Checking exception message in JUnit - Test Execution Trace

Test Overview

This test verifies that a method throws an IllegalArgumentException with the expected message when given invalid input.

Test Code - JUnit 5

JUnit

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

public class CalculatorTest {

    @Test
    void testDivideByZeroThrowsExceptionWithMessage() {
        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class, () -> {
            Calculator.divide(10, 0);
        });
        assertEquals("Cannot divide by zero", exception.getMessage());
    }
}

class Calculator {
    public static int divide(int a, int b) {
        if (b == 0) {
            throw new IllegalArgumentException("Cannot divide by zero");
        }
        return a / b;
    }
}

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initialized	-	PASS
2	Calls Calculator.divide(10, 0)	Method invoked with divisor zero	-	PASS
3	Calculator.divide throws IllegalArgumentException with message 'Cannot divide by zero'	Exception thrown and caught by assertThrows	Exception type is IllegalArgumentException	PASS
4	Assert exception message equals 'Cannot divide by zero'	Exception message retrieved	assertEquals("Cannot divide by zero", exception.getMessage())	PASS
5	Test ends successfully	Test passed with expected exception and message	-	PASS

Failure Scenario

Failing Condition: Calculator.divide does not throw exception or throws with wrong message

Execution Trace Quiz - 3 Questions

What does the test verify about the exception?

A The exception type and its message

B Only that an exception is thrown

C That no exception is thrown

D That the method returns zero

Key Result

Always verify both the type and the message of exceptions to ensure your code fails as expected with clear error information.