Java - Map Interface

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.

Map Interface Methods

Classes that Implement Map

The following are the classes that implement a Map to use the functionalities of a Map -

• HashMap
• EnumMap
• LinkedHashMap
• WeakHashMap
• TreeMap

Interfaces that Extend Map

The following are the interfaces that extend the Map interface -

• SortedMap
• NavigableMap
• ConcurrentMap

Examples of Map Interface

Example 1

Map has its implementation in various classes like HashMap. Following is an example to explain map functionality −

import java.util.HashMap;
import java.util.Map;
public class CollectionsDemo {

   public static void main(String[] args) {
      Map<String, String> 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);
   }
}

Output

Map Elements
	{Daisy = 14, Ayan = 12, Zara = 8, Mahnaz = 31}

Example 2

Map has its implementation in various classes like TreeMap which sorts the entries based on keys. Following is an example to explain map functionality using TreeMap −

import java.util.Map;
import java.util.TreeMap;

public class CollectionsDemo {

   public static void main(String[] args) {
      Map<String, String> m1 = new TreeMap<>(); 
      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);
   }
}

Output

 Map Elements
	{Ayan=12, Daisy=14, Mahnaz=31, Zara=8}

Example 3

Map has its implementation in various classes like HashMap. Following is an example to explain map functions using HashMap to add and remove elements to the map−

import java.util.HashMap;
import java.util.Map;
public class CollectionsDemo {

   public static void main(String[] args) {
      Map<String, String> 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);
	  
      m1.remove("Daisy");
	  System.out.println(" Map Elements");
      System.out.print("\t" + m1);
   }
}

Output

 Map Elements
	{Daisy=14, Ayan=12, Zara=8, Mahnaz=31} Map Elements
	{Ayan=12, Zara=8, Mahnaz=31}