1.HashMap数据结构

首先先看下HashMap的数据结构图

我们都知道数组的储存方式在是连续的，查找速度比较快，但插入和删除数据比较慢

而链表的存储方式是非连续的，所以插入和删除速度较快，但查找速度就比较慢，HashMap在数据结构上两种都采用了。

    /**
     * The table, initialized on first use, and resized as
     * necessary. When allocated, length is always a power of two.
     * (We also tolerate length zero in some operations to allow
     * bootstrapping mechanics that are currently not needed.)
     */
    transient Node<K,V>[] table;

再看下Node中的变量（Node中含有hash值、key、value以及指向下个节点的next）

        final int hash;
        final K key;
        V value;
        Node<K,V> next;

        Node(int hash, K key, V value, Node<K,V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }

通过这个table变量我们就能理解上面那个数据结构图，它不仅使用了数组，而且还采用了链表结构。

2.HashMap变量

首先我们看下HashMap中的变量，在看之前思考几个问题：

1. HashMap数据结构既然使用了数组，那么数组默认内存大小是多少？

2.当已使用内存大小超过总内存的一个百分比后，需要继续增大内存，这个百分比是多少？

3.当链表节点长多超过多少时储存结构变成红黑树？

    /**
     * The default initial capacity - MUST be a power of two.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

由此可以知道数组的默认大小是16

扫描二维码关注公众号，回复： 2756261 查看本文章

    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

当数组不够大的时候，就会增加内存大小，大小的上限为1<<30,也就是2的30次方

    /**
     * The load factor used when none specified in constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

这个变量就是问题2的答案，也被称为负载因子，默认为0.75

    /**
     * The bin count threshold for using a tree rather than list for a
     * bin.  Bins are converted to trees when adding an element to a
     * bin with at least this many nodes. The value must be greater
     * than 2 and should be at least 8 to mesh with assumptions in
     * tree removal about conversion back to plain bins upon
     * shrinkage.
     */
    static final int TREEIFY_THRESHOLD = 8;

这个变量就是问题三的答案，当节点数量超过8时，储存结构转换成红黑树

  /**
     * The bin count threshold for untreeifying a (split) bin during a
     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
     * most 6 to mesh with shrinkage detection under removal.
     */
    static final int UNTREEIFY_THRESHOLD = 6;

当bin的个数小于6时由树形结构调整为列表

    /**
     * The number of key-value mappings contained in this map.
     */
    transient int size;

当前HashMap中储存key-value的大小

    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    transient int modCount;

HashMap被修改和删除的次数

3.HashMap的储存过程

        HashMap hashMap = new HashMap();
        hashMap.put("1","1230");
        System.out.print(hashMap.get("1"));

我们从hashMap.put()方法开始

    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

这里详细讲下hash()函数，首先先看源码：

    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

如果key的值不为空的话，就把变量h等于key值的hashCode，然后把h的高16位与低16位进行一个异或运算，这样得到的值和数组的默认长度16相对应，方便后续计算该Node存储的数组下标是多少。

未完。。。。待续

深入理解HashMap源码

1.HashMap数据结构

2.HashMap变量

3.HashMap的储存过程

猜你喜欢