doReturn and doThrow in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does doReturn() do in JUnit mocking?

doReturn() tells a mock object to return a specific value when a method is called. It is useful when you want to avoid calling the real method.

beginner

How does doThrow() work in JUnit mocking?

doThrow() makes a mock method throw an exception when called. This helps test how your code handles errors.

intermediate

Why use doReturn() instead of when().thenReturn() in JUnit?

doReturn() is safer when mocking methods that are final, private, or have side effects. It avoids calling the real method during setup.

beginner

Show a simple example of doThrow() usage in JUnit.

doThrow(new RuntimeException()).when(mockedObject).someMethod();

This makes someMethod() throw a RuntimeException when called.

intermediate

What is the difference between doReturn() and thenReturn() in JUnit mocking?

thenReturn() is used with when() and calls the real method during setup, which can cause issues. doReturn() does not call the real method and is safer for certain cases.

What does doReturn(value).when(mock).method() do?

A Calls the real method <code>method()</code> on <code>mock</code>.

B Throws an exception when <code>method()</code> is called.

C Returns <code>value</code> when <code>method()</code> is called on <code>mock</code>.

D Does nothing.

doReturn() sets the return value for the mocked method call.

Which method is used to make a mock throw an exception when a method is called?

A doThrow()

B thenReturn()

C doReturn()

D when()

doThrow() makes the mock throw an exception on method call.

Why might you prefer doReturn() over when().thenReturn()?

A Because <code>doReturn()</code> avoids calling the real method during setup.

B Because <code>doReturn()</code> is slower.

C Because <code>when().thenReturn()</code> throws exceptions always.

D Because <code>doReturn()</code> calls the real method.

doReturn() avoids calling the real method, which is safer for some methods.

What happens if you use doThrow(new Exception()).when(mock).method()?

A The method returns normally.

B The method throws the specified exception when called.

C The method is never called.

D The mock is reset.

The mock method throws the exception when called.

Which of these is NOT a reason to use doReturn()?

A Mocking private methods.

B Avoiding side effects during setup.

C Mocking final methods.

D Calling the real method during setup.

doReturn() avoids calling the real method; calling it is a reason to avoid when().thenReturn().

Explain how doReturn() and doThrow() help in mocking with JUnit.

Think about how you control what a mock does when a method is called.

Describe a situation where doReturn() is better than when().thenReturn().

Consider methods that are hard or risky to call during test setup.