Mocking Void Methods with Mockito

In this post, We will learn How to mock void methods with Mockito

Usually most of the time Mockito’s org.mockito.stubbing.Stubber.when(T) method is really good enough to mock an object’s behavior but when we need to mock a void method then we can’t use when(T) method.

Mockito Mock Void Method

Mockito provides the following methods that can be used to mock void methods.

org.mockito.Mockito.doAnswer(Answer): We can use this method in Mockito when we want to stub a void method with generic Answer
org.mockito.Mockito.doThrow(Throwable...): We can use this method when we want to stub a void method with an exception
org.mockito.Mockito.doNothing(): We can use this method for setting void methods to do nothing. We should be beware that the void method on mocks does nothing by default. However, there are very rare situations when the doNothing() method comes handy
org.mockito.Mockito.doCallRealMethod(): We can use this method when we want to call the real implementation of a method

Mockito mock void method example

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<groupId>com.kkjavatutorials</groupId>
	<artifactId>MockingVoidMethodsWithMockitoExample</artifactId>
	<version>0.0.1-SNAPSHOT</version>

	<properties>
		<maven.compiler.target>8</maven.compiler.target>
		<maven.compiler.source>8</maven.compiler.source>
	</properties>

	<dependencies>

		<dependency>
			<groupId>org.mockito</groupId>
			<artifactId>mockito-junit-jupiter</artifactId>
			<version>3.5.7</version>
			<scope>test</scope>
		</dependency>

	</dependencies>
</project>

<project xmlns="http://maven.apache.org/POM/4.0.0"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">

<groupId>com.kkjavatutorials</groupId>

<artifactId>MockingVoidMethodsWithMockitoExample</artifactId>

<version>0.0.1-SNAPSHOT</version>

<maven.compiler.target>8</maven.compiler.target>

<maven.compiler.source>8</maven.compiler.source>

</properties>

<groupId>org.mockito</groupId>

<artifactId>mockito-junit-jupiter</artifactId>

</dependency>

</dependencies>

</project>

Dictionary.java

package com.kkjavatutorials;

public interface Dictionary {

	public abstract void add(String word, String meaning);
	public abstract String getMeaning(String word);
}

package com.kkjavatutorials;

public interface Dictionary {

public abstract void add(String word, String meaning);

public abstract String getMeaning(String word);

}

WordDictionary.java

package com.kkjavatutorials;

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

public class WordDictionary implements Dictionary {
	
    private Map<String, String> wordsMap;
 
    public WordDictionary() {
        wordsMap = new HashMap<String, String>();
    }
    public void add(String word, String meaning) {
        wordsMap.put(word, meaning);
    }
    public String getMeaning(String word) {
        return wordsMap.get(word);
    }
}

package com.kkjavatutorials;

import java.util.HashMap;

import java.util.Map;

public class WordDictionary implements Dictionary {

private Map<String, String> wordsMap;

public WordDictionary() {

wordsMap = new HashMap<String, String>();

}

public void add(String word, String meaning) {

wordsMap.put(word, meaning);

}

public String getMeaning(String word) {

return wordsMap.get(word);

}

WordDictionaryTest.java

package com.kkjavatutorials;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertThrows;
import static org.mockito.ArgumentMatchers.anyString;
import static org.mockito.Mockito.*;

import org.junit.jupiter.api.Test;
import org.mockito.ArgumentCaptor;
/**
 * Mocking Void Methods with Mockito Examples
 * @author KK JavaTutorials
 */
public class WordDictionaryTest {
	
	@Test
	public void whenAddCalledWithVerify() {
		
		WordDictionary wordDictionary = mock(WordDictionary.class);
	  
		doNothing().when(wordDictionary).add(anyString(), anyString());
	    wordDictionary.add("awesome", "Very Good");
	 
	    verify(wordDictionary, times(1)).add("awesome", "Very Good");
	    
	    System.out.println(wordDictionary.getMeaning("awesome"));
	}
	
	@Test
	public void whenAddCalledValueCaptured() {
		
		WordDictionary wordDictionary = mock(WordDictionary.class);
	    ArgumentCaptor<String> valueCapture = ArgumentCaptor.forClass(String.class);
	    
	    doNothing().when(wordDictionary).add(anyString(), valueCapture.capture());
	 
	    wordDictionary.add("awesome", "Very Good");
	    
	    assertEquals("Very Good", valueCapture.getValue());
	}
	
	@Test
	public void whenAddCalledRealMethodUsingSpy() {
		
		WordDictionary wordDictionary = spy(WordDictionary.class);
	    doCallRealMethod().when(wordDictionary).add(anyString(),anyString());
	    
	    wordDictionary.add("awesome", "Very Good");
	 
	    verify(wordDictionary, times(1)).add("awesome", "Very Good");
	    assertEquals("Very Good", wordDictionary.getMeaning("awesome"));
	    //System.out.println(wordDictionary.getMeaning("awesome"));
	}
	
	/**
	 * Test for void Return Type method with Exception
	 */
	@Test
	public void voidMethodThrowingExceptionMockitoTest() {
		WordDictionary wordDictionary = mock(WordDictionary.class);
		
		doThrow(new IllegalStateException("Error occurred!!"))
	      .when(wordDictionary)
	      .add(anyString(), anyString());
	    
	    assertThrows(IllegalStateException.class, ()->wordDictionary.add("awesome", "Very Good"));
	}

}

package com.kkjavatutorials;

import static org.junit.jupiter.api.Assertions.assertEquals;

import static org.junit.jupiter.api.Assertions.assertThrows;

import static org.mockito.ArgumentMatchers.anyString;

import static org.mockito.Mockito.*;

import org.junit.jupiter.api.Test;

import org.mockito.ArgumentCaptor;

/**

* Mocking Void Methods with Mockito Examples

* @author KK JavaTutorials

public class WordDictionaryTest {

@Test

public void whenAddCalledWithVerify() {

WordDictionary wordDictionary = mock(WordDictionary.class);

doNothing().when(wordDictionary).add(anyString(), anyString());

wordDictionary.add("awesome", "Very Good");

verify(wordDictionary, times(1)).add("awesome", "Very Good");

System.out.println(wordDictionary.getMeaning("awesome"));

}

@Test

public void whenAddCalledValueCaptured() {

WordDictionary wordDictionary = mock(WordDictionary.class);

ArgumentCaptor<String> valueCapture = ArgumentCaptor.forClass(String.class);

doNothing().when(wordDictionary).add(anyString(), valueCapture.capture());

wordDictionary.add("awesome", "Very Good");

assertEquals("Very Good", valueCapture.getValue());

}

@Test

public void whenAddCalledRealMethodUsingSpy() {

WordDictionary wordDictionary = spy(WordDictionary.class);

doCallRealMethod().when(wordDictionary).add(anyString(),anyString());

wordDictionary.add("awesome", "Very Good");

verify(wordDictionary, times(1)).add("awesome", "Very Good");

assertEquals("Very Good", wordDictionary.getMeaning("awesome"));

//System.out.println(wordDictionary.getMeaning("awesome"));

}

/**

* Test for void Return Type method with Exception

@Test

public void voidMethodThrowingExceptionMockitoTest() {

WordDictionary wordDictionary = mock(WordDictionary.class);

doThrow(new IllegalStateException("Error occurred!!"))

.when(wordDictionary)

.add(anyString(), anyString());

assertThrows(IllegalStateException.class, ()->wordDictionary.add("awesome", "Very Good"));

}

The Output of the above project:

You May Also Like:

What is Mocking?
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What are the Benefits of Mockito?
How to mock interface using Mockito example?
Mockito and JUnit Integration Using Maven Example
@Mock and @Spy Mockito Annotations With Example
@InjectMocks Annotation in Mockito with Example
Mockito – Verifying Method Calls
@Captor Annotation in Mockito with Example
Adding behavior to mocked object in Mockito
Mocking Exception Throwing using Mockito
Mockito’s mock Overloaded Methods

That’s all about Mocking Void Methods with Mockito
If you have any feedback or suggestion please feel free to drop in below comment box.

Mockito Mock Void Method

Mockito mock void method example

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