JUnit Assert.assertNull() Method Example

In this post, we will demonstrate how to use Assert.assertNull() method with an example.
Check out JUnit 5 tutorials and examples at JUnit 5 Tutorial
In JUnit 5,  all JUnit 4 assertion methods are moved to org.junit.jupiter.api.Assertions class.

When to use assertNull() method or assertion

When we want to test if an object is null we can use the assertNull assertion.

void org.junit.Assert.assertNull(Object object)

Asserts that an object is null. If it isn't an AssertionError is thrown. 
  • object - Object to check or null

Assert.assertNull(Object object) Method Example

import static org.junit.Assert.assertArrayEquals;
import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertNull;

import org.junit.Test;

public class AssertNullExample {
  * Concatenate the given {@code String} arrays into one,
  * with overlapping array elements included twice.
  * <p>The order of elements in the original arrays is preserved.
  * @param array1 the first array (can be {@code null})
  * @param array2 the second array (can be {@code null})
  * @return the new array ({@code null} if both given arrays were {@code null})
 public static String[] concatenateStringArrays( final String[] array1,  final String[] array2) {
     if (array1 == null || array1.length == 0) {
          return array2;
     if (array2 == null || array2.length == 0) {
          return array1;

     final String[] newArr = new String[array1.length + array2.length];
     System.arraycopy(array1, 0, newArr, 0, array1.length);
     System.arraycopy(array2, 0, newArr, array1.length, array2.length);
     return newArr;
 public void testConcatenateStringArrays() {
     final String[] input1 = new String[] {"myString2"};
     final String[] input2 = new String[] {"myString1", "myString2"};
     final String[] result = concatenateStringArrays(input1, input2);
     assertNull(concatenateStringArrays(null, null));
     assertEquals(3, result.length);
     assertEquals("myString2", result[0]);
     assertEquals("myString1", result[1]);
     assertEquals("myString2", result[2]);

     assertArrayEquals(input1, concatenateStringArrays(input1, null));
     assertArrayEquals(input2, concatenateStringArrays(null, input2));  


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