Unit testing is the foundation of reliable software development. Before exploring advanced mocking frameworks like Mockito, it’s crucial to understand how to write basic unit tests with JUnit 5.

1. Create a Simple Business Class

Let’s create a simple class called SomeBusinessImpl that contains a method to calculate the sum of integers in an array.

Directory Structure

src
├── main
│   └── java
│       └── com.learnitweb.unittesting.business
│           └── SomeBusinessImpl.java
└── test
    └── java
        └── com.learnitweb.unittesting.business
            └── SomeBusinessImplTest.java

SomeBusinessImpl.java

package com.in28minutes.unittesting.business;

public class SomeBusinessImpl {
    public int calculateSum(int[] data) {
        int sum = 0;
        for (int value : data) {
            sum += value;
        }
        return sum;
    }
}

Explanation:

• The method calculateSum takes an int[] as input.
• It iterates through the array and computes the total sum.
• The method returns the final result.

2. Writing Unit Tests with JUnit 5

Now, let’s write a unit test class to test calculateSum with different scenarios.

SomeBusinessImplTest.java

package com.learnitweb.unittesting.business;

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

public class SomeBusinessImplTest {

    @Test
    void calculateSum_basicScenario() {
        SomeBusinessImpl business = new SomeBusinessImpl();
        int result = business.calculateSum(new int[] {1, 2, 3});
        int expectedResult = 6;
        assertEquals(expectedResult, result);
    }

    @Test
    void calculateSum_emptyArray() {
        SomeBusinessImpl business = new SomeBusinessImpl();
        int result = business.calculateSum(new int[] {});
        int expectedResult = 0;
        assertEquals(expectedResult, result);
    }

    @Test
    void calculateSum_oneValue() {
        SomeBusinessImpl business = new SomeBusinessImpl();
        int result = business.calculateSum(new int[] {5});
        int expectedResult = 5;
        assertEquals(expectedResult, result);
    }
}

Explanation of Key Concepts

• The @Test annotation tells JUnit that the method is a test method.
• assertEquals(expected, actual) checks if the actual value matches the expected value.
• If they don’t match, JUnit throws an AssertionError, and the test fails.
• If the actual output doesn’t match the expected result, the test fails and shows a clear error:
  

org.opentest4j.AssertionFailedError: 
Expected :8
Actual   :6

This helps you identify bugs early.