Checking exception type hierarchy in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to check exception type hierarchy in testing?

It means verifying that the thrown exception is of the expected type or a subtype, ensuring the test catches the correct error class.

beginner

How can you check for a specific exception type in JUnit 5?

Use the assertThrows method with the expected exception class and a lambda that runs the code that should throw the exception.

intermediate

Why is it important to check the exception type hierarchy rather than just the base exception?

Because catching only the base exception might hide specific errors; checking the exact type or subtype ensures precise error handling and test accuracy.

intermediate

Example: What exception type will assertThrows catch if the thrown exception is a subclass of the expected exception?

assertThrows will pass if the thrown exception is the expected type or any subclass of it, because it checks the exception type hierarchy.

beginner

How to write a JUnit 5 test that verifies a method throws IOException or any subclass of IOException?

Use assertThrows(IOException.class, () -> methodThatThrows()); This passes if the thrown exception is IOException or any subclass like FileNotFoundException.

In JUnit 5, which method is used to check if a specific exception or its subclass is thrown?

A assertTrue

B assertEquals

C assertThrows

D assertNull

assertThrows checks if the code throws the expected exception type or any subclass of it.

If a method throws FileNotFoundException, which is a subclass of IOException, what happens when you assertThrows(IOException.class, ...)?

A Test fails because exception types must match exactly

B Test fails because IOException is a superclass

C Test passes only if FileNotFoundException is caught explicitly

D Test passes because FileNotFoundException is a subclass of IOException

assertThrows accepts the expected exception type or any subclass, so the test passes.

Why should you avoid catching only the base Exception class in tests?

A It can hide specific exceptions and make tests less precise

B It makes tests run slower

C It causes syntax errors

D It is required to catch only base Exception

Catching only base Exception hides specific exception types, reducing test accuracy.

Which JUnit 5 feature helps you check the exception message along with the type?

A assertTrue

B assertThrows returns the exception object for further checks

C assertEquals

D assertNull

assertThrows returns the caught exception so you can assert its message or other properties.

What happens if the tested code does not throw any exception but assertThrows expects one?

A The test fails because no exception was thrown

B The test passes

C The test is skipped

D The test throws a compilation error

assertThrows fails the test if no exception is thrown.

Explain how to use JUnit 5 to check that a method throws a specific exception or any of its subclasses.

Think about how assertThrows works with exception types.

Why is it important to consider exception type hierarchy when writing tests for exceptions?

Consider how Java exception classes relate to each other.

Practice

1. In JUnit, when you want to check if a method throws an exception of a specific type or any of its subclasses, which assertion method should you use?

easy

A. assertThrows()

B. assertEquals()

C. assertTrue()

D. assertNull()

Solution

1. Step 1: Understand the purpose of assertThrows()

   assertThrows() is designed to check if a block of code throws an exception of a specified type or its subclass.

2. Step 2: Compare with other assertions

   assertEquals() checks equality, assertTrue() checks boolean conditions, and assertNull() checks for null values, none of which check exceptions.

3. Final Answer:

   assertThrows() -> Option A

4. Quick Check:

   Exception type check = assertThrows() [OK]

Hint: Use assertThrows() to check exceptions and their subclasses [OK]

Common Mistakes:

• Using assertEquals() to check exceptions
• Confusing assertTrue() with exception checks
• Expecting assertNull() to verify exceptions

2. Which of the following is the correct syntax to assert that a method throws a NullPointerException in JUnit 5?

easy

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

B. assertThrows(NullPointerException, method());

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

D. assertThrows(method(), NullPointerException);

Solution

1. Step 1: Recall assertThrows() method signature

   assertThrows() takes the exception class as the first argument and a lambda expression as the second.

2. Step 2: Match the syntax

   assertThrows(NullPointerException.class, () -> method()); correctly uses assertThrows(NullPointerException.class, () -> method()); which matches the required syntax.

3. Final Answer:

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

4. Quick Check:

   Correct syntax = assertThrows(NullPointerException.class, () -> method()); [OK]

Hint: Exception class first, then lambda expression in assertThrows() [OK]

Common Mistakes:

• Passing exception instance instead of class
• Swapping argument order in assertThrows()
• Not using lambda expression for the executable

3. Consider the following JUnit 5 test code:

assertThrows(RuntimeException.class, () -> { throw new IllegalArgumentException(); });

What will be the result of this test?

medium

A. Test fails with a compilation error

B. Test fails because IllegalArgumentException is not RuntimeException

C. Test passes because IllegalArgumentException is a subclass of RuntimeException

D. Test passes but warns about unchecked exception

Solution

1. Step 1: Check exception hierarchy

   IllegalArgumentException extends RuntimeException, so it is a subclass.

2. Step 2: Understand assertThrows behavior

   assertThrows(RuntimeException.class, ...) passes if the thrown exception is RuntimeException or any subclass.

3. Final Answer:

   Test passes because IllegalArgumentException is a subclass of RuntimeException -> Option C

4. Quick Check:

   Subclass exception passes assertThrows superclass check [OK]

Hint: assertThrows accepts subclass exceptions of the specified class [OK]

Common Mistakes:

• Thinking exact exception type is required
• Confusing subclass with unrelated exceptions
• Expecting compilation error for subclass exceptions

4. Given this JUnit 5 test:

assertThrows(RuntimeException.class, () -> { throw new FileNotFoundException(); });

The test fails. What is the most likely reason?

medium

A. The lambda expression does not throw any exception

B. The test method does not declare throws IOException

C. FileNotFoundException is a checked exception and must be declared

D. FileNotFoundException is not a subclass of RuntimeException

Solution

1. Step 1: Check exception hierarchy

   FileNotFoundException extends IOException, not RuntimeException.

2. Step 2: Understand assertThrows behavior

   assertThrows(RuntimeException.class, ...) fails because the thrown exception is not RuntimeException or any subclass. Other options are incorrect: checked exceptions like FileNotFoundException do not require declaration in the test method when using Executable lambdas.

3. Final Answer:

   FileNotFoundException is not a subclass of RuntimeException -> Option D

4. Quick Check:

   Non-subclass exception causes assertThrows to fail [OK]

Hint: assertThrows requires the thrown exception to be the specified class or subclass [OK]

Common Mistakes:

• Misidentifying exception subclass relationships
• Ignoring checked exception declaration rules
• Expecting runtime exceptions to behave like checked

5. You want to write a JUnit 5 test that passes if a method throws either an IOException or any of its subclasses, but fails for other exceptions. Which of the following is the best approach?

hard

A. Use assertThrows(Exception.class, () -> method()); and check instance with instanceof IOException

B. Use assertThrows(IOException.class, () -> method());

C. Use try-catch inside test and assert exception type manually

D. Use assertThrows(Throwable.class, () -> method());

Solution

1. Step 1: Understand assertThrows behavior with superclass

   assertThrows(IOException.class, ...) passes if the thrown exception is IOException or any subclass.

2. Step 2: Evaluate other options

   Use assertThrows(Exception.class, () -> method()); and check instance with instanceof IOException requires manual checks and complicates test; Use try-catch inside test and assert exception type manually is verbose; Use assertThrows(Throwable.class, () -> method()); is too broad and accepts all throwables.

3. Final Answer:

   Use assertThrows(IOException.class, () -> method()); -> Option B

4. Quick Check:

   Superclass in assertThrows covers subclasses [OK]

Hint: Use superclass exception in assertThrows to cover subclasses [OK]

Common Mistakes:

• Using too broad exception classes
• Manually checking exception types unnecessarily
• Ignoring assertThrows built-in subclass support