How to Compute the Union of Two Arrays in Java Using HashSet

The article explains how to compute the union of two arrays in Java, ensuring that all distinct elements from both arrays are retained. The common approach uses a HashSet and its addAll() method to perform deduplication.

Primitive Data Types

For primitive int arrays, the example converts the arrays to object form with ArrayUtils.toObject, adds them to a HashSet<Integer>, and prints the result.

package com.fun.ztest.java;

import com.fun.frame.SourceCode;
import org.apache.commons.lang3.ArrayUtils;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;

public class Tdd extends SourceCode {
    public static void main(String[] args) {
        int[] array1 = {1, 2, 3};
        int[] array2 = {2, 4, 6};
        Set<Integer> union = new HashSet<>();
        union.addAll(Arrays.asList(ArrayUtils.toObject(array1)));
        union.addAll(Arrays.asList(ArrayUtils.toObject(array2)));
        output(union);
    }
}

Console output shows the merged set:

INFO-> [1, 2, 3, 4, 6]
Process finished with exit code 0

Object Data Types

The same technique works for String arrays, directly adding the arrays to a HashSet<String> without conversion.

package com.fun.ztest.java;

import com.fun.frame.SourceCode;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;

public class Tdd extends SourceCode {
    public static void main(String[] args) {
        String[] array1 = {"A", "B", "C", "D"};
        String[] array2 = {"C", "D", "E", "F"};
        Set<String> union = new HashSet<>();
        union.addAll(Arrays.asList(array1));
        union.addAll(Arrays.asList(array2));
        output(union);
    }
}

The resulting set printed to the console is:

INFO-> [A, B, C, D, E, F]
Process finished with exit code 0

Groovy Shortcut

Using Groovy, the union can be expressed more concisely by concatenating two lists and casting the result to a Set:

public static void main(String[] args) {
    def a = [3, 4, 6]
    def b = [4, 5, 6]
    def integers = a + b as Set
    output(integers)
}

The Groovy version prints:

INFO-> [3, 4, 6, 5]
Process finished with exit code 0

These examples illustrate a straightforward, language‑agnostic method for merging arrays and removing duplicates, suitable for both primitive and object data types.