Collections.newSetFromMap(Map) in Java

Collections.newSetFromMap(Map): This method is available in java.util.Collections class of Java.

Syntax:

<E> Set<E> java.util.Collections.newSetFromMap(Map<E, Boolean> map)

This method takes one argument. This method returns a set backed by the specified map. The resulting set displays the same ordering, concurrency, and performance characteristics as the backing map.

Parameters: One parameter is required for this method.

map: the backing map.

Returns: the set backed by the map.

Throws:

1. IllegalArgumentException - if map is not empty.

Approach 1: When no exception

Java

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class CollectionsnewSetFromMap {
    public static void main(String[] args) {

        Map<Integer, Boolean> map = new HashMap<Integer, 
                                            Boolean>();
        Set<Integer> set = Collections.newSetFromMap(map);

        set.add(10);
        set.add(20);
        set.add(6);
        System.out.println(map);

    }
}

Output:

{20=true, 6=true, 10=true}

Approach 2: IllegalArgumentException 

Java

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class CollectionsnewSetFromMap {
    public static void main(String[] args) {

        Map<Integer, Boolean> map = new HashMap<Integer, 
                                              Boolean>();
        map.put(8, true);

        Set<Integer> set = Collections.newSetFromMap(map);

        set.add(10);
        set.add(20);
        set.add(6);
        System.out.println(map);

    }
}

Output:

Exception in thread "main" java.lang.IllegalArgumentException: Map is non-empty
	at java.base/java.util.Collections$SetFromMap.<init>(Collections.java:5644)
	at java.base/java.util.Collections.newSetFromMap(Collections.java:5629)