JUnit assertNull example

Table of Contents

testIndia – Pass
testUSA – Pass
testJapan – Fail

Junit 5’s org.junit.jupiter.Assertions class provides different static assertions method to write test cases.

Please note that you need to use JUnit’s org.junit.Assert class in case of JUnit 4 or JUnit 3 to assert using assertNull method.

Assertions.assertNull() checks that object is null. In case, object is not null, it will through AssertError.

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

Here is simple example


package org.arpit.java2blog;

import java.util.HashMap;
import java.util.Map;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
 
public class AssertNullTest {
 
	AssertNullTest ant;
	
    public String getCapital(String country){
        Map<String,String> countryCapitalMap = new HashMap<String,String>();
        countryCapitalMap.put("Italy", "Rome");
        countryCapitalMap.put("Japan", "Tokyo");
        countryCapitalMap.put("Zimbabwe", "Harare");
        countryCapitalMap.put("Belgiaum", "Brussels");
        return countryCapitalMap.get(country);
    }
     
    @BeforeEach
	public void beforeEachTest()
	{
    	ant = new AssertNullTest();
	}
    /*
     * Examples for each overloaded methods of assertNull
     */
    
    //public static void assertNull(Object actual)
    @Test
    public void testIndia(){
    	String capitalIndia=ant.getCapital("India");
    	Assertions.assertNull(capitalIndia);
    }
    
    //public static void assertNull(Object actual, String message)
    @Test
    public void testUSA(){
    	String capitalUSA=ant.getCapital("USA");
    	// You can pass message as well
    	Assertions.assertNull(capitalUSA,"Capital should be null");
    }
    
    //public static void assertNull(Object actual, Supplier messageSupplier)
    @Test
    public void testJapan(){
    	String capitalJapan=ant.getCapital("Japan");
    	Assertions.assertNull(capitalJapan,() -> "Capital should be null");
    }
}

package org.arpit.java2blog;

import java.util.HashMap;

import java.util.Map;

import org.junit.jupiter.api.Assertions;

import org.junit.jupiter.api.BeforeEach;

import org.junit.jupiter.api.Test;

public class AssertNullTest {

AssertNullTest ant;

public String getCapital(String country){

Map<String,String> countryCapitalMap = new HashMap<String,String>();

countryCapitalMap.put("Italy", "Rome");

countryCapitalMap.put("Japan", "Tokyo");

countryCapitalMap.put("Zimbabwe", "Harare");

countryCapitalMap.put("Belgiaum", "Brussels");

return countryCapitalMap.get(country);

}

@BeforeEach

public void beforeEachTest()

{

ant = new AssertNullTest();

}

* Examples for each overloaded methods of assertNull

//public static void assertNull(Object actual)

@Test

public void testIndia(){

String capitalIndia=ant.getCapital("India");

Assertions.assertNull(capitalIndia);

}

//public static void assertNull(Object actual, String message)

@Test

public void testUSA(){

String capitalUSA=ant.getCapital("USA");

// You can pass message as well

Assertions.assertNull(capitalUSA,"Capital should be null");

}

//public static void assertNull(Object actual, Supplier messageSupplier)

@Test

public void testJapan(){

String capitalJapan=ant.getCapital("Japan");

Assertions.assertNull(capitalJapan,() -> "Capital should be null");

}

When you run above testcase, you will get below output:
AssertNullTestOutput
Let’s understand output of each testcase:

testIndia – Pass

As countryCapitalMap does not contain key “India”, assertNull will return true and this testcase will pass.

testUSA – Pass

As countryCapitalMap does not contain key “USA”, assertNull will return true and this testcase will pass.

testJapan – Fail

As countryCapitalMap contains key “Japan”, assertNull will return false and this testcase will fail.

That’s all about Assertions.assertNull() method.

Was this post helpful?

Let us know if this post was helpful. Feedbacks are monitored on daily basis. Please do provide feedback as that\'s the only way to improve.