Map Interface in JAVA

A Map is an object that maps keys to values.Each key can map to at most one value. Map interface starts of it’s own interface hierarchy, for maintaining key-value associations. This interface describes a mapping from keys to values, without duplicate keys, by definition.

The Map interface provides three collection views(HashMap, TreeMap, and LinkedHashMap), which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The order of a map is defined as the order in which the iterators on the map's collection views return their elements.

Given a key and a value, we can store the value in a Map object. After the value is stored, you can retrieve it by using its key.

Following are list of exception we may get while implementing Map Interface:

• throw a NoSuchElementException when no items exist in the invoking map.
• throw a ClassCastException when an object is incompatible with the elements in a map.
• throw a NullPointerException if an attempt is made to use a null object and null is not allowed in the map.
• throw An UnsupportedOperationException when an attempt is made to change an unmodifiable map.

Following is the Map Interface:

public interface Map<K,V> {

    // Basic operations
    V put(K key, V value);
    V get(Object key);
    V remove(Object key);
    boolean containsKey(Object key);
    boolean containsValue(Object value);
    int size();
    boolean isEmpty();

    // Bulk operations
    void putAll(Map<? extends K, ? extends V> m);
    void clear();

    // Collection Views
    public Set<K> keySet();
    public Collection<V> values();
    public Set<Map.Entry<K,V>> entrySet();

    // Interface for entrySet elements
    public interface Entry {
        K getKey();
        V getValue();
        V setValue(V value);
    }
}

Following table shows the list of methods available in Map interface:

Example:

package com.sample.javase.testing;

import java.util.Iterator;

import java.util.Map;

import java.util.Map.Entry;

import java.util.Set;

import java.util.TreeMap;

public class MapInterfaceExample {

/**

* @param args

*/

public static void main(String[] args) {

Map<String, String> map = new TreeMap<>();

map.put("framework1", "struts");

map.put("framework2", "hibernate");

map.put("framework3", "spring");

System.out.println(map);

/**

* Following lines of code specifies hoe to get the map values form

* iterator. For this first we need to get entry set or key set from

* map.Here we are using entryset() method.

*/

Set set = map.entrySet();

for (Iterator iterator = set.iterator(); iterator.hasNext();) {

Map.Entry<String, String> object = (Entry<String, String>) iterator

.next();

System.out.println("key is " + object.getKey());

System.out.println("value is " + object.getValue());

}

/**

* Following lines of code specifies hoe to get the map values form

* iterator. For this first we need to get entry set or key set from

* map.Here we are using keyset() method.

*/

Set set1 = map.keySet();

for (Iterator iterator = set1.iterator(); iterator.hasNext();) {

Object object = iterator.next();

System.out.println("key is " + object);

System.out.println("value is " + map.get(object));

}

}

}