How to Test Abstract Classes with Mockito: A Step-by-Step Guide

Testing Abstract Classes with Mockito

Abstract classes pose challenges for unit testing due to their lack of concrete implementations. While manual mock creation is an option, it can be time-consuming and complex.

Mockito offers an elegant solution for testing abstract classes without manual mock creation. Here's how it works:

1. Mock the Abstract Class:

   <code class="java">My mock = Mockito.mock(My.class, Answers.CALLS_REAL_METHODS);</code>

   By using Answers.CALLS_REAL_METHODS, you instruct Mockito to execute the actual implementation of non-overridden methods, allowing you to test the behavior of the abstract methods.

2. Stub Abstract Methods as Needed:

   For abstract methods that you want to control during testing, use the following syntax:

   <code class="java">Mockito.when(mock.methodIDontCareAbout()).thenReturn(null);</code>

   This stubs the specific abstract method, providing the desired behavior.

Example:

Consider the following scenario:

<code class="java">public abstract class My {
    public Result methodUnderTest() { ... }
    protected abstract void methodIDontCareAbout();
}

@Test
public void shouldFailOnNullIdentifiers() {
    My my = Mockito.mock(My.class, Answers.CALLS_REAL_METHODS);
    Assert.assertSomething(my.methodUnderTest());
}</code>

In this example, the abstract method methodIDontCareAbout() is not used in the method under test. By mocking it with thenReturn(null), you effectively ignore it during testing.

This approach provides a concise and convenient way to test abstract classes, eliminating the need for hand-crafted mocks and facilitating efficient testing of non-concrete classes.

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