# HashMap

### Initialization

```java
// must use object, can't use primitive type
// array based data structure
Map<Object, Object> map = new HashMap<>();
```

### Useful methods

```java
map.size();
map.isEmpty();   // return boolean
map.put(key, value);  // return value, O(1)
map.get(key);  // return value, O(1)
map.containsKey(key); // boolean, O(1)
map.remove(key); // return value, O(1)

map.keySet();   // return a set of key, Set<key>
for (String str: map.KeySet()) {} // 遍历key


map.entrySet();   // return a set of entry, 
```


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