Fixing NullPointerException In Mockito When Using When().thenReturn()

Are you encountering a NullPointerException while stubbing methods with Mockito’s when().thenReturn()? This common issue often arises when chaining method calls on mocked objects. Let’s explore how to resolve this efficiently.

Understanding the Problem

Consider this scenario:

You have an interface BindingStubHelper with a method getBindingStub() that returns a BindingStub object.
In your test, you mock BindingStubHelper and attempt to stub generatePayments() on the result of getBindingStub().

Example Code:

when(bindingStubHelper.getBindingStub().generatePayments(paymentRequest))
    .thenReturn(payments); // Throws NullPointerException

Why Does This Fail?
By default, mocked objects return null for unstubbed methods. Here, getBindingStub() returns null, causing .generatePayments() to throw a NullPointerException.

Solution: Stub Intermediate Method Calls

To resolve this, ensure all method calls in the chain are properly mocked.

Step 1: Mock the Intermediate Object

Create a mock of BindingStub to return when getBindingStub() is called:

BindingStub mockedBindingStub = Mockito.mock(BindingStub.class);
when(bindingStubHelper.getBindingStub()).thenReturn(mockedBindingStub);

Step 2: Stub the Final Method

Now, define the behavior for generatePayments() on the mocked BindingStub:

when(mockedBindingStub.generatePayments(paymentRequest)).thenReturn(payments);

Complete Example

@Mock
private BindingStubHelper bindingStubHelper;

@Test
public void testGeneratePayments() {
    // Create mock objects
    BindingStub mockedBindingStub = Mockito.mock(BindingStub.class);
    PaymentRequest paymentRequest = new PaymentRequest(...);
    Payment expectedPayment = new Payment(...);

    // Stub method chain
    when(bindingStubHelper.getBindingStub()).thenReturn(mockedBindingStub);
    when(mockedBindingStub.generatePayments(paymentRequest)).thenReturn(expectedPayment);

    // Execute test and verify
    Payment result = bindingStubHelper.getBindingStub().generatePayments(paymentRequest);
    assertEquals(expectedPayment, result);
}

Best Practices

Avoid Chaining Mocks: Break down method chains to isolate dependencies.
Use Argument Matchers: For flexibility, use any() or eq() for method parameters:

   when(mockedBindingStub.generatePayments(any(PaymentRequest.class))).thenReturn(payments);

Leverage Annotations: Simplify setup with @Mock and @InjectMocks.

Why This Works

By stubbing each method in the chain, you ensure no null values disrupt the flow. This approach mimics real object behavior while maintaining test isolation.

Keywords

Mockito NullPointerException fix
Stubbing method chains in Mockito
How to mock nested method calls
Mockito when().thenReturn() example
Java unit testing best practices

By following these steps, you’ll eliminate NullPointerExceptions in Mockito stubs and write cleaner, more reliable tests. For more tips, explore Mockito documentation or check out our Java testing guides! 🚀

Fixing NullPointerException in Mockito When Using when().thenReturn()