Testing Mastery: JUnit, Mockito & Integration Tests · Aula

Configurando Valores de Retorno

Configure objetos simulados para retornarem valores específicos ou lançarem exceções quando seus métodos forem invocados.

Configurando Valores de Retorno é uma aula grátis de Testing Mastery: JUnit, Mockito & Integration Tests no CoddyKit. Esta é a aula 1 de 4. Você pode ler a aula completa abaixo gratuitamente — depois pratica ao vivo no navegador com um editor de código integrado e um tutor de IA 24/7. Faz parte do caminho de aprendizado de Testing Mastery: JUnit, Mockito & Integration Tests, e seu progresso é sincronizado entre a web e o app CoddyKit. O curso de Testing Mastery: JUnit, Mockito & Integration Tests inclui 4 aulas no total.

Partes desta aula ainda não foram traduzidas e aparecem em inglês.

Intro to Stubbing Mocks

When you create a mock object, it doesn't have any real behavior. Its methods do nothing by default, often returning null, 0, or empty collections.

Stubbing is the process of teaching a mock object how to respond to specific method calls. It's like giving your mock a script to follow!

This allows you to control the exact conditions for your tests, isolating the code you're actually testing.

Making Mocks Return Values

The most common way to stub a mock is using when().thenReturn(). This tells Mockito: "When this specific method is called on the mock, return this predefined value."

when(mock.someMethod()).thenReturn(value);

The value can be any object or primitive type that matches the method's return type.

thenReturn() Code Example

Try running this simple example. Notice how the mock's add method returns our stubbed value only when called with (1, 1).

import static org.mockito.Mockito.*;

interface CalculatorService {
    int add(int a, int b);
}

public class Main {
    public static void main(String[] args) {
        // Create a mock of CalculatorService
        CalculatorService mockCalc = mock(CalculatorService.class);

        // Stub the add(1, 1) method to return 5
        when(mockCalc.add(1, 1)).thenReturn(5);

        // Call the stubbed method
        System.out.println("1 + 1 = " + mockCalc.add(1, 1));

        // Call a method that was NOT stubbed
        System.out.println("2 + 2 = " + mockCalc.add(2, 2));
    }
}

Mocks Throwing Errors

Sometimes you need to test how your code handles errors. You can configure a mock to throw an exception when a specific method is called, using when().thenThrow().

when(mock.failingMethod()).thenThrow(new SomeException("Error!"));

This is crucial for testing error paths and ensuring your application gracefully handles unexpected situations.

thenThrow() Code Demo

In this example, our DataService mock is configured to throw an IllegalArgumentException if we try to fetch data with an "invalid" ID.

import static org.mockito.Mockito.*;

interface DataService {
    String fetchData(String id);
}

public class Main {
    public static void main(String[] args) {
        DataService mockData = mock(DataService.class);

        // Stub fetchData("invalid") to throw an exception
        when(mockData.fetchData("invalid"))
            .thenThrow(new IllegalArgumentException("ID not found!"));

        // Call a method that was NOT stubbed (returns null by default)
        System.out.println("Fetching 'valid': " + mockData.fetchData("valid"));

        // Call the stubbed method that throws an exception
        try {
            System.out.println("Fetching 'invalid': " + mockData.fetchData("invalid"));
        } catch (IllegalArgumentException e) {
            System.out.println("Caught error: " + e.getMessage());
        }
    }
}

Dynamic Responses: thenAnswer()

What if you need the mock's response to depend on the arguments passed to it, or to perform some custom logic?

when().thenAnswer() is your solution. It takes an Answer object (often a lambda or anonymous class) which provides more control:

You get access to the actual method arguments.
You can perform calculations or complex logic.
You can return a value or throw an exception dynamically.

thenAnswer() Code Demo

Here, the multiply method on our mock actually performs the multiplication based on the arguments it receives. This is more dynamic than a fixed return value.

import static org.mockito.Mockito.*;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;

interface MathService {
    int multiply(int a, int b);
}

public class Main {
    public static void main(String[] args) {
        MathService mockMath = mock(MathService.class);

        // Stub multiply for any two integers to perform actual multiplication
        when(mockMath.multiply(anyInt(), anyInt())).thenAnswer(
            new Answer<Integer>() {
                public Integer answer(InvocationOnMock invocation) {
                    Object[] args = invocation.getArguments();
                    Integer arg1 = (Integer) args[0];
                    Integer arg2 = (Integer) args[1];
                    return arg1 * arg2; // Custom logic based on arguments
                }
            }
        );

        System.out.println("3 * 4 = " + mockMath.multiply(3, 4));
        System.out.println("5 * 2 = " + mockMath.multiply(5, 2));
        System.out.println("7 * 0 = " + mockMath.multiply(7, 0));
    }
}

Multiple Returns: Chaining

What if you want a mock method to return different values on successive calls?

Mockito allows you to chain stubbing methods. You can provide multiple return values to thenReturn(), or chain multiple thenReturn(), thenThrow(), or thenAnswer() calls.

when(mock.method()).thenReturn(val1, val2, val3);
when(mock.method()).thenReturn(val1).thenThrow(ex).thenReturn(val3);

After all specified values or actions are exhausted, the last one will be repeated for subsequent calls.

Chained Stubbing Demo

Observe how our mock Queue returns "Item A", then "Item B", and then throws an exception on subsequent poll() calls.

import static org.mockito.Mockito.*;
import java.util.Queue;
import java.util.LinkedList;

public class Main {
    public static void main(String[] args) {
        Queue<String> mockQueue = mock(Queue.class);

        // Stub poll() to return different values/actions on successive calls
        when(mockQueue.poll())
            .thenReturn("Item A")
            .thenReturn("Item B")
            .thenThrow(new IllegalStateException("Queue is empty!"));

        System.out.println("Poll 1: " + mockQueue.poll()); // Returns "Item A"
        System.out.println("Poll 2: " + mockQueue.poll()); // Returns "Item B"

        try {
            System.out.println("Poll 3: " + mockQueue.poll()); // Throws exception
        } catch (IllegalStateException e) {
            System.out.println("Caught error on Poll 3: " + e.getMessage());
        }

        // Subsequent calls will repeat the last action (throw exception)
        try {
            System.out.println("Poll 4: " + mockQueue.poll());
        } catch (IllegalStateException e) {
            System.out.println("Caught error on Poll 4: " + e.getMessage());
        }
    }
}

Check Your Understanding

You are testing a method that processes items from a queue. You need the queue's poll() method to return "Task 1", then "Task 2", and finally null (indicating an empty queue) for subsequent calls. Which Mockito stubbing approach would you use?

Stubbing Return Values: Recap

In this lesson, you learned how to control the behavior of your mock objects by stubbing their method calls. This is fundamental for isolating the unit under test and creating predictable test environments.

when().thenReturn(): To make a mock method return a specific value.
when().thenThrow(): To make a mock method throw an exception.
when().thenAnswer(): For dynamic responses based on method arguments or custom logic.
Chaining: To define a sequence of different behaviors for successive calls to the same method.

Next, we'll explore Mockito's argument matchers for even more flexible stubbing and verification!

Perguntas Frequentes

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O que vou aprender em “Configurando Valores de Retorno”?

Configure objetos simulados para retornarem valores específicos ou lançarem exceções quando seus métodos forem invocados. Você pratica Testing Mastery: JUnit, Mockito & Integration Tests com código prático que executa diretamente no navegador, e um tutor de IA 24/7 responde suas dúvidas enquanto trabalha na aula.

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Todas as aulas deste curso

Configurando Valores de Retorno
Correspondedores de Argumentos do Mockito
Espionando Objetos Reais
Lançando exceções e fazendo chamadas consecutivas

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