How to use junit on inteliJ IDEA

Prerequisites

I am using Windows8.1 for my OS.
Suppose we are using Windows 8.1.

JUnit

At first, we will add junit library. Open File -> Project Structure.

Then open "Library". Click on green + . Search for junit and find something like "junit:junit:4.12". Click on it and add it as library.

We need to add a directory for tests. Create the directory:

Click OK.

Now you have a "Test" directory.

Right click on it and mark the directory as "Test Sources Root"

We will add a test now. We will make a test for this method:

public class TestJunit {
    public int test(){
        return 10;
    }
}

This is supposed to return 10.

You can use the "Create Test" intention action by pressing Alt+Enter.

Press Alt+Enter on the class name.

You will see that this will appear.

Click "Create Test". Then you will see this:

If you opened this dialog for the fist time, to add the required library jars to the module dependencies, you need to click "Fix".

Original image is from: https://www.jetbrains.com/help/idea/configuring-testing-libraries.html

Now tick methods for which you want to test and then click OK.

Your groovy script for test is added here:

Write as follows in the TestTestJunit.groovy:

import TestJunit;
import org.junit.Assert;
import org.junit.Test;

public class TestTestJunit {
    @Test
    public void unit_test1(){
        TestJunit jtest = new TestJunit();
        Assert.assertEquals(10, jtest.test());
        Assert.assertEquals("This is not 10!!!",10, jtest.test());
       
        System.out.println("Unit test1 finished without error.");
    }
}

Now run the test (right click inside unit_test1 method):

Test finished.

Just for a try, we can change the returned value and test the method again:

Test result:

You can see it is correctly detecting that the returned value is different from the expected value.