When to Use Mockito's doAnswer vs. thenReturn in Unit Testing

When should I use doAnswer versus thenReturn in Mockito for unit testing?

Mockito is a powerful framework for creating and using mock objects in unit tests, particularly for Java applications. In this guide, we will explore the differences between the `doAnswer` and `thenReturn` methods, when to use each, and provide code examples to clarify their use cases.

import static org.mockito.Mockito.*;

// Example using thenReturn
List mockList = mock(List.class);
when(mockList.get(0)).thenReturn("first");

// Example using doAnswer
doAnswer(invocation -> {
    Object arg0 = invocation.getArgument(0);
    return "Hello " + arg0;
}).when(mockList).get(anyInt());
// This will modify behavior dynamically.

Causes

• Understanding the context of your tests
• Identifying the behavior you want to simulate
• Determining whether you need to capture arguments or the return value

Solutions

• Use `thenReturn` for straightforward stubbing of return values.
• Opt for `doAnswer` when you need to execute code when the method is called beyond just returning a static value, such as capturing input or executing a side effect.

Mistake: Using thenReturn when you need to perform an action on method call.

Solution: Consider using doAnswer to execute logic on method invocation.

Mistake: Overcomplicating tests by using doAnswer when a simple return suffices.

Solution: Use thenReturn for simple stubbing to keep tests clean and readable.