Top 12 Mockito Interview Questions: Mastering the Mocking Framework

Ace your Mockito interview with these frequently asked questions and in-depth answers!

This complete guide goes over the most important Mockito interview questions, giving you the knowledge and confidence to do well in your interview.

Why Mocking?

Mocking plays a crucial role in unit testing, enabling you to isolate and test specific code components independently It’s particularly valuable when

• Dependencies are not yet implemented or under development.
• Component updates modify system state (e.g., database calls).

DoReturn vs ThenReturn Understanding the Difference

Both doReturn and thenReturn create stubs, but they differ in their behavior:

• thenReturn: Type-safe, ensuring compatibility with the return type of the mocked method.
• doReturn: Less strict, allowing you to return any type, but potentially leading to runtime errors.

Spies: Partial Mocks for Enhanced Flexibility

Spies allow you to partially mock an object calling real methods when no mock is set up while enabling verification of interactions. This is particularly useful when you want to mock specific methods within an object.

Static Methods: Beyond Mockito’s Reach

Mockito cannot mock static methods due to their association with the class itself. Frameworks like PowerMock provide this capability through bytecode manipulation at runtime.

Verification: Ensuring Mock Invocations

Verifying mock invocations is crucial to ensure that your test setup and expected outcomes align. Mockito’s “verification” matchers allow you to validate whether a mock was called, how many times and with what arguments.

Testable Code: Principles for Easy Unit Testing

Testable code adheres to principles that facilitate unit testing, such as:

• Reduced dependencies and tight coupling.
• Single Responsibility Principle (SRP) adherence.
• Minimal usage of static methods and final classes.

Limitations of Mockito:

While Mockito is a popular choice, it has limitations:

• Inability to mock static methods.
• Cannot mock constructors, private methods, or final classes.

Alternatives for Mocking Private and Static Methods:

Frameworks like PowerMockito and JMockit offer support for mocking these elements.

Mocking Default Methods in Java 8 Interfaces:

Mockito supports mocking default methods introduced in Java 8 interfaces, providing out-of-the-box functionality.

Verifying Order of Mock Invocations:

Mockito’s InOrder interface helps you verify the order in which mocks were called. This is particularly useful when multiple mocks are involved and the order of their interactions is critical.

Returning Multiple Values for Consecutive Calls:

Mockito provides three approaches for returning different values for consecutive calls to the same stubbed method:

• Using comma-separated values with thenReturn.
• Chaining thenReturn statements.
• Chaining doReturn statements.

Types of Mocking Frameworks:

Mocking frameworks fall into two categories:

• Proxy-based: Mockito, EasyMock
• Bytecode-based: PowerMock, JMockit

Mastering Mockito requires a solid understanding of its capabilities and limitations. This guide provides you with the essential knowledge to confidently answer Mockito interview questions and demonstrate your expertise in this valuable mocking framework.