assertThrows for exceptions in JUnit - Test Execution Trace

Test Overview

This test checks that a method throws the expected exception when given invalid input. It verifies that the exception type and message are correct.

Test Code - JUnit 5

JUnit

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

class Calculator {
    int divide(int a, int b) {
        if (b == 0) {
            throw new IllegalArgumentException("Divider cannot be zero");
        }
        return a / b;
    }
}

public class CalculatorTest {
    @Test
    void testDivideByZeroThrowsException() {
        Calculator calc = new Calculator();
        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class, () -> {
            calc.divide(10, 0);
        });
        assertEquals("Divider cannot be zero", exception.getMessage());
    }
}

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initialized	-	PASS
2	Calculator object created	Calculator instance ready	-	PASS
3	assertThrows called with IllegalArgumentException.class and lambda calling divide(10, 0)	Executing divide method with b=0	Exception of type IllegalArgumentException is thrown	PASS
4	Exception caught by assertThrows and stored	Exception message: 'Divider cannot be zero'	Exception message equals 'Divider cannot be zero'	PASS
5	Test ends successfully	Test passed with expected exception	-	PASS

Failure Scenario

Failing Condition: divide method does not throw IllegalArgumentException when dividing by zero

Execution Trace Quiz - 3 Questions

What does assertThrows verify in this test?

A That the divide method does not throw any exception

B That the divide method returns zero when dividing by zero

C That the divide method throws IllegalArgumentException when dividing by zero

D That the divide method throws NullPointerException

Key Result

Use assertThrows to verify that your code throws the correct exception type and then check the exception message to ensure the error is exactly what you expect.