Java中的HashMap的格式为<Key, Value>
和hashtable相比是unsynchronized的，同时也允许null值
常用method：
void |
clear()
Removes all of the mappings from this map. |
boolean |
containsKey(Object key)
Returns true if this map contains a mapping for the specified key. |
boolean |
containsValue(Object value)
Returns true if this map maps one or more keys to the specified value. |
V |
get(Object key)
Returns the value to which the specified key is mapped, or |
boolean |
isEmpty()
Returns tr |
V |
put(K key, V value)
Associates the specified value with the specified key in this map. |
V |
remove(Object key)
Removes the mapping for the specified key from this map if present. |
boolean |
remove(Object key, Object value)
Removes the entry for the specified key only if it is currently mapped to the specified value. |
V |
replace(K key, V value)
Replaces the entry for the specified key only if it is currently mapped to some value. |
boolean |
replace(K key, V oldValue, V newValue)
Replaces the entry for the specified key only if currently mapped to the specified value. |
int |
size()
Returns the number of key-value mappings in this map. |
Set<Map.Entry<K,V>> |
entrySet()
Returns a |
需要注意的是各种输入输出的类型（type），比如get方法返回的直接就是value的type。
关于Map.entry：
| Modifier and Type | Method and Description |
|---|---|
boolean |
equals(Object o)
Compares the specified object with this entry for equality. |
K |
getKey()
Returns the key corresponding to this entry. |
V |
getValue()
Returns the value corresponding to this entry. |
int |
hashCode()
Returns the hash code value for this map entry. |
V |
setValue(V value)
Replaces the value corresponding to this entry with the specified value (optional operation). |
关于set：
Iterator<E> |
iterator()
Returns an iterator over the elements in this set. |
关于Iterator：
boolean |
hasNext()
Returns |
E |
next()
Returns the next element in the iteration. |
void |
remove()
Removes from the underlying collection the last element returned by this iterator (optional operation). |
例：以下为某次写的一小段代码，对于返回值类型为object还是key，value本身的类型非常迷茫，于是有很多冗余的cast：
Iterator iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
Map.Entry pairs = (Map.Entry)iterator.next();
if (Integer.parseInt(pairs.getValue().toString()) % 2 == 1) {
iterator.remove();
return pairs.getKey().toString();
}
}
我用的HashMap<String, Integer>
其中pairs.getValue()已经是所需的东西了，并不需要用
Integer.parseInt(pairs.getValue().toString())
如此复杂的东西来表示~ 同理pairs.getKey()也不需要pairs.getKey().toString();