Merge two sets in Java

Last Updated : 11 Dec, 2018

Prerequisites : Sets in Java

Following are the various ways to merge two sets in Java:

Double brace Initialization :

Examples:

Input : a = [1, 3, 5, 7, 9]
        b = [0, 2, 4, 6, 8]
Output : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

// Java program to demonstrate 
// merging of two sets in Java 
// using Double brace Initialization 
import java.util.stream.*; 
import java.util.*; 
import java.io.*; 
  
public class GfG { 
  
    // Function merging two sets using DoubleBrace Initialisation 
    public static <T> Set<T> mergeSet(Set<T> a, Set<T> b) 
    { 
        return new HashSet<T>() {{ 
                      addAll(a); 
                       addAll(b); 
        } }; 
    } 
  
    public static void main(String[] args) 
    { 
  
        // Creating the sets to be merged 
        // First set 
        Set<Integer> a = new HashSet<Integer>(); 
        a.addAll(Arrays.asList(new Integer[] { 1, 3, 5, 7, 9 })); 
  
        // Second set 
        Set<Integer> b = new HashSet<Integer>(); 
        b.addAll(Arrays.asList(new Integer[] { 0, 2, 4, 6, 8 })); 
  
        // Printing the sets 
        System.out.println("Set a: " + a); 
        System.out.println("Set b: " + b); 
  
        // calling mergeSets() 
        System.out.println("Merged Set: " + mergeSet(a, b)); 
    } 
} 

Output:

Set a: [1, 3, 5, 7, 9]
Set b: [0, 2, 4, 6, 8]
Merged Set: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Set addAll() : The addAll() method is provided by the Set interface. It adds the elements passed as parameter at the last of this set.

Using user-defined method

Examples:

Input : a = [1, 3, 5, 7, 9]
        b = [0, 2, 4, 6, 8]
Output : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

// Java program to demonstrate 
// merging of two sets in Java 
// using SetAll() method 
import java.util.*; 
  
public class GfG { 
  
    // Function merging two sets using addAll() 
    public static <T> Set<T> mergeSet(Set<T> a, Set<T> b) 
    { 
  
        // Creating an empty set 
        Set<T> mergedSet = new HashSet<T>(); 
  
        // add the two sets to be merged 
        // into the new set 
        mergedSet.addAll(a); 
        mergedSet.addAll(b); 
  
        // returning the merged set 
        return mergedSet; 
    } 
  
    public static void main(String[] args) 
    { 
  
        // Creating the sets to be merged 
        // First set 
        Set<Integer> a = new HashSet<Integer>(); 
        a.addAll(Arrays.asList(new Integer[] { 1, 3, 5, 7, 9 })); 
  
        // Second set 
        Set<Integer> b = new HashSet<Integer>(); 
        b.addAll(Arrays.asList(new Integer[] { 0, 2, 4, 6, 8 })); 
  
        // Printing the sets 
        System.out.println("Set a: " + a); 
        System.out.println("Set b: " + b); 
  
        // calling mergeSets() 
        System.out.println("Merged Set: " + mergeSet(a, b)); 
    } 
} 

Output:

Set a: [1, 3, 5, 7, 9]
Set b: [0, 2, 4, 6, 8]
Merged Set: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Using Java 8 stream in the user defined function

Examples:

Input : a = [1, 3, 5, 7, 9]
        b = [0, 2, 4, 6, 8]
Output : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

// Java program to demonstrate 
// merging of two sets in Java 
// using stream 
  
import java.util.stream.*; 
import java.util.*; 
import java.io.*; 
  
public class GfG { 
  
    // Function merging two sets using addAll() 
    public static <T> Set<T> mergeSet(Set<T> a, Set<T> b) 
    { 
  
        // Creating a set with 'a' 
        Set<T> mergedSet = a.stream() 
                               .collect(Collectors.toSet()); 
  
        // add the second set to be merged 
        mergedSet.addAll(b); 
  
        // returning the merged set 
        return mergedSet; 
    } 
  
    public static void main(String[] args) 
    { 
  
        // Creating the sets to be merged 
        // First set 
        Set<Integer> a = new HashSet<Integer>(); 
        a.addAll(Arrays.asList(new Integer[] { 1, 3, 5, 7, 9 })); 
  
        // Second set 
        Set<Integer> b = new HashSet<Integer>(); 
        b.addAll(Arrays.asList(new Integer[] { 0, 2, 4, 6, 8 })); 
  
        // Printing the sets 
        System.out.println("Set a: " + a); 
        System.out.println("Set b: " + b); 
  
        // calling mergeSets() 
        System.out.println("Merged Set: " + mergeSet(a, b)); 
    } 
} 

Output:

Set a: [1, 3, 5, 7, 9]
Set b: [0, 2, 4, 6, 8]
Merged Set: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Collections.addAll() :

Examples:

Input : a = [1, 3, 5, 7, 9]
        b = [0, 2, 4, 6, 8]
Output : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

// Java program to merge two arrays of 
// same type into an Object array. 
  
import java.util.*; 
import java.io.*; 
  
class GFG { 
  
    // Function merging two sets using addAll() 
    public static Set<Integer> mergeSet(Set<Integer> a, Set<Integer> b) 
    { 
  
        // Creating an empty set 
        Set<Integer> mergedSet = new HashSet<>(); 
  
        // add the two sets to be merged 
        // into the new set 
        Collections.addAll(mergedSet, a.toArray(new Integer[0])); 
        Collections.addAll(mergedSet, b.toArray(new Integer[0])); 
  
        // returning the merged set 
        return mergedSet; 
    } 
  
    public static void main(String[] args) 
    { 
  
        // Creating the sets to be merged 
        // First set 
        Set<Integer> a = new HashSet<Integer>(); 
        a.addAll(Arrays.asList(new Integer[] { 1, 3, 5, 7, 9 })); 
  
        // Second set 
        Set<Integer> b = new HashSet<Integer>(); 
        b.addAll(Arrays.asList(new Integer[] { 0, 2, 4, 6, 8 })); 
  
        // Printing the sets 
        System.out.println("Set a: " + a); 
        System.out.println("Set b: " + b); 
  
        // calling mergeSets() 
        System.out.println("Merged Set: " + mergeSet(a, b)); 
    } 
} 

Output:

Set a: [1, 3, 5, 7, 9]
Set b: [0, 2, 4, 6, 8]
Merged Set: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Stream.of() + Stream.forEach():

Examples:

Input : a = [1, 3, 5, 7, 9]
        b = [0, 2, 4, 6, 8]
Output : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

// Java program to demonstrate 
// merging of two sets in Java 
// using stream 
  
import java.util.stream.*; 
import java.util.*; 
import java.io.*; 
  
public class GfG { 
  
    // Function merging two sets 
    // using Stream of() and forEach() methods 
    public static <T> Set<T> mergeSet(Set<T> a, Set<T> b) 
    { 
  
        // Creating an empty set 
        Set<T> mergedSet = new HashSet<T>(); 
  
        // add the two sets to be merged 
        // into the new set 
        Stream.of(a, b).forEach(mergedSet::addAll); 
  
        // returning the merged set 
        return mergedSet; 
    } 
  
    public static void main(String[] args) 
    { 
  
        // Creating the sets to be merged 
        // First set 
        Set<Integer> a = new HashSet<Integer>(); 
        a.addAll(Arrays.asList(new Integer[] { 1, 3, 5, 7, 9 })); 
  
        // Second set 
        Set<Integer> b = new HashSet<Integer>(); 
        b.addAll(Arrays.asList(new Integer[] { 0, 2, 4, 6, 8 })); 
  
        // Printing the sets 
        System.out.println("Set a: " + a); 
        System.out.println("Set b: " + b); 
  
        // calling mergeSets() 
        System.out.println("Merged Set: " + mergeSet(a, b)); 
    } 
} 

Output:

Set a: [1, 3, 5, 7, 9]
Set b: [0, 2, 4, 6, 8]
Merged Set: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Stream.of() + flatMap() + Collector:

Examples:

Input : a = [1, 3, 5, 7, 9]
        b = [0, 2, 4, 6, 8]
Output : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

// Java program to demonstrate 
// merging of two sets in Java 
// using stream 
  
import java.util.stream.*; 
import java.util.*; 
import java.io.*; 
  
public class GfG { 
  
    // Function merging two sets 
    // using Stream of(), flatMap() and Collector 
    public static <T> Set<T> mergeSet(Set<T> a, Set<T> b) 
    { 
  
        // add the two sets to be merged 
        // into the new set and 
        // return the merged set 
        return Stream.of(a, b) 
            .flatMap(x -> x.stream()) 
            .collect(Collectors.toSet()); 
    } 
  
    public static void main(String[] args) 
    { 
  
        // Creating the sets to be merged 
        // First set 
        Set<Integer> a = new HashSet<Integer>(); 
        a.addAll(Arrays.asList(new Integer[] { 1, 3, 5, 7, 9 })); 
  
        // Second set 
        Set<Integer> b = new HashSet<Integer>(); 
        b.addAll(Arrays.asList(new Integer[] { 0, 2, 4, 6, 8 })); 
  
        // Printing the sets 
        System.out.println("Set a: " + a); 
        System.out.println("Set b: " + b); 
  
        // calling mergeSets() 
        System.out.println("Merged Set: " + mergeSet(a, b)); 
    } 
} 

Output:

Set a: [1, 3, 5, 7, 9]
Set b: [0, 2, 4, 6, 8]
Merged Set: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Stream.concat() + Collector :

Examples:

Input : a = [1, 3, 5, 7, 9]
        b = [0, 2, 4, 6, 8]
Output : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

The concatenate function is used to merge to string and make a single string that contains both the string. Stream.concat() method creates a lazily concatenated stream whose elements are all the elements of the first stream followed by all the elements of the second stream.

// Java program to demonstrate 
// merging of two sets in Java 
// using stream 
  
import java.util.stream.*; 
import java.util.*; 
import java.io.*; 
  
public class GfG { 
  
    // Function merging two sets 
    // using Stream concat() and Collectors 
    public static <T> Set<T> mergeSet(Set<T> a, Set<T> b) 
    { 
  
        // add the two sets to be merged 
        // into the new set and 
        // return the merged set 
        return Stream.concat(a.stream(), b.stream()) 
            .collect(Collectors.toSet()); 
    } 
  
    public static void main(String[] args) 
    { 
  
        // Creating the sets to be merged 
        // First set 
        Set<Integer> a = new HashSet<Integer>(); 
        a.addAll(Arrays.asList(new Integer[] { 1, 3, 5, 7, 9 })); 
  
        // Second set 
        Set<Integer> b = new HashSet<Integer>(); 
        b.addAll(Arrays.asList(new Integer[] { 0, 2, 4, 6, 8 })); 
  
        // Printing the sets 
        System.out.println("Set a: " + a); 
        System.out.println("Set b: " + b); 
  
        // calling mergeSets() 
        System.out.println("Merged Set: " + mergeSet(a, b)); 
    } 
} 

Output:

Set a: [1, 3, 5, 7, 9]
Set b: [0, 2, 4, 6, 8]
Merged Set: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Apache Common Collections:

Examples:

Input : a = [1, 3, 5, 7, 9]
        b = [0, 2, 4, 6, 8]
Output : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

// Java program to demonstrate 
// merging of two sets in Java 
// using Apache Common Collection 
import org.apache.commons.collections4.SetUtils; 
import java.util.*; 
import java.io.*; 
  
public class GfG { 
  
    // Function merging two sets using addAll() 
    public static <T> Set<T> mergeSet(Set<T> a, Set<T> b) 
    { 
  
        // add the two sets to be merged 
        // into the new set and 
        // return the merged set 
        return SetUtils.union(a, b); 
    } 
  
    public static void main(String[] args) 
    { 
  
        // Creating the sets to be merged 
        // First set 
        Set<Integer> a = new HashSet<Integer>(); 
        a.addAll(Arrays.asList(new Integer[] { 1, 3, 5, 7, 9 })); 
  
        // Second set 
        Set<Integer> b = new HashSet<Integer>(); 
        b.addAll(Arrays.asList(new Integer[] { 0, 2, 4, 6, 8 })); 
  
        // Printing the sets 
        System.out.println("Set a: " + a); 
        System.out.println("Set b: " + b); 
  
        // calling mergeSets() 
        System.out.println("Merged Set: " + mergeSet(a, b)); 
    } 
} 

Output:

Set a: [1, 3, 5, 7, 9]
Set b: [0, 2, 4, 6, 8]
Merged Set: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Guava Iterables.concat():

Examples:

Input : a = [1, 3, 5, 7, 9]
        b = [0, 2, 4, 6, 8]
Output : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

// Java program to demonstrate 
// merging of two sets in Java 
// using Guava library 
  
import com.google.common.collect.Iterables; 
import com.google.common.collect.Sets; 
import java.util.*; 
import java.io.*; 
  
public class GfG { 
  
    // Function merging two sets 
    // using Guava Iterables.concat() 
    public static <T> Set<T> mergeSet(Set<T> a, Set<T> b) 
    { 
  
        // add the two sets to be merged 
        // into the new set and 
        // return the merged set 
        return Sets.newHashSet(Iterables.concat(a, b)); 
    } 
  
    public static void main(String[] args) 
    { 
  
        // Creating the sets to be merged 
        // First set 
        Set<Integer> a = new HashSet<Integer>(); 
        a.addAll(Arrays.asList(new Integer[] { 1, 3, 5, 7, 9 })); 
  
        // Second set 
        Set<Integer> b = new HashSet<Integer>(); 
        b.addAll(Arrays.asList(new Integer[] { 0, 2, 4, 6, 8 })); 
  
        // Printing the sets 
        System.out.println("Set a: " + a); 
        System.out.println("Set b: " + b); 
  
        // calling mergeSets() 
        System.out.println("Merged Set: " + mergeSet(a, b)); 
    } 
} 

Output:

Set a: [1, 3, 5, 7, 9]
Set b: [0, 2, 4, 6, 8]
Merged Set: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Note : Any duplicate element presents in the sets will be discarded during the merge in all the above methods.

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