointerException();
            if (est < 0)
                getEstimate(); // force initialization
            TreeMap.Entry<K,V> f = fence, e, p, pl;
            if ((e = current) != null && e != f) {
                current = f; // exhaust
                do {
                    action.accept(e);
                    if ((p = e.right) != null) {
                        while ((pl = p.left) != null)
                            p = pl;
                    }
                    else {
                        while ((p = e.parent) != null && e == p.right)
                            e = p;
                    }
                } while ((e = p) != null && e != f);
                if (tree.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
            }
        }

        public boolean tryAdvance(Consumer<? super Map.Entry<K,V>> action) {
            TreeMap.Entry<K,V> e;
            if (action == null)
                throw new NullPointerException();
            if (est < 0)
                getEstimate(); // force initialization
            if ((e = current) == null || e == fence)
                return false;
            current = successor(e);
            action.accept(e);
            if (tree.modCount != expectedModCount)
                throw new ConcurrentModificationException();
            return true;
        }

        public int characteristics() {
            return (side == 0 ? Spliterator.SIZED : 0) |
                    Spliterator.DISTINCT | Spliterator.SORTED | Spliterator.ORDERED;
        }

        @Override
        public Comparator<Map.Entry<K, V>> getComparator() {
            // Adapt or create a key-based comparator
            if (tree.comparator != null) {
                return Map.Entry.comparingByKey(tree.comparator);
            }
            else {
                return (Comparator<Map.Entry<K, V>> & Serializable) (e1, e2) -> {
                    @SuppressWarnings("unchecked")
                    Comparable<? super K> k1 = (Comparable<? super K>) e1.getKey();
                    return k1.compareTo(e2.getKey());
                };
            }
        }
    }
}