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;  
}