Java - 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.
Several methods throw a NoSuchElementException when no items exist in the invoking map.
A ClassCastException is thrown when an object is incompatible with the elements in a map.
A NullPointerException is thrown if an attempt is made to use a null object and null is not allowed in the map.
An UnsupportedOperationException is thrown when an attempt is made to change an unmodifiable map.
SN | Methods with Description |
---|---|
1 | void clear( )
Removes all key/value pairs from the invoking map. |
2 | boolean containsKey(Object k)
Returns true if the invoking map contains k as a key. Otherwise, returns false. |
3 | boolean containsValue(Object v)
Returns true if the map contains v as a value. Otherwise, returns false |
4 | Set entrySet( )
Returns a Set that contains the entries in the map. The set contains objects of type Map.Entry. This method provides a set-view of the invoking map. |
5 | boolean equals(Object obj)
Returns true if obj is a Map and contains the same entries. Otherwise, returns false. |
6 | Object get(Object k)
Returns the value associated with the key k. |
7 | int hashCode( )
Returns the hash code for the invoking map. |
8 | boolean isEmpty( )
Returns true if the invoking map is empty. Otherwise, returns false. |
9 | Set 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. |
10 | Object put(Object k, Object 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. |
11 | void putAll(Map m)
Puts all the entries from m into this map. |
12 | Object remove(Object k)
Removes the entry whose key equals k. |
13 | int size( )
Returns the number of key/value pairs in the map. |
14 | Collection values( )
Returns a collection containing the values in the map. This method provides a collection-view of the values in the map. |
Example:
Map has its implementation in various classes like HashMap. Following is the example to explain map functionality:
import java.util.*; public class CollectionsDemo { public static void main(String[] args) { Map m1 = new HashMap(); m1.put("Zara", "8"); m1.put("Mahnaz", "31"); m1.put("Ayan", "12"); m1.put("Daisy", "14"); System.out.println(); System.out.println(" Map Elements"); System.out.print("\t" + m1); } }
This would produce the following result:
Map Elements {Mahnaz=31, Ayan=12, Daisy=14, Zara=8}