/** 
 * Returns the value to which the specified key is mapped, 
 * or if this map contains no mapping for the key. 
 * 
 * 獲取key對應的value 
 */  
public V get(Object key) {  
    if (key == null)  
        return getForNullKey();  
    //獲取key的hash值  
    int hash = hash(key.hashCode());  
    // 在“該hash值對應的連結串列”上查詢“鍵值等於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.   
 * 獲取key為null的鍵值對，HashMap將此鍵值對儲存到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.   
 * 返回鍵為key的鍵值對 
 */  
final Entry<K,V> getEntry(Object key) {  
    //先獲取雜湊值。如果key為null，hash = 0；這是因為key為null的鍵值對儲存在table[0]的位置。  
    int hash = (key == null) ? 0 : hash(key.hashCode());  
    //在該雜湊值對應的連結串列上查詢鍵值與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. 
 * 
 * 將“key-value”新增到HashMap中，如果hashMap中包含了key，那麼原來的值將會被新值取代 
 */  
public V put(K key, V value) {  
    //如果key是null，那麼呼叫putForNullKey(),將該鍵值對新增到table[0]中  
    if (key == null)  
        return putForNullKey(value);  
    //如果key不為null，則計算key的雜湊值，然後將其新增到雜湊值對應的連結串列中  
    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;  
        //如果這個key對應的鍵值對已經存在，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;  
}