Java Collections newSetFromMap() Method

The newSetFromMap() method of Java Collections class is used to return a set backed by the specified map. The resulting set displays the same ordering, concurrency, and performance characteristics as the backing map.

Syntax

Following is the declaration of newSetFromMap() method:

Parameter

Parameter	Description	Required/Optional
map	It is the backing map.	Required

Returns

The newSetFromMap() method returns the set backed by the specified map.

Exceptions

IllegalArgumentException- It throws this exception if map is not empty.

Compatibility Version

Java 1.6 and above

Example 1

import java.util.*;
public class CollectionsNewSetFromMapExample1 {
	public static void main(String[] args) {		
		// create map
	      Map<String, Boolean> map = new WeakHashMap<String, Boolean>();
	      // create a set from map
	      Set<String> set = Collections.newSetFromMap(map); 
	      // add values in set
	      set.add("Raj"); 
	      set.add("Rahul");
	      set.add("Karan");
	      // set and map values are
	      System.out.println("Set is: " + set);
	      System.out.println("Map is: " + map);
	      }
}

Test it Now

Output:

Set is: [Karan, Raj, Rahul]
Map is: {Karan=true, Raj=true, Rahul=true}

Example 2

import java.util.*;
public class CollectionsNewSetFromMapExample2 {
	public static void main(String[] args) {		
		Map<Integer, Boolean> map = new IdentityHashMap<Integer, Boolean>();
	      Set<Integer> set = Collections.newSetFromMap(map);
	      set.add(1);
	      set.add(1);
	      set.add(1);
	      // set and map values are
		System.out.println("Set is: " + set);
		System.out.println("Map is: " + map);
	      }
}

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Output:

Set is: [1]
Map is: {1=true}

Example 3

import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
public class CollectionsNewSetFromMapExample3 {
	public static void main(String[] args) {		
		Set<String> users = Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>());
            System.out.println("Users: " + users);
            users.add("Java");
            users.add("PHP");
            System.out.println("Users: " + users);
	      }
}

Test it Now

Output: