Java Collections newSetFromMap() Method

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Description

The Java Collections newSetFromMap(Map<E, Boolean>) method is used to return a set backed by the specified map.

Declaration

Following is the declaration for java.util.Collections.newSetFromMap() method.

public static <E> Set<E> newSetFromMap(Map<E, Boolean> map)

Parameters

map − The backing map

Return Value

The method call returns the set backed by the map.

Exception

IllegalArgumentException − This is thrown if map is not empty.

Getting a Set of Strings Backed by Map Example

The following example shows the usage of Java Collection newSetFromMap(Map) method. We've created a Map object of String and Boolean. Then a set is created using newSetFromMap(Map) method. Values are added to set and then both set and map are printed.

package com.tutorialspoint;

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

public class CollectionsDemo {
   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("Java"); 
      set.add("C");
      set.add("C++");

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

Let us compile and run the above program, this will produce the following result.

Set is: [Java, C++, C]
Map is: {Java=true, C++=true, C=true}

Getting a Set of Integers Backed by Map Example

The following example shows the usage of Java Collection newSetFromMap(Map) method. We've created a Map object of Integer and Boolean. Then a set is created using newSetFromMap(Map) method. Values are added to set and then both set and map are printed.

package com.tutorialspoint;

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

public class CollectionsDemo {
   public static void main(String args[]) { 
      
      // create map
      Map<Integer, Boolean> map = new WeakHashMap<Integer, Boolean>();

      // create a set from map
      Set<Integer> set = Collections.newSetFromMap(map); 

      // add values in set
      set.add(1); 
      set.add(2);
      set.add(3);

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

Let us compile and run the above program, this will produce the following result.

Set is: [3, 2, 1]
Map is: {3=true, 2=true, 1=true}

Getting a Set of Objects Backed by Map Example

The following example shows the usage of Java Collection newSetFromMap(Map) method. We've created a Map object of Student object and Boolean. Then a set is created using newSetFromMap(Map) method. Values are added to set and then both set and map are printed.

package com.tutorialspoint;

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

public class CollectionsDemo {
   public static void main(String args[]) { 
      
      // create map
      Map<Student, Boolean> map = new WeakHashMap<Student, Boolean>();

      // create a set from map
      Set<Student> set = Collections.newSetFromMap(map); 

      // add values in set
      set.add(new Student(1, "Julie"));  
      set.add(new Student(2, "Robert"));
      set.add(new Student(3, "Adam"));

      // set and map values are
      System.out.println("Set is: " + set);
      System.out.println("Map is: " + map);
   }
}
class Student {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
}

Let us compile and run the above program, this will produce the following result.

Set is: [[ 1, Julie ], [ 3, Adam ], [ 2, Robert ]]
Map is: {[ 1, Julie ]=true, [ 3, Adam ]=true, [ 2, Robert ]=true}