How to write a unit test using JUnit

Introduction

This shot is intended for Java developers with basic knowledge of core Java and Object-Oriented Principles.

This shot provides an introduction to the test-first or test-driven development (TDD) philosophy, which recommends that unit testing and coding go hand-in-hand to ensure your code’s stability.

We will be writing a very simple unit test while developing a simple application.

What is JUnit?

JUnit is a simple, free, open-source framework used to write repeatable unit tests with Java – Erick Gamma and Kert Beck originally wrote it. JUnit is a regression-testing framework that allows developers to write high-quality code faster; its tests increase the stability of the software.

The main philosophy behind this testing framework is to make coding and testing move hand-in-hand.

package io.educative.junit5;

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

import org.junit.jupiter.api.Test;



/* class to be tested */

class HelloWorld {

    private String hello; 

    private String world;



    HelloWorld() {

      hello = "Hello"; 

      world = "World";

    }



    public String formMessage() {

      String message = hello + " " + world;

      return message;

    }

}



/* automated unit test */

class HelloWorldTest {

    @Test

    void checkHelloMsg() {

        HelloWorld helloWorldInstance = new HelloWorld();

        assertEquals(helloWorldInstance.formMessage(), "Hello World");

    }

}

Congrats, the green checkmark indicates that the test was successful!

If the test case fails, a red checkmark and error message will be displayed (execute the code below to see this):

package io.educative.junit5;

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

import org.junit.jupiter.api.Test;



/* class to be tested */

class HelloWorld {

    private String hello; 

    private String world;



    HelloWorld() {

      hello = "Hello"; 

      world = "World";

    }



    public String formMessage() {

      String message = hello + " " + world;

      return message;

    }

}



/* automated unit test */

class HelloWorldTest {

    @Test

    void checkHelloMsg() {

        HelloWorld helloWorldInstance = new HelloWorld();

        assertEquals(helloWorldInstance.formMessage(), "Hello World!");

    }

}

JUnit best practices

Below are some best practices I would recommend while using JUnit in production:

  • Run all the tests in the system at least once per day (or night).
  • If you find yourself debugging using System.out.println(), write a test to automatically check the result instead.
  • When a bug is reported, write a test to expose the bug.
  • Write unit tests before writing the code – only write new code when a test is failing.
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