HashMap作为Map接口的一个具体实现类，我想很多人都在使用。但是不知道有没有人发现，这个实现并没有完全按照接口说明来实现。比如：方法 get。
接口说明如下：
get
V get(Object key)Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
More formally, if this map contains a mapping from a key k to a value v such that (key==null ? k==null : key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

If this map permits null values, then a return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.

Parameters:
key - the key whose associated value is to be returned
Returns:
the value to which the specified key is mapped, or null if this map contains no mapping for the key
Throws:
ClassCastException - if the key is of an inappropriate type for this map (optional)
NullPointerException - if the specified key is null and this map does not permit null keys (optional)
这部分翻译过来说的是：如果此映射包含一个满足 (key==null ? k==null : key.equals(k)) 的从 k 键到 v 值的映射关系，则此方法返回 v；否则返回 null。（最多只能有一个这样的映射关系。） 但是经本人测试发现，无论怎样重载key的equals方法，就是不能得到相应的结果，后来查询源代码，发现HashMap首先要验证hashcode是否相同，如果相同，才验证equals。实现如下：
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;
}
同样，对于containsKey也存在相同的实现。 不知道这是sun有心为之，还是说明文档没有写全，亦或是我的理解有误。