How to Avoid NullPointerException When Stubbing Methods with Mockito?

What are the best practices to prevent NullPointerExceptions when using Mockito for stubbing methods in Java-Spring projects?

when(myService.getListWithData(inputdata)).thenReturn(Optional.of(expectedOutputList));

When working with Mockito in Java-Spring projects, encountering NullPointerExceptions during method stubbing is a common issue, mainly due to uninitialized objects or incorrectly mocked dependencies. This guide helps you understand how to correctly set up your mocks to avoid such errors.

@Test
public void test_sort() {
    InputData inputdata = new InputData(); // Initialize input data
    List<ItemData> expectedOutputList = new ArrayList<>();
    // Assuming you have added some mock ItemData to expectedOutputList here

    when(myService.getListWithData(inputdata)).thenReturn(Optional.of(expectedOutputList)); // Stubbing the method

    Optional<OutputData> returnedData = classUnderTest.getInformations(inputdata);

    // Add assertions here as needed
}

Causes

• The object being mocked is null before the method call.
• Not mocking the method properly which results in the default behavior (null) being executed instead.
• Not returning the expected object from the previous method call when using method chaining.

Solutions

• Ensure all dependencies are mocked before use, particularly the object that calls the method being stubbed.
• Properly set up your mocks using Mockito, especially ensuring the return values for chained calls are correctly linked.
• Use Optional.of() to ensure the method returns a non-null Optional object.

Mistake: Not initializing objects before use, leading to NullPointerExceptions when methods are called.

Solution: Always ensure that all necessary objects are initialized and set up correctly in your test cases.

Mistake: Mocking methods expecting parameters without setting the parameters correctly beforehand.

Solution: Set up the input parameters correctly and mock return values using the correct Mockito syntax.