JAVA 常用 Map

• HashMap
• ConcurrentHashMap
• TreeMap
• LinkedHashMap
• WeakHashMap
• IdentityHashMap

HashMap

https://www.jianshu.com/p/fde06b74c1d4

ConcurrentHashMap

https://www.jianshu.com/p/fde06b74c1d4

TreeMap

非哈希表，为红黑树结构，即平衡二叉排序树。时间复杂度为 O(logN)
key 需要实现 Comparable 接口

• TreeSet 使用了 TreeMap

    // 二叉排序树遍历
    Comparable k = (Comparable) 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);   // 值相同则覆盖并结束
    } while (t != null);
    
    // 新增节点
     Entry e = new Entry<>(key, value, parent);
    if (cmp < 0)
        parent.left = e;
    else
        parent.right = e;
    
    // 平衡树
    fixAfterInsertion(e);

LinkedHashMap

继承自 HashMap 并重写 HashMap 中的方法，实现对链表的维护

• LinkedHashSet 使用了 LinkedHashMap

   static class Entry extends HashMap.Node {
        Entry before, after;
        Entry(int hash, K key, V value, Node next) {
            super(hash, key, value, next);
        }
    }

    /**
     * The head (eldest) of the doubly linked list.
     */
    transient LinkedHashMap.Entry head;

    /**
     * The tail (youngest) of the doubly linked list.
     */
    transient LinkedHashMap.Entry tail;

WeakHashMap

Entry 继承自 WeakReference，通过查询 ReferenceQueue 移除被回收的 Entry
get、put、size 等方法均会触发对 ReferenceQueue 的查询与移除 Entry

    
    private static class Entry extends WeakReference