Following methods assert that a given object is an instance of a specific class or interface. The primary purpose is to explicitly verify the type of an object.

public static <T> T assertInstanceOf(Class<T> expectedType, Object actualValue)
public static <T> T assertInstanceOf(Class<T> expectedType, Object actualValue)
public static <T> T assertInstanceOf(Class<T> expectedType, Object actualValue, String message)
public static <T> T assertInstanceOf(Class<T> expectedType, Object actualValue, Supplier<String> messageSupplier)
public static <T> T assertInstanceOf(Class<T> expectedType, Object actualValue, String message)
public static <T> T assertInstanceOf(Class<T> expectedType, Object actualValue, Supplier<String> messageSupplier)

Example

assertInstanceOf Examples

package com.logicbig.example;

import org.junit.jupiter.api.Test;
import java.util.ArrayList;
import java.util.List;
import static org.junit.jupiter.api.Assertions.assertInstanceOf;

class AssertInstanceOfExamples {

  @Test
  void testStringInstance() {
      Object obj = "Hello World";
      assertInstanceOf(String.class, obj);
  }

  @Test
  void testIntegerInstance() {
      Object obj = 42;
      assertInstanceOf(Integer.class, obj);
  }

  @Test
  void testListInstance() {
      Object obj = new ArrayList<String>();
      assertInstanceOf(List.class, obj);
  }

  @Test
  void testArrayInstance() {
      Object obj = new int[]{1, 2, 3};
      assertInstanceOf(int[].class, obj);
  }
}

mvn test -Dtest=AssertInstanceOfExamples

Output

D:\example-projects\junit-5\junit-5-assertions\junit-5-asserting-instance-of>mvn test -Dtest=AssertInstanceOfExamples
[INFO] Scanning for projects...
[INFO] 
[INFO] ---------< com.logicbig.example:junit-5-asserting-instance-of >---------
[INFO] Building junit-5-asserting-instance-of 1.0-SNAPSHOT
[INFO]   from pom.xml
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- resources:3.3.1:resources (default-resources) @ junit-5-asserting-instance-of ---
[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-instance-of\src\main\resources
[INFO] 
[INFO] --- compiler:3.14.1:compile (default-compile) @ junit-5-asserting-instance-of ---
[INFO] Nothing to compile - all classes are up to date.
[INFO] 
[INFO] --- resources:3.3.1:testResources (default-testResources) @ junit-5-asserting-instance-of ---
[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-instance-of\src\test\resources
[INFO] 
[INFO] --- compiler:3.14.1:testCompile (default-testCompile) @ junit-5-asserting-instance-of ---
[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-instance-of ---
[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.AssertInstanceOfExamples
[INFO] Tests run: 4, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.053 s -- in com.logicbig.example.AssertInstanceOfExamples
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 4, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  3.213 s
[INFO] Finished at: 2025-12-01T11:26:48+08:00
[INFO] ------------------------------------------------------------------------

Example Project

Dependencies and Technologies Used:

• junit-jupiter-engine 6.0.1 (Module "junit-jupiter-engine" of JUnit)
   Version Compatibility: 5.8.0 - 6.0.1Version List
  ×

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

  • 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