Maps in java

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

MapMaps unique keys to values.
Map.EntryDescribes an element (a key/value pair) in a map. This is an inner class of Map.
NavigableMapExtends SortedMap to handle the retrieval of entries based on closest-match
 searches. (Added by Java SE 6.)
SortedMapExtends Map so that the keys are maintained in ascending order.
The Map Interfaces

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, specifies the type of keys, and specifies the type of values.

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. 
The Methods Defined by Map

Leave a Comment