when().thenThrow() for exceptions in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does when().thenThrow() do in JUnit testing?

when().thenThrow() is used to tell a mock object to throw an exception when a specific method is called. It helps test how your code handles errors.

beginner

How do you use when().thenThrow() to simulate an exception?

You write when(mock.method()).thenThrow(new ExceptionType()). This means when method() is called on the mock, it throws the specified exception.

beginner

Why is it useful to throw exceptions in mocks during testing?

Throwing exceptions in mocks lets you check if your code correctly handles errors, like catching exceptions or cleaning up resources.

intermediate

Example: How to mock a method readFile() to throw IOException?

when(mock.readFile()).thenThrow(new IOException("File not found"));

This simulates a file read error in your test.

intermediate

What happens if you forget to handle the exception thrown by when().thenThrow() in your test?

Your test will fail with an unexpected exception unless you catch it or declare it with @Test(expected=Exception.class).

What is the purpose of when().thenThrow() in JUnit mocking?

A To create a real object

B To make a mock method return a value

C To verify a method call

D To make a mock method throw an exception

when().thenThrow() tells the mock to throw an exception when a method is called.

Which exception type can you throw using thenThrow()?

A Any exception type

B Only runtime exceptions

C Only checked exceptions

D No exceptions

You can throw any exception type with thenThrow(), checked or unchecked.

What happens if the mocked method does not throw the exception as expected?

A Test passes silently

B Test ignores the mock

C Test fails because behavior is not as mocked

D Mock throws a different exception

The test fails if the mock does not behave as specified, including throwing exceptions.

How do you write a test that expects an exception thrown by a mock method?

A Use <code>@Test(expected=Exception.class)</code>

B Ignore exceptions

C Use <code>assertEquals()</code>

D No special handling needed

Use @Test(expected=Exception.class) or try-catch to handle expected exceptions in tests.

Which mocking framework commonly uses when().thenThrow() syntax?

A JUnit only

B Mockito

C Selenium

D JUnit Jupiter

when().thenThrow() is a Mockito syntax for mocking exceptions.

Explain how when().thenThrow() helps test exception handling in your code.

Think about how you test code reacting to errors.

Describe the steps to mock a method to throw a specific exception using when().thenThrow().

Focus on the syntax and purpose.