Following methods of Assertions class are to validate the nullity of objects.

public static void assertNull(Object actual)
public static void assertNull(Object actual, String message)
public static void assertNull(Object actual, Supplier<String> messageSupplier)
public static void assertNotNull(Object actual)
public static void assertNotNull(Object actual, String message)
public static void assertNotNull(Object actual, Supplier<String> messageSupplier)

Example

package com.logicbig.example;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertNull;
import static org.junit.jupiter.api.Assertions.assertNotNull;

class NullAssertionsTest {

  @Test
  void testAssertNull() {
      String nullValue = null;
      Integer nullNumber = null;

      assertNull(nullValue); 
      assertNull(nullNumber);
  }

  @Test
  void testAssertNotNull() {
      String name = "John";
      Integer age = 25;
      Object obj = new Object();

      assertNotNull(name);
      assertNotNull(age);
      assertNotNull(obj);
  }

  @Test
  void testArrayAndCollections() {
      String[] emptyArray = new String[0];
      String[] nullArray = null;

      assertNotNull(emptyArray);
      assertNull(nullArray);   
  }

  @Test
  void testWithCustomMessages() {
      String nullString = null;
      String actualString = "Hello";

      assertNull(nullString, "This string should be null");
      assertNotNull(actualString, "This string should not be null");
  }
}

mvn test -Dtest=NullAssertionsTest

Output

D:\example-projects\junit-5\junit-5-assertions\junit-5-asserting-nullability>mvn test -Dtest=NullAssertionsTest
[INFO] Scanning for projects...
[INFO] 
[INFO] ---------< com.logicbig.example:junit-5-asserting-nullability >---------
[INFO] Building junit-5-asserting-nullability 1.0-SNAPSHOT
[INFO]   from pom.xml
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- resources:3.3.1:resources (default-resources) @ junit-5-asserting-nullability ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory D:\example-projects\junit-5\junit-5-assertions\junit-5-asserting-nullability\src\main\resources
[INFO] 
[INFO] --- compiler:3.14.1:compile (default-compile) @ junit-5-asserting-nullability ---
[INFO] Nothing to compile - all classes are up to date.
[INFO] 
[INFO] --- resources:3.3.1:testResources (default-testResources) @ junit-5-asserting-nullability ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory D:\example-projects\junit-5\junit-5-assertions\junit-5-asserting-nullability\src\test\resources
[INFO] 
[INFO] --- compiler:3.14.1:testCompile (default-testCompile) @ junit-5-asserting-nullability ---
[INFO] Recompiling the module because of added or removed source files.
[INFO] Compiling 1 source file with javac [debug target 25] to target\test-classes
[INFO] 
[INFO] --- surefire:3.2.5:test (default-test) @ junit-5-asserting-nullability ---
[INFO] Using auto detected provider org.apache.maven.surefire.junitplatform.JUnitPlatformProvider
[INFO] 
[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running com.logicbig.example.NullAssertionsTest
[INFO] Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.061 s -- in com.logicbig.example.NullAssertionsTest
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 4, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  2.722 s
[INFO] Finished at: 2025-12-01T11:56:15+08:00
[INFO] ------------------------------------------------------------------------

Example Project

Dependencies and Technologies Used:

• junit-jupiter-engine 6.0.1 (Module "junit-jupiter-engine" of JUnit)
   Version Compatibility: 5.0.0 - 6.0.1Version List
  ×

  Version compatibilities of junit-jupiter-engine with this example:

  • 5.0.0
  • 5.0.1
  • 5.0.2
  • 5.0.3
  • 5.1.0
  • 5.1.1
  • 5.2.0
  • 5.3.0
  • 5.3.1
  • 5.3.2
  • 5.4.0
  • 5.4.1
  • 5.4.2
  • 5.5.0
  • 5.5.1
  • 5.5.2
  • 5.6.0
  • 5.6.1
  • 5.6.2
  • 5.6.3
  • 5.7.0
  • 5.7.1
  • 5.7.2
  • 5.8.0
  • 5.8.1
  • 5.8.2
  • 5.9.0
  • 5.9.1
  • 5.9.2
  • 5.9.3
  • 5.10.0
  • 5.10.1
  • 5.10.2
  • 5.10.3
  • 5.10.4
  • 5.10.5
  • 5.11.0
  • 5.11.1
  • 5.11.2
  • 5.11.3
  • 5.11.4
  • 5.12.0
  • 5.12.1
  • 5.12.2
  • 5.13.0
  • 5.13.1
  • 5.13.2
  • 5.13.3
  • 5.13.4
  • 5.14.0
  • 5.14.1
  • 6.0.0
  • 6.0.1

  Versions in green have been tested.

• JDK 25
• Maven 3.9.11