【java源码一带一路系列】之TreeMap

TreeMap基于红黑树实现，在之前HashMap篇章中有所涉及，所以本篇重点不在此。上路~

containsKey() --> getEntry() --> getEntryUsingComparator()

/**
 * Returns {@code true} if this map contains a mapping for the specified
 * key.
 *
 * @param key key whose presence in this map is to be tested
 * @return {@code true} if this map contains a mapping for the
 *         specified key
 * @throws ClassCastException if the specified key cannot be compared
 *         with the keys currently in the map
 * @throws NullPointerException if the specified key is null
 *         and this map uses natural ordering, or its comparator
 *         does not permit null keys
 */
public boolean containsKey(Object key) {
    return getEntry(key) != null; // Key不能为null 
}


/**
 * Returns this map's entry for the given key, or {@code null} if the map
 * does not contain an entry for the key.
 *
 * @return this map's entry for the given key, or {@code null} if the map
 *         does not contain an entry for the key
 * @throws ClassCastException if the specified key cannot be compared
 *         with the keys currently in the map
 * @throws NullPointerException if the specified key is null
 *         and this map uses natural ordering, or its comparator
 *         does not permit null keys
 */
final Entry<K,V> getEntry(Object key) {
    // Offload comparator-based version for sake of performance
    if (comparator != null)
        return getEntryUsingComparator(key);
    if (key == null)
        throw new NullPointerException();
    @SuppressWarnings("unchecked")
        Comparable<? super K> k = (Comparable<? super K>) key;
    Entry<K,V> p = root;
    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;
}

/**
 * Version of getEntry using comparator. Split off from getEntry
 * for performance. (This is not worth doing for most methods,
 * that are less dependent on comparator performance, but is
 * worthwhile here.)
 */
final Entry<K,V> getEntryUsingComparator(Object key) {
    @SuppressWarnings("unchecked")
        K k = (K) key;
    Comparator<? super K> cpr = comparator;
    if (cpr != null) {
        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;
}

虽然明面上是获取值的方法，本质却是比出个高低等。这里将Java的java.util.Comparator（比较器排序）、java.lang.Comparable（自然排序）都用到了。顺便补了两者知识点(见文末①)。当然这里好奇的是源码中将使用comparator比较独立提成方法，说是能提高性能。why?反向思考下，假使将getEntryUsingComparator()方法内代码放回getEntry()似乎也就多了一对“{}”。费解- -

顺带一提，如果你还记得之前文章中的HashMap也用到了红黑树，而它先比较的hash再比key值，这比较的是key值。

put() --> compare() --> fixAfterInsertion()

/**
 * Associates the specified value with the specified key in this map.
 * If the map previously contained a mapping for the key, the old
 * value is replaced.
 *
 * @param key key with which the specified value is to be associated
 * @param value value to be associated with the specified key
 *
 * @return the previous value associated with {@code key}, or
 *         {@code null} if there was no mapping for {@code key}.
 *         (A {@code null} return can also indicate that the map
 *         previously associated {@code null} with {@code key}.)
 * @throws ClassCastException if the specified key cannot be compared
 *         with the keys currently in the map
 * @throws NullPointerException if the specified key is null
 *         and this map uses natural ordering, or its comparator
 *         does not permit null keys
 */
public V put(K key, V value) {
    Entry<K,V> t = root;
    if (t == null) {
        compare(key, key); // type (and possibly null) check

        root = new Entry<>(key, value, null);
        size = 1;
        modCount++;
        return null;
    }
    int cmp;
    Entry<K,V> parent;
    // split comparator and comparable paths
    Comparator<? super K> cpr = comparator;
    if (cpr != null) {
        do {
            parent = t;
            cmp = cpr.compare(key, t.key);
            if (cmp < 0)
                t = t.left;
            else if (cmp > 0)
                t = t.right;
            else
                return t.setValue(value);
        } while (t != null);
    }
    else {
        if (key == null)
            throw new NullPointerException();
        @SuppressWarnings("unchecked")
            Comparable<? super K> k = (Comparable<? super K>) key;
        do {
            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);
    }
    Entry<K,V> e = new Entry<>(key, value, parent);
    if (cmp < 0)
        parent.left = e;
    else
        parent.right = e;
    fixAfterInsertion(e);
    size++;
    modCount++;
    return null;
}

/**
 * Compares two keys using the correct comparison method for this TreeMap.
 */
@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);
}


/** 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;
}

"compare(key, key);"是一个有意思写法。从注释直译就是类型（为null可能性）检查。为空检查很好理解，因为null.xx()肯定跑异常，至于类型检查笔者理解是要求键值实现Comparable接口。

"from CLR"是指Cormen, Leiserson, Rivest，他们是算法导论的作者，也就是说TreeMap里面算法都是参照算法导论的伪代码。

由于TreeMap的有序性，使其增删查都是先进行比较，找到合适的位置。fixAfterInsertion()在这的作用类似HashMap中的balanceInsertion()，修复红黑树性质。

deleteEntry() --> successor() --> fixAfterDeletion()

/**
 * 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;
        }
    }
}

/**
 * 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) { // ① ②
        Entry<K,V> p = t.right;
        while (p.left != null)
            p = p.left;
        return p;
    } else { //③ ④ ⑤
        Entry<K,V> p = t.parent;
        Entry<K,V> ch = t;
        while (p != null && ch == p.right) {
            ch = p;
            p = p.parent;
        }
        return p;
    }
}

successor()可以简单的理解为“一个升序数组a[index],successor即为a[index+1]”。相对的还有prodecessor()。源码中可能出现的情况抽象如下图（while只举一次循环为例）。

deleteEntry调用successor时，由于right != null，所以不会出现③ ④ ⑤的情况。基本思路就是找到“a[index+1]”（p）替换待删节点，然后使“a[index+1]”的子节点（replacement）指向其父节点（Link replacement to parent）,最后清p、fixAfterDeletion修复红黑树性。

如果觉得这个看懂了，可以挑战下HashMap.TreeNode.removeTreeNode()。

说点什么：

TreeMap 有序；非线程安全；key值不支持null...;

实现了NavigableMap接口(见文末②)，NavigableMap具有了针对给定搜索目标返回最接近匹配项的导航方法。

如： lowerEntry、floorEntry、ceilingEntry 和 higherEntry 分别返回与小于、小于等于、大于等于、大于给定键的 Map.Entry对象，如果不存在这样的键，则返回 null。

实现了SortedMap接口：它用来保持键的有序顺序,也提供了范围检索的一些方法;

如： headMap、subMap、tailMap分别返回小于结束键、大于或等于开始和小于结束键、大于或等于开始键的Map.Entry对象。

添加到SortedMap实现类的元素必须实现Comparable接口，否则您必须给它的构造函数提供一个Comparator接口的实现。TreeMap类是它的唯一一份实现。

更多有意思的内容，欢迎访问笔者小站： rebey.cn

知识点：

①Java中自然排序和比较器排序详解:Comparable与Comparator；

②计算机程序的思维逻辑 (43) - 剖析TreeMap：方法应用举例;