A map is an object that stores associations between keys and values, or key/value pairs. Given a key, you can find its value. Both keys and values are objects. The keys must be unique, but the values may be duplicated. Some maps can accept a null key and null values, others cannot.
The Map Interfaces
| Interface | Description |
| Map | Maps unique keys to values. |
| Map.Entry | Describes an element (a key/value pair) in a map. This is an inner class of Map. |
| NavigableMap | Extends SortedMap to handle the retrieval of entries based on closest-match |
| searches. (Added by Java SE 6.) | |
| SortedMap | Extends Map so that the keys are maintained in ascending order. |
The Map Interface
The Map interface maps unique keys to values. A key is an object that you use to retrieve a value at a later date. Given a key and a value, you can store the value in a Map object. After the value is stored, you can retrieve it by using its key. Map is generic and is declared as shown here:
interface Map<K, V>
Here, K specifies the type of keys, and V specifies the type of values.
| Method | Description | ||
| void clear( ) | Removes all key/value pairs from the invoking map. | ||
| boolean containsKey(Object k) | Returns true if the invoking map contains k as a key. Otherwise, | ||
| returns false. | |||
| boolean containsValue(Object v) | Returns true if the map contains v as a value. Otherwise, returns false. | ||
| Set<Map.Entry<K, V>> entrySet( ) | Returns a Set that contains the entries in the map. The set contains | ||
| objects of type Map.Entry. Thus, this method provides a set-view of the | |||
| invoking map. | |||
| boolean equals(Object obj) | Returns true if obj is a Map and contains the same entries. Otherwise, | ||
| returns false. | |||
| V get(Object k) | Returns the value associated with the key k. Returns null if the key is | ||
| not found. | |||
| int hashCode( ) | Returns the hash code for the invoking map. | ||
| boolean isEmpty( ) | Returns true if the invoking map is empty. Otherwise, returns false. | ||
| Set<K> keySet( ) | Returns a Set that contains the keys in the invoking map. This method | ||
| provides a set-view of the keys in the invoking map. | |||
| V put(K k, V v) | Puts an entry in the invoking map, overwriting any previous value | ||
| associated with the key. The key and value are k and v, respectively. | |||
| Returns null if the key did not already exist. Otherwise, the previous | |||
| value linked to the key is returned. | |||
| void putAll(Map<? extends K, | Puts all the entries from m into this map. | ||
| void putAll(Map<? extends V> m) | |||
| V remove(Object k) | Removes the entry whose key equals k. | ||
| int size( ) | Returns the number of key/value pairs in the map. | ||
| Collection<V> values( ) | Returns a collection containing the values in the map. This method | ||
| provides a collection-view of the values in the map. |