Why Checking exception message in JUnit? - Purpose & Use Cases

The Big Idea

What if your tests could instantly catch the exact error message your users see?

The Scenario

Imagine you are testing a program manually and want to make sure it shows the right error message when something goes wrong, like dividing by zero.

You run the program, see the error popup, and try to remember the exact message to check later.

The Problem

Manually checking error messages is slow and easy to forget or misread.

You might miss small differences in wording that matter, or spend too much time repeating the same checks.

The Solution

Using JUnit to check exception messages automatically catches the exact error text during tests.

This saves time, avoids mistakes, and makes sure your program tells users the right thing when errors happen.

Before vs After

✗ Before

try {
  divide(10, 0);
  fail("Expected exception");
} catch (Exception e) {
  if (!e.getMessage().equals("Cannot divide by zero")) {
    fail("Wrong message");
  }
}

✓ After

assertThrows(ArithmeticException.class, () -> divide(10, 0), "Cannot divide by zero");

What It Enables

You can trust your tests to catch exact error messages, making your software more reliable and user-friendly.

Real Life Example

When a banking app prevents a withdrawal that exceeds the balance, checking the exception message ensures the user sees a clear, correct warning.

Key Takeaways

Manual error message checks are slow and error-prone.

JUnit lets you verify exact exception messages automatically.

This improves test accuracy and saves time.

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