Merge two sets in Java

Prerequisites : Sets in Java

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

1. 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]
   

2. 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]
     

3. 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]
   

4. 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]
   

5. 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]
   

6. 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]
   

7. 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]
   

8. 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.