HashMap用法總結

Java中的HashMap的格式爲<Key, Value>

和hashtable相比是unsynchronized的，同時也允許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.

需要注意的是各種輸入輸出的類型（type），比如get方法返回的直接就是value的type。

關於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).

關於set：

Iterator<E>

iterator() Returns an iterator over the elements in this set.

關於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：以下爲某次寫的一小段代碼，對於返回值類型爲object還是key，value本身的類型非常迷茫，於是有很多冗餘的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()已經是所需的東西了，並不需要用

Integer.parseInt(pairs.getValue().toString())

如此複雜的東西來表示~ 同理pairs.getKey()也不需要pairs.getKey().toString();