Collections newSetFromMap() method in Java with Examples

The newSetFromMap() method of java.util.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. In essence, this factory method provides a Set implementation corresponding to any Map implementation. There is no need to use this method on a Map implementation that already has a corresponding Set implementation (such as HashMap or TreeMap).

Syntax:

public static  Set newSetFromMap(Map map)

Parameters: This method takes the backing map as a parameter

Return Value: This method returns the set backed by the map

Exception: This method throws IllegalArgumentException, if map is not empty.

Below are the examples to illustrate the newSetFromMap() method

Example 1:

// Java program to demonstrate
// newSetFromMap() method

import java.util.*;

public class GFG1 {
    public static void main(String[] argv)
        throws Exception
    {

        try {

            // creating object of Map<String, Boolean>
            Map<String, Boolean>
                map = new WeakHashMap<String, Boolean>();

            // creating object of LinkedList
            Set<String> set = Collections.newSetFromMap(map);

            // add values in set
            set.add("A");
            set.add("B");
            set.add("C");
            set.add("D");

            // set and map values are
            System.out.println("Map is: " + map);
            System.out.println("Set from Map is: " + set);
        }

        catch (IllegalArgumentException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

Output:

Map is: {C=true, B=true, A=true, D=true}
Set from Map is: [C, B, A, D]

Example 2: for IllegalArgumentException

// Java program to demonstrate
// newSetFromMap() method
// for IllegalArgumentException

import java.util.*;
<div class = 'outputDiv'>
<b>Output:</b>
<pre>
Set is: [C, B, A, D]
Map is: {C=true, B=true, A=true, D=true}
</pre>
</div>

public class GFG1 {
    public static void main(String[] argv) throws Exception
    {
        try {

            // creating object of Map<String, Boolean>
            Map<String, Boolean>
                map = new WeakHashMap<String, Boolean>();

            // putting value in map
            map.put("1", true);

            // creating object of LinkedList
            Set<String> set = Collections.newSetFromMap(map);

            // add values in set
            set.add("A");
            set.add("B");
            set.add("C");
            set.add("D");

            // set and map values are
            System.out.println("Map is: " + map);
            System.out.println("Set from Map is: " + set);
        }

        catch (IllegalArgumentException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

Output:

Exception thrown : java.lang.IllegalArgumentException: Map is non-empty