Explanation:

Java allows using the JUnit test framework to write and execute automated tests. It contains a set of assert methods to test method code. It uses @Test annotation to define the test case. It also includes two methods:

• setUp() method that runs before every test invocation.
• tearDown() method that runs after every test method.

Program:

The required Java code for JUnit test case is given below:

import static org.junit.Assert.*;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
// The test class SalesItemTest.
public class SalesItemTest
{
    // Default constructor 
     public SalesItemTest()
    {
    }
  // Sets up the test fixture.
   @Before
    public void setUp()
    {
    }

    // Tears down the test fixture.
    @After
    public void tearDown()
   {
    }
  /&
   * Test case to add two comments and that check the size of ArrayList is correct afterwards.       
  */
    @Test
    public void testAddComments()
    {
       SalesItem salesItem1 = new SalesItem("Java for complete Idiots", 21998);
       assertEquals(true,  salesItem1.addComment("James Duckling", "This book is great.", 4));
       assertEquals(true,  salesItem1.addComment("Jhoky Jonathan", "Super Java book.", 5));
       assertEquals(2, salesItem1.getNumberOfComments());
    }
}

In the above JUnit test case, the two comments are added then test the size of ArrayList by comparing the return value of getNumberOfComments() method with 2.

The inspector to check for the comment list, their size, and elementData field of ArrayList object is as follows: