JavaSE_坚持读源码_HashMap对象_get_Java1.7

 1 /**
 2      * Returns the value to which the specified key is mapped,
 3      * or {@code null} if this map contains no mapping for the key.
 4      *
 5      * <p>More formally, if this map contains a mapping from a key
 6      * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
 7      * key.equals(k))}, then this method returns {@code v}; otherwise
 8      * it returns {@code null}.  (There can be at most one such mapping.)
 9      *
10      * <p>A return value of {@code null} does not <i>necessarily</i>
11      * indicate that the map contains no mapping for the key; it‘s also
12      * possible that the map explicitly maps the key to {@code null}.
13      * The {@link #containsKey containsKey} operation may be used to
14      * distinguish these two cases.
15      *
16      * @see #put(Object, Object)
17      */
18     public V get(Object key) {
19     　　//如果key是null，调用getForNullKey，这个方法会去获取table[0]元素
20         if (key == null)
21             return getForNullKey();
22     　　//否则根据key调用getEntry方法
23         Entry<K,V> entry = getEntry(key);
24 
25         return null == entry ? null : entry.getValue();
26     }

 1 /**
 2      * Returns the entry associated with the specified key in the
 3      * HashMap.  Returns null if the HashMap contains no mapping
 4      * for the key.
 5      */
 6     final Entry<K,V> getEntry(Object key) {
 7     //这个size是在当前map中的key-value的映射数量
 8         if (size == 0) {
 9             return null;
10         }
11     
12     //这里又对key进行了下非空校验，如果不为空，通过hash()获取key对应的hash值
13         int hash = (key == null) ? 0 : hash(key);
14     //取出table指定索引处的值，如果hash和key都能匹配上
15         for (Entry<K,V> e = table[indexFor(hash, table.length)];
16              e != null;
17              e = e.next) {
18             Object k;
19             if (e.hash == hash &&
20                 ((k = e.key) == key || (key != null && key.equals(k))))
21                 return e;
22         }
23         return null;
24     }