Verify that a method throws the expected exception

Preconditions (2)

Step 1: Call the method with a negative input value

Step 2: Verify that the method throws IllegalArgumentException

✅ Expected Result: The test should pass only if IllegalArgumentException is thrown when the method is called with negative input

Automation Requirements - JUnit 5

Assertions Needed:

assertThrows to verify IllegalArgumentException is thrown

Best Practices:

Use lambda expression inside assertThrows

Check exception message if relevant

Keep test method names descriptive

Automated Solution

JUnit

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

class Calculator {
    int squareRoot(int number) {
        if (number < 0) {
            throw new IllegalArgumentException("Negative input not allowed");
        }
        return (int) Math.sqrt(number);
    }
}

public class CalculatorTest {

    @Test
    void testSquareRootThrowsExceptionForNegativeInput() {
        Calculator calculator = new Calculator();

        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class, () -> {
            calculator.squareRoot(-5);
        });

        // Optional: verify exception message
        org.junit.jupiter.api.Assertions.assertEquals("Negative input not allowed", exception.getMessage());
    }
}

This test uses assertThrows from JUnit 5 to check that the squareRoot method throws an IllegalArgumentException when called with a negative number.

The lambda expression () -> calculator.squareRoot(-5) is passed to assertThrows. This means the code inside the lambda is executed and JUnit checks if the expected exception is thrown.

If the exception is thrown, the test passes. We also capture the exception object to verify its message matches the expected text. This helps ensure the exception is thrown for the right reason.

The test method name clearly describes what is being tested, which is a good practice.

Common Mistakes - 3 Pitfalls

Using try-catch block instead of assertThrows

Not using a lambda expression inside assertThrows

Not verifying the exception message

Bonus Challenge

Now add data-driven testing with 3 different negative inputs to verify the exception is thrown each time

Show Hint