How to Write Tests in JUnit: Simple Guide with Examples

To write a test in JUnit, create a public method annotated with @Test inside a class. Use assertions like Assertions.assertEquals() to check expected results. Run the test with a JUnit runner to see if it passes or fails.

Syntax

In JUnit, a test method must be annotated with @Test. The method should be public, return void, and contain assertions to verify behavior.

• @Test: Marks the method as a test.
• Method name: Describes what is tested.
• Assertions: Check if the actual result matches the expected.

java

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

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

Example

This example shows a simple test that checks if adding two numbers returns the correct sum. It uses assertEquals to compare the expected value with the actual result.

java

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

public class CalculatorTest {
    @Test
    public void testAdd() {
        Calculator calc = new Calculator();
        int result = calc.add(10, 20);
        assertEquals(30, result, "10 + 20 should equal 30");
    }
}

class Calculator {
    public int add(int a, int b) {
        return a + b;
    }
}

Output

Test passed: testAdd()

Common Pitfalls

Common mistakes when writing JUnit tests include:

• Forgetting the @Test annotation, so the method does not run as a test.
• Using assertions incorrectly, such as reversing expected and actual values.
• Writing tests that depend on external state or order, causing flaky results.
• Not making test methods public void.

java

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

public class WrongTest {
    // Missing @Test annotation - this test will not run
    public void testFail() {
        assertEquals(5, 2 + 2); // Wrong expected value
    }

    @Test
    public void testRight() {
        assertEquals(4, 2 + 2); // Correct assertion
    }
}

Output

Test passed: testRight()
Test not run: testFail() (missing @Test annotation)

Quick Reference

Remember these tips when writing JUnit tests:

• Always annotate test methods with @Test.
• Use assertEquals(expected, actual) for checking values.
• Keep tests independent and repeatable.
• Name tests clearly to describe their purpose.

Key Takeaways

Annotate test methods with @Test to make JUnit recognize them as tests.

Use assertions like assertEquals to verify expected outcomes.

Test methods must be public and return void.

Avoid dependencies between tests to keep results reliable.

Name tests clearly to understand what they check at a glance.