Java集合源码学习笔记（四）HashMap分析

/**
     * 内部实际的存储数组，如果需要调整，容量必须是2的幂
     */
    transient Entry[] table;

static class Entry<K,V> implements Map.Entry<K,V> {
        final K key;//Key-value结构的key
        V value;//存储值
        Entry<K,V> next;//指向下一个链表节点
        final int hash;//哈希值

        /**
         * Creates new entry.
         */
        Entry(int h, K k, V v, Entry<K,V> n) {
            value = v;
            next = n;
            key = k;
            hash = h;
        }
		......
  }

public interface Map<K,V> {
	public static interface Entry<K,V> {
		//获取该Entry的key 
		public abstract Object getKey();
		//获取该Entry的value
		public abstract Object getValue();
		//设置Entry的value  
		public abstract Object setValue(Object obj);
		public abstract boolean equals(Object obj);
		public abstract int hashCode();
	}
	
	//返回键值对的数目  
	int size();
	//判断容器是否为空  
	boolean isEmpty();
	//判断容器是否包含关键字key  
	boolean containsKey(Object key);
	//判断容器是否包含值value 
	boolean containsValue(Object value);
	 //根据key获取value  
	Object get(Object key);
	 //向容器中加入新的key-value对  
	Object put(Object key, Object value);
	//根据key移除相应的键值对  
	Object remove(Object key);
	//将另一个Map中的所有键值对都添加进去  
	void putAll(Map<? extends K, ? extends V> m);
	//清除容器中的所有键值对  
	void clear();
	//返回容器中所有的key组成的Set集合  
	Set keySet();
	//返回所有的value组成的集合  
	Collection values();
	 //返回所有的键值对  
	Set<Map.Entry<K, V>> entrySet();
	
	//继承自Object的方法
	boolean equals(Object obj);
	int hashCode();
}

DEFAULT_LOAD_FACTOR = 0.75f;

public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
    table = new Entry[DEFAULT_INITIAL_CAPACITY];
     init();
    }
	
public HashMap(Map<? extends K, ? extends V> m) {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
                      DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
        putAllForCreate(m);
    }
 
 public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }
 
  public HashMap(int initialCapacity, float loadFactor) {
	  ......
  }

public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key.hashCode());
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }

public V get(Object key) {
        if (key == null)
            return getForNullKey();
        int hash = hash(key.hashCode());
        for (Entry<K,V> e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
                return e.value;
        }
        return null;
    }

private V putForNullKey(V value) {
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
		......
    }

/**
     * 哈希函数
     * 看一下具体的操作，首先对h分别进行无符号右移20位和12位，
     * 然后对两个值进行按位异或，最后再与h进行按位异或，
     * 得到新的h后再进行一次同样的操作，分别右移7位和4位，具体的哈希函数优劣就不去研究了
	 * 这个方法可以尽量减少碰撞
     */
    static int hash(int h) { 
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }

/**
     * 再散列过程
     * Rehashes the contents of this map into a new array with a
     * larger capacity.  This method is called automatically when the
     * number of keys in this map reaches its threshold.
     */
    void resize(int newCapacity) {
        Entry[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }

        Entry[] newTable = new Entry[newCapacity];
        transfer(newTable);
        table = newTable;
        threshold = (int)(newCapacity * loadFactor);
    }

    /**
     * 把当前Entry[]表的全部元素转移到新的table中
     */
    void transfer(Entry[] newTable) {
        Entry[] src = table;
        int newCapacity = newTable.length;
        for (int j = 0; j < src.length; j++) {
            Entry<K,V> e = src[j];
            if (e != null) {
                src[j] = null;
                do {
                    Entry<K,V> next = e.next;
                    int i = indexFor(e.hash, newCapacity);
                    e.next = newTable[i];
                    newTable[i] = e;
                    e = next;
                } while (e != null);
            }
        }
    }