Checking exception type hierarchy in JUnit - Build an Automation Script

Verify that a method throws the correct exception type and its hierarchy

Preconditions (2)

Step 1: Call the method 'processData' with invalid input 'null'

Step 2: Catch the exception thrown

Step 3: Verify that the exception is an instance of IllegalArgumentException

Step 4: Verify that the exception is also an instance of RuntimeException

Step 5: Verify that the exception is not an instance of IOException

✅ Expected Result: The test confirms that the thrown exception is IllegalArgumentException and it correctly inherits from RuntimeException but not from IOException

Automation Requirements - JUnit 5

Assertions Needed:

assertThrows to check exception type

assertTrue with instanceof to check exception hierarchy

assertFalse with instanceof to check negative hierarchy

Best Practices:

Use assertThrows for exception testing

Use clear and descriptive assertion messages

Avoid catching exceptions manually inside test methods

Use meaningful test method names

Automated Solution

JUnit

import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.Test;

class ExceptionHierarchyTest {

    void processData(String input) {
        if (input == null) {
            throw new IllegalArgumentException("Input cannot be null");
        }
        // processing logic
    }

    @Test
    void testProcessDataThrowsCorrectException() {
        IllegalArgumentException thrown = assertThrows(IllegalArgumentException.class, () -> {
            processData(null);
        }, "Expected processData to throw IllegalArgumentException for null input");

        assertTrue(thrown instanceof RuntimeException, "Exception should be instance of RuntimeException");
        assertFalse(thrown instanceof java.io.IOException, "Exception should not be instance of IOException");
    }
}

The processData method throws IllegalArgumentException when given null input.

The test method testProcessDataThrowsCorrectException uses assertThrows to check that the exception thrown is exactly IllegalArgumentException.

Then, assertTrue checks that this exception is also a RuntimeException, confirming the inheritance hierarchy.

Finally, assertFalse confirms that the exception is not an IOException, ensuring the exception type is correct and not from an unrelated hierarchy.

This approach avoids manual try-catch blocks and uses clear assertions with messages for easy debugging.

Common Mistakes - 3 Pitfalls

{'mistake': 'Catching exceptions manually inside the test method instead of using assertThrows', 'why_bad': 'This makes tests longer, harder to read, and can miss assertion failures if exceptions are not properly handled.', 'correct_approach': "Use JUnit 5's assertThrows to automatically check for exceptions."}

Checking exception type with getClass() equality instead of instanceof

Not providing assertion messages

Bonus Challenge

Now add tests to verify that processData throws NullPointerException for empty string input and check its exception hierarchy

Show Hint

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