Checking exception message in JUnit - Build an Automation Script

Verify exception message when invalid input is provided

Preconditions (2)

Step 1: Call the method parseNumber with input 'abc'

Step 2: Catch the IllegalArgumentException thrown

Step 3: Verify the exception message is exactly 'Invalid number format'

✅ Expected Result: The test should pass only if the exception is thrown and the message matches 'Invalid number format'

Automation Requirements - JUnit 5

Assertions Needed:

Assert that IllegalArgumentException is thrown

Assert that exception message equals 'Invalid number format'

Best Practices:

Use assertThrows to check for exceptions

Capture the exception object to verify its message

Keep test method names descriptive

Avoid catching exceptions manually inside test

Automated Solution

JUnit

import static org.junit.jupiter.api.Assertions.assertThrows;
import static org.junit.jupiter.api.Assertions.assertEquals;

import org.junit.jupiter.api.Test;

public class NumberParserTest {

    // Method under test
    public int parseNumber(String input) {
        try {
            return Integer.parseInt(input);
        } catch (NumberFormatException e) {
            throw new IllegalArgumentException("Invalid number format");
        }
    }

    @Test
    void testParseNumberThrowsExceptionWithCorrectMessage() {
        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class, () -> {
            parseNumber("abc");
        });
        assertEquals("Invalid number format", exception.getMessage());
    }
}

The test method testParseNumberThrowsExceptionWithCorrectMessage uses assertThrows to check that calling parseNumber("abc") throws an IllegalArgumentException. The thrown exception is captured in the exception variable. Then assertEquals verifies the exception message matches exactly "Invalid number format". This approach avoids manual try-catch blocks and clearly expresses the intent to check both the exception type and its message.

The parseNumber method simulates the behavior that throws the exception with the expected message when input is invalid.

Common Mistakes - 3 Pitfalls

Catching exception manually inside test and using fail() instead of assertThrows

Not verifying the exception message, only the exception type

Using assertEquals with exception.toString() instead of getMessage()

Bonus Challenge

Now add data-driven testing with 3 different invalid inputs to verify the exception message each time

Show Hint

Practice

1. In JUnit 5, which method is used to check that a specific exception is thrown with a particular message?

easy

A. assertThrows with a lambda and then assert on the exception message

B. assertEquals with the exception class

C. assertTrue with exception message string

D. assertNull with exception object

Solution

1. Step 1: Understand how to catch exceptions in JUnit 5

   JUnit 5 uses assertThrows to check if a block of code throws an exception.

2. Step 2: Check the exception message

   After assertThrows returns the exception, you can assert its message with assertEquals or other assertions.

3. Final Answer:

   assertThrows with a lambda and then assert on the exception message -> Option A

4. Quick Check:

   assertThrows + message check = A [OK]

Hint: Use assertThrows and then check message from returned exception [OK]

Common Mistakes:

• Trying to check exception message inside assertEquals directly
• Using assertTrue with exception message string only
• Expecting assertNull to check exceptions

2. Which of the following is the correct syntax to assert that a method throws IllegalArgumentException with message "Invalid input" in JUnit 5?

easy

A. assertThrows(IllegalArgumentException.class, () -> method(), "Invalid input");

B. assertEquals("Invalid input", assertThrows(IllegalArgumentException.class, () -> method()).getMessage());

C. assertThrows(() -> method(), IllegalArgumentException.class).getMessage().equals("Invalid input");

D. assertThrows(IllegalArgumentException.class, () -> method()).getMessage() == "Invalid input";

Solution

1. Step 1: Use assertThrows with exception class and lambda

   The correct order is assertThrows(ExceptionClass.class, executable lambda).

2. Step 2: Get the exception message and compare with equals()

   Use getMessage() and equals() to compare strings, not ==.

3. Final Answer:

   assertEquals("Invalid input", assertThrows(IllegalArgumentException.class, () -> method()).getMessage()); -> Option B

4. Quick Check:

   assertThrows + getMessage + equals = B [OK]

Hint: Use assertEquals("msg", assertThrows(class, lambda).getMessage()) [OK]

Common Mistakes:

• Passing message as third argument to assertThrows
• Swapping lambda and exception class order
• Using == to compare strings instead of equals()

3. What will be the result of this JUnit 5 test code?

Exception exception = assertThrows(NullPointerException.class, () -> { throw new NullPointerException("Null value found"); });
assertEquals("Null value found", exception.getMessage());

medium

A. Test fails because exception type does not match

B. Test fails because exception message does not match

C. Test passes because exception type and message match

D. Test throws compilation error

Solution

1. Step 1: Check exception type thrown matches assertThrows

   The lambda throws NullPointerException, which matches the expected type.

2. Step 2: Check the exception message matches the assertEquals

   The message "Null value found" matches exactly, so assertion passes.

3. Final Answer:

   Test passes because exception type and message match -> Option C

4. Quick Check:

   Exception type and message match = Pass [OK]

Hint: Exception type and message must both match for assertEquals pass [OK]

Common Mistakes:

• Assuming assertThrows only checks type, not message
• Confusing assertEquals order of arguments
• Thinking test fails if exception is thrown

4. Identify the error in this JUnit 5 test code that checks exception message:

assertThrows(IllegalStateException.class, () -> { throw new IllegalStateException("Error occurred"); }, "Error occurred");

medium

A. Exception message should be checked with assertTrue, not assertThrows

B. Exception type IllegalStateException is incorrect

C. Lambda syntax is invalid

D. assertThrows does not accept a message as third argument for checking exception message

Solution

1. Step 1: Review assertThrows method signature

   assertThrows expects (Class, Executable) or (Class, Executable, String) where String is failure message, not exception message.

2. Step 2: Understand how to check exception message

   Exception message must be checked separately after assertThrows returns the exception.

3. Final Answer:

   assertThrows does not accept a message as third argument for checking exception message -> Option D

4. Quick Check:

   Third argument in assertThrows is failure message, not exception message [OK]

Hint: Third argument in assertThrows is test failure message, not exception message [OK]

Common Mistakes:

• Passing expected exception message as third argument to assertThrows
• Confusing failure message with exception message
• Not capturing exception to check message separately

5. You want to test a method that throws different exceptions with different messages based on input. Which JUnit 5 approach correctly checks both exception type and message for input "bad" that throws IllegalArgumentException with message "Bad input"?

hard

A. var ex = assertThrows(IllegalArgumentException.class, () -> method("bad")); assertEquals("Bad input", ex.getMessage());

B. assertThrows(IllegalArgumentException.class, () -> method("bad"), "Bad input");

C. assertTrue(() -> method("bad").getMessage().equals("Bad input"));

D. try { method("bad"); fail(); } catch (Exception e) { assertEquals("Bad input", e.getMessage()); }

Solution

1. Step 1: Use assertThrows to capture the exception for input "bad"

   assertThrows returns the exception object for further checks.

2. Step 2: Assert the exception message matches expected "Bad input"

   Use assertEquals on ex.getMessage() to verify the message.

3. Final Answer:

   var ex = assertThrows(IllegalArgumentException.class, () -> method("bad")); assertEquals("Bad input", ex.getMessage()); -> Option A

4. Quick Check:

   Capture exception then check message separately = D [OK]

Hint: Capture exception with assertThrows, then assert message separately [OK]

Common Mistakes:

• Passing message as third argument to assertThrows
• Using assertTrue incorrectly with lambda
• Not failing test if exception not thrown