@Test annotation in JUnit

Introduction

The @Test annotation tells JUnit which methods are test cases. It helps run tests automatically without manual calls.

When you want to check if a method in your code works correctly.

When you want to run multiple tests automatically in one go.

When you want to mark a method as a test so JUnit can report pass or fail.

When you want to organize your tests clearly in your test class.

When you want to use assertions inside a method to verify expected results.

Syntax

JUnit

@Test
public void testMethodName() {
    // test code here
}

The method annotated with @Test must be public and void.

No arguments are allowed in the test method.

Examples

This test checks if 2 + 3 equals 5.

JUnit

@Test
public void additionTest() {
    int sum = 2 + 3;
    assertEquals(5, sum);
}

This test checks that the string is not null.

JUnit

@Test
public void stringTest() {
    String text = "hello";
    assertNotNull(text);
}

Sample Program

This test class has one test method multiplyTest that checks if 4 times 5 equals 20.

JUnit

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

public class CalculatorTest {

    @Test
    public void multiplyTest() {
        int result = 4 * 5;
        assertEquals(20, result);
    }
}

Important Notes

Use @Test to mark each test method clearly.

JUnit 5 uses org.junit.jupiter.api.Test package.

Assertions like assertEquals help verify expected results inside @Test methods.

Summary

@Test marks methods as test cases for JUnit to run.

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

Use assertions inside @Test methods to check results.

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