Java ConcurrentHashMap是什么

/**

     * Creates a new, empty map with the default initial table size (16).

     */

    public ConcurrentHashMap() {

    }

    /**

     * Creates a new, empty map with an initial table size

     * accommodating the specified number of elements without the need

     * to dynamically resize.

     *

     * @param initialCapacity The implementation performs internal

     * sizing to accommodate this many elements.

     * @throws IllegalArgumentException if the initial capacity of

     * elements is negative

     */

    public ConcurrentHashMap(int initialCapacity) {

        if (initialCapacity < 0)

            throw new IllegalArgumentException();

        int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?

                   MAXIMUM_CAPACITY :

                   tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));

        this.sizeCtl = cap;

    }

    /**

     * Creates a new map with the same mappings as the given map.

     *

     * @param m the map

     */

    public ConcurrentHashMap(Map<? extends K, ? extends V> m) {

        this.sizeCtl = DEFAULT_CAPACITY;

        putAll(m);

    }