@Test annotation in JUnit - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple JUnit test with @Test

What will be the result of running this JUnit test class?

JUnit

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

public class SampleTest {
    @Test
    void testSum() {
        assertEquals(5, 2 + 3);
    }
}

A Compilation error due to missing main method.

B Test fails with an AssertionError.

C Test passes successfully with no errors.

D Runtime exception due to null pointer.

💡 Hint

Remember that @Test marks a method as a test and assertEquals checks expected vs actual values.

The testSum method checks if 2 + 3 equals 5. Since it does, the assertion passes and the test passes without errors.

❓ assertion

intermediate

Identify the failing assertion in a JUnit test

Given this test method, which assertion will cause the test to fail?

JUnit

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

public class AssertionTest {
    @Test
    void testValues() {
        assertTrue(3 > 2);
        assertFalse(5 < 1);
        assertEquals("hello", "hello");
        assertNotNull(null);
    }
}

A assertNotNull(null);

B assertFalse(5 < 1);

C assertEquals("hello", "hello");

D assertTrue(3 > 2);

💡 Hint

Check which assertion conditions are false or invalid.

assertNotNull(null) fails because the argument is null, so the assertion fails and the test fails at that point.

🧠 Conceptual

advanced

Purpose of the @Test annotation in JUnit

What is the main purpose of the @Test annotation in a JUnit test class?

A To ignore a test method during execution.

B To specify the order in which tests should run.

C To provide setup code before each test method.

D To mark a method as a test method that JUnit should run.

💡 Hint

Think about how JUnit knows which methods to execute as tests.

The @Test annotation tells JUnit that the method is a test method to be executed during testing.

🔧 Debug

advanced

Identify the error in this JUnit test method

What error will occur when running this test class?

JUnit

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

public class DebugTest {
    @Test
    void testDivision() {
        int result = 10 / 0;
        assertEquals(0, result);
    }
}

A Compilation error due to division by zero.

B ArithmeticException: division by zero at runtime.

C AssertionError because result is not zero.

D Test passes successfully.

💡 Hint

Division by zero is not allowed in Java and causes an exception.

The code divides 10 by 0, which causes an ArithmeticException at runtime before the assertion is reached.

❓ framework

expert

Behavior of @Test with expected exception in JUnit 5

Consider this test method in JUnit 5. What is the test result?

JUnit

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

public class ExceptionTest {
    @Test
    void testException() {
        assertThrows(IllegalArgumentException.class, () -> {
            throw new IllegalArgumentException("Invalid argument");
        });
    }
}

A Test passes because the expected exception is thrown.

B Test fails because no exception is thrown.

C Test fails due to a compilation error in assertThrows usage.

D Test fails because a different exception is thrown.

💡 Hint

assertThrows expects the specified exception to be thrown inside the lambda.

The lambda throws IllegalArgumentException as expected, so assertThrows passes and the test passes.

Practice

1. What is the main purpose of the @Test annotation in JUnit?

easy

A. To mark a method as a test case that JUnit will run

B. To declare a method as private

C. To specify the return type of a method

D. To import JUnit libraries

Solution

1. Step 1: Understand the role of @Test

   The @Test annotation tells JUnit which methods are test cases to execute.

2. Step 2: Eliminate unrelated options

   Options B, C, and D do not relate to marking test methods.

3. Final Answer:

   To mark a method as a test case that JUnit will run -> Option A

4. Quick Check:

   @Test marks test methods [OK]

Hint: Remember: @Test means 'run this method as test' [OK]

Common Mistakes:

• Thinking @Test changes method visibility
• Confusing @Test with import statements
• Assuming @Test sets return type

2. Which of the following is the correct syntax for a JUnit test method using @Test?

easy

A. public void testMethod(int x) {}

B. private int testMethod() {}

C. public void testMethod() {}

D. public String testMethod() {}

Solution

1. Step 1: Recall JUnit test method rules

   Test methods must be public, return void, and take no parameters.

2. Step 2: Check each option

   public void testMethod() {} matches all rules. Options B, C, and D violate visibility, return type, or parameters.

3. Final Answer:

   public void testMethod() {} -> Option C

4. Quick Check:

   Public, void, no params = correct test method [OK]

Hint: Test methods: public + void + no parameters [OK]

Common Mistakes:

• Making test methods private
• Adding parameters to test methods
• Using non-void return types

3. Given the code below, what will be the test result when running with JUnit?

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

public class SampleTest {
    @Test
    public void checkSum() {
        int result = 2 + 3;
        assertEquals(5, result);
    }
}

medium

A. Runtime exception thrown

B. Test passes successfully

C. Compilation error due to missing return

D. Test fails with assertion error

Solution

1. Step 1: Analyze the test method logic

   The method calculates 2 + 3 and stores in result, then asserts result equals 5.

2. Step 2: Check assertion correctness

   Since 2 + 3 is 5, the assertion assertEquals(5, result) will pass.

3. Final Answer:

   Test passes successfully -> Option B

4. Quick Check:

   2 + 3 = 5, assertion true [OK]

Hint: If assertion matches calculation, test passes [OK]

Common Mistakes:

• Assuming test fails without checking assertion
• Confusing compilation errors with test failures
• Ignoring that test methods return void

4. Identify the error in the following JUnit test method:

import org.junit.jupiter.api.Test;

public class ErrorTest {
    @Test
    private void testHidden() {
        // test code
    }
}

medium

A. Test method must be public, not private

B. Missing return statement in test method

C. Test method should have parameters

D. Missing @BeforeEach annotation

Solution

1. Step 1: Check method visibility

   JUnit requires test methods to be public so it can access and run them.

2. Step 2: Validate other method properties

   Test methods must be void and parameterless; private visibility is the error here.

3. Final Answer:

   Test method must be public, not private -> Option A

4. Quick Check:

   Test methods must be public [OK]

Hint: Test methods must be public, not private [OK]

Common Mistakes:

• Making test methods private
• Adding parameters to test methods
• Confusing lifecycle annotations with test method requirements

5. You want to write a JUnit test method that checks if a method throws an exception. Which is the correct way to use @Test for this purpose in JUnit 5?

hard

A. Use @Test(expected = Exception.class) annotation parameter

B. Add throws Exception to the test method signature

C. Write a try-catch block without @Test

D. Use @Test with assertThrows() inside the method

Solution

1. Step 1: Recall JUnit 5 exception testing

   JUnit 5 uses assertThrows() inside @Test methods to check exceptions.

2. Step 2: Eliminate outdated or incorrect options

   Use @Test(expected = Exception.class) annotation parameter is from JUnit 4 and invalid in JUnit 5. Add throws Exception to the test method signature only declares exception, not tests it. Write a try-catch block without @Test misses @Test.

3. Final Answer:

   Use @Test with assertThrows() inside the method -> Option D

4. Quick Check:

   JUnit 5 uses assertThrows() inside @Test [OK]

Hint: JUnit 5 tests exceptions with assertThrows() inside @Test [OK]

Common Mistakes:

• Using JUnit 4 style @Test(expected=) in JUnit 5
• Only declaring throws without assertion
• Not using @Test annotation