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HashMap用法总结
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 null if this map contains no mapping for the key. |
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 Set view of the mappings contained in this map. |
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). |
Iterator<E> | iterator()Returns an iterator over the elements in this set. |
关于Iterator：
boolean | hasNext()Returns true if the iteration has more elements. |
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();
HashMap用法总结
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