TreeMap源码分析

TreeMap是基于红黑树来实现的，它是根据键的自然顺序，或者创建时提供的comparator来排序的。

1.属性

     /**
     * The comparator used to maintain order in this tree map, or
     * null if it uses the natural ordering of its keys.
     * TreeMap中的映射排序是根据此comparator来排序的,如果此comparator为null，则根据键的自然顺序排序
     * @serial
     */
    private final Comparator<? super K> comparator;
    // 根元素
    private transient Entry<K,V> root = null;

    /**
     * The number of entries in the tree
     * 元素的数量
     */
    private transient int size = 0;

    /**
     * The number of structural modifications to the tree.
     * TreeMap修改的次数
     */
    private transient int modCount = 0;

2.构造器

  /**
     * Constructs a new, empty tree map, using the natural ordering of its
     * keys.  All keys inserted into the map must implement the {@link
     * Comparable} interface.  Furthermore, all such keys must be
     * <em>mutually comparable</em>: {@code k1.compareTo(k2)} must not throw
     * a {@code ClassCastException} for any keys {@code k1} and
     * {@code k2} in the map.  If the user attempts to put a key into the
     * map that violates this constraint (for example, the user attempts to
     * put a string key into a map whose keys are integers), the
     * {@code put(Object key, Object value)} call will throw a
     * {@code ClassCastException}.
     */
    public TreeMap() {
        comparator = null;
    }

    /**
     * Constructs a new, empty tree map, ordered according to the given
     * comparator.  All keys inserted into the map must be <em>mutually
     * comparable</em> by the given comparator: {@code comparator.compare(k1,
     * k2)} must not throw a {@code ClassCastException} for any keys
     * {@code k1} and {@code k2} in the map.  If the user attempts to put
     * a key into the map that violates this constraint, the {@code put(Object
     * key, Object value)} call will throw a
     * {@code ClassCastException}.
     *
     * @param comparator the comparator that will be used to order this map.
     *        If {@code null}, the {@linkplain Comparable natural
     *        ordering} of the keys will be used.
     * 构造一个空的TreeMap，根据指定的comparator来排序 
     */
    public TreeMap(Comparator<? super K> comparator) {
        this.comparator = comparator;
    }

    /**
     * Constructs a new tree map containing the same mappings as the given
     * map, ordered according to the <em>natural ordering</em> of its keys.
     * All keys inserted into the new map must implement the {@link
     * Comparable} interface.  Furthermore, all such keys must be
     * <em>mutually comparable</em>: {@code k1.compareTo(k2)} must not throw
     * a {@code ClassCastException} for any keys {@code k1} and
     * {@code k2} in the map.  This method runs in n*log(n) time.
     *
     * @param  m the map whose mappings are to be placed in this map
     * @throws ClassCastException if the keys in m are not {@link Comparable},
     *         or are not mutually comparable
     * @throws NullPointerException if the specified map is null
     * 构造一个TreeMap，包含给定的Map，按照key的自然顺序排序，key要实现Comparable接口
     */
    public TreeMap(Map<? extends K, ? extends V> m) {
        comparator = null;
        putAll(m);
    }

    /**
     * Constructs a new tree map containing the same mappings and
     * using the same ordering as the specified sorted map.  This
     * method runs in linear time.
     *
     * @param  m the sorted map whose mappings are to be placed in this map,
     *         and whose comparator is to be used to sort this map
     * @throws NullPointerException if the specified map is null
     * 构造一个TreeMap，包含指定的Map，按照给定的SortedMap的排序规则进行排序
     */
    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) {
        }
    }

 3.方法

3.1：V put(K key,V value):将指定值与此映射中的指定键进行关联。如果该映射以前包含此键的映射关系，那么将替换旧值

插入操作可以分为两步：一是将该TreeMap看做一个二叉树来操作，找到将要插入的位置，然后进行插入操作。二是当插入操作完成后，为了保证红黑树的特性，需要对红黑树调整：节点旋转和颜色改变

 public V put(K key, V value) {
        Entry<K,V> t = root;
        // 如果根节点为null，那么这个插入的数据将是根节点  
        if (t == null) {
            compare(key, key); // type (and possibly null) check

            root = new Entry<>(key, value, null);
            size = 1;
            modCount++;
            return null;
        }
        // key值比较大小返回的比较结果
        int cmp;
        Entry<K,V> parent;
        // split comparator and comparable paths
        Comparator<? super K> cpr = comparator;
        // Comparator不为空的情况下，使用comparator来比较key值的大小，同时获取新插入节点的父节点parent
        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);
        }
        // Comparator为null的情况下，使用key的自然顺序进行排序,同时获取新插入节点的父节点parent
        else {
            if (key == null)
                throw new NullPointerException();
            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);
        }
        //构造新插入的节点，然后设置其父节点
        Entry<K,V> e = new Entry<>(key, value, parent);
        if (cmp < 0)
            parent.left = e;
        else
            parent.right = e;
        // 为了保证红黑树的特性，需要进行红黑树调整：节点旋转和颜色改变
        fixAfterInsertion(e);
        size++;
        modCount++;
        return null;
    }

进入fixAfterInsertion方法：进行节点旋转和颜色改变。可以参照我的另一篇文章：红黑树详解 

 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;
    }

3.2:V get(Object key):返回指定键所映射的值，如果对于该键而言，此映射不包含任何映射关系，则返回 null。

  public V get(Object key) {
        Entry<K,V> p = getEntry(key);
        return (p==null ? null : p.value);
    }
    
    // 对于查找方法，完全是依赖二叉搜索树的特点来进行的，并没有用到红黑树的特性 
    final Entry<K,V> getEntry(Object key) {
        // Offload comparator-based version for sake of performance
        if (comparator != null)
            // 如果Comparator不为null，则比较器使用Comparator，来获取对应的Value
            return getEntryUsingComparator(key);
        if (key == null)
            throw new NullPointerException();
        Comparable<? super K> k = (Comparable<? super K>) key;
        Entry<K,V> p = root;
        // 否则使用key的自然排序来获取Value
        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;
    }

3.3 remove (Object key):如果此 TreeMap 中存在该键的映射关系，则将其删除。