Exception Test in JUnit

  

What is Exception Test in JUnit?

In JUnit, an exception test is used to verify that a method throws a specific exception under certain conditions. It's a type of negative test to ensure your code fails gracefully when given invalid inputs or when something unexpected occurs.

Why Use Exception Tests?

Exception tests are useful to:

• Validate input checks.

• Ensure robust error handling.

• Test edge cases and defensive programming logic.

How to Write Exception Tests in JUnit

JUnit 4: Using @Test(expected = Exception.class)







import org.junit.Test;

public class ExceptionTestExample {

    @Test(expected = ArithmeticException.class)
    public void testDivideByZero() {
        int result = 10 / 0;  // This will throw ArithmeticException
    }
}

Explanation:

• If ArithmeticException is thrown, the test passes.

• If no exception or a different one is thrown, the test fails.

JUnit 5: Using Assertions.assertThrows()







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

public class ExceptionTestExample {

    @Test
    void testDivideByZero() {
        assertThrows(ArithmeticException.class, () -> {
            int result = 10 / 0;
        });
    }
}

Explanation:

• assertThrows() checks whether the given lambda throws the specified exception.

• It gives more flexibility (example: checking exception message or stack trace).

Example: Custom Exception Test (JUnit 5)

class InvalidAgeException extends RuntimeException {
    public InvalidAgeException(String message) {
        super(message);
    }
}

class Person {
    public void setAge(int age) {
        if (age < 0) {
            throw new InvalidAgeException("Age cannot be negative");
        }
    }
}

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

public class PersonTest {

    @Test
    void testInvalidAgeException() {
        Person person = new Person();
        Exception exception = assertThrows(InvalidAgeException.class, () -> {
            person.setAge(-5);
        });

        assertEquals("Age cannot be negative", exception.getMessage());
    }
}

At a glance:

Feature	JUnit 4	JUnit 5
Exception Test Syntax	@Test(expected = Exception.class)	assertThrows(Exception.class, () -> {...})
Can check message	No	Yes
Recommended for new code	No, because syntax is old	Yes, because of flexibility