JUnit Assert.assertNull() Method Example

assertNull() method belongs to JUnit 4 org.junit.Assert class. In JUnit 5 all JUnit 4 assertion methods are moved to org.junit.jupiter.api.Assertions class.

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

Asserts that an object is null. If it isn't an AssertionError is thrown. Parameters:
  • 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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