HashMap.put()和get()原理

/**

 * Returns the value to which the specified key is mapped,

 * or if this map contains no mapping for the key.

 *

 * 获取key对应的value

 */

public V get(Object key) {

    if (key == null)

        return getForNullKey();

    //获取key的hash值

    int hash = hash(key.hashCode());

    // 在“该hash值对应的链表”上查找“键值等于key”的元素

    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;

}  

/**

 * Offloaded version of get() to look up null keys.  Null keys map

 * to index 0.

 * 获取key为null的键值对，HashMap将此键值对存储到table[0]的位置

 */

private V getForNullKey() {

    for (Entry<K,V> e = table[0]; e != null; e = e.next) {

        if (e.key == null)

            return e.value;

    }

    return null;

}  

/**

 * Returns <tt>true</tt> if this map contains a mapping for the

 * specified key.

 *

 * HashMap是否包含key

 */

public boolean containsKey(Object key) {

    return getEntry(key) != null;

}  

/**

 * Returns the entry associated with the specified key in the

 * HashMap.

 * 返回键为key的键值对

 */

final Entry<K,V> getEntry(Object key) {

    //先获取哈希值。如果key为null，hash = 0；这是因为key为null的键值对存储在table[0]的位置。

    int hash = (key == null) ? 0 : hash(key.hashCode());

    //在该哈希值对应的链表上查找键值与key相等的元素。

    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 != null && key.equals(k))))

            return e;

    }

    return null;

}  

/**

 * Associates the specified value with the specified key in this map.

 * If the map previously contained a mapping for the key, the old

 * value is replaced.

 *

 * 将“key-value”添加到HashMap中，如果hashMap中包含了key，那么原来的值将会被新值取代

 */

public V put(K key, V value) {

    //如果key是null，那么调用putForNullKey(),将该键值对添加到table[0]中

    if (key == null)

        return putForNullKey(value);

    //如果key不为null，则计算key的哈希值，然后将其添加到哈希值对应的链表中

    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;

        //如果这个key对应的键值对已经存在，就用新的value代替老的value。

        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;

}