How to Use Mockito with List Matchers when Generics Are Involved?

How can I avoid warnings in Mockito when using List matchers with generics, specifically for List<Bar>?

when(mock.process(Matchers.any(List.class)));

When dealing with generic types in Mockito, it’s common to encounter warnings due to type erasure. Java’s lists work well with generics, but Mockito's default matchers may not recognize the specific type of List you're using (like List<Bar>). This guide will help you use Mockito's ArgumentMatchers effectively to avoid warnings when working with generic lists.

import static org.mockito.Mockito.*;
import static org.mockito.ArgumentMatchers.*;

when(mock.process(anyList())); // Correct approach to handle generics properly

Causes

• Java's type erasure means that generic type information is not available at runtime, leading to warnings.
• Using ArgumentMatchers.any(List.class) does not specify the generic type, causing type mismatches.

Solutions

• Use ArgumentCaptor to capture the argument type explicitly and check its type in the test cases.
• Employ ArgumentMatchers with type tokens or a custom matcher that clearly defines the expected generic type.

Mistake: Using Matchers.any(List.class) directly with generic types.

Solution: Instead, use anyList() or create a custom matcher for specific types.

Mistake: Ignoring compilation warnings about unchecked conversions.

Solution: Refactor to use generic-aware matchers to avoid these issues.