assertThrows for exceptions in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of assertThrows in JUnit?

assertThrows checks if a specific exception is thrown by a block of code during a test. It helps verify that error handling works as expected.

beginner

How do you use assertThrows to test that a method throws IllegalArgumentException?

assertThrows(IllegalArgumentException.class, () -> {
    methodThatShouldThrow();
});

This runs methodThatShouldThrow() and passes if it throws IllegalArgumentException.

intermediate

What does assertThrows return and how can it be used?

assertThrows returns the thrown exception object. You can use it to check the exception message or other properties.

Exception e = assertThrows(Exception.class, () -> { ... });
assertEquals("Error message", e.getMessage());

intermediate

Why is assertThrows better than using try-catch in tests?

assertThrows makes tests cleaner and clearer by directly expressing the expected exception. It avoids manual try-catch blocks and reduces boilerplate code.

beginner

Can assertThrows check for multiple exception types at once?

No, assertThrows checks for one specific exception type per call. To test multiple exceptions, write separate assertThrows calls for each.

What does assertThrows verify in a JUnit test?

A That no exceptions are thrown

B That a specific exception is thrown

C That a method returns true

D That a method completes within time

assertThrows checks if the expected exception is thrown by the tested code.

Which of the following is the correct syntax to test for NullPointerException using assertThrows?

A assertThrows(() -> method(), NullPointerException.class);

B assertThrows(method(), NullPointerException.class);

C assertThrows(NullPointerException.class, () -> method());

D assertThrows(NullPointerException, method());

The first argument is the exception class, second is a lambda with the code to test.

What can you do with the exception object returned by assertThrows?

A Check its message or properties

B Ignore it

C Change its type

D Throw it again automatically

You can inspect the exception details to verify error messages or other info.

If the tested code does NOT throw the expected exception, what happens to the test with assertThrows?

A The test fails

B The test passes

C The test is skipped

D The test throws a runtime error

The test fails because the expected exception was not thrown.

Is it possible to test multiple exceptions in one assertThrows call?

A Yes, by using a wildcard exception

B Yes, by listing exceptions separated by commas

C Yes, by using an array of exceptions

D No, test each exception separately

assertThrows accepts only one exception class per call.

Explain how to use assertThrows to verify that a method throws an exception and how to check the exception message.

Think about how you write the lambda and what assertThrows returns.

Describe why using assertThrows is preferred over try-catch blocks in JUnit tests.

Consider readability and test clarity.