JUnit - Basic Usage

Let us have a basic example to demonstrate the step-by-step process of using JUnit.

1. Create a Class :-

Create a java class to be tested, say, MessageUtil.java in C:\>JUNIT_WORKSPACE

/*
* This class prints the given message on console.
*/
public class MessageUtil {
   private String message;

   //Constructor
   //@param message to be printed
   public MessageUtil(String message){
      this.message = message;
   }
      
   // prints the message
   public String printMessage(){
      System.out.println(message);
      return message;
   }   
}  

2. Create Test Case Class :-

• Create a java test class, say, TestJunit.java.
• Add a test method testPrintMessage() to your test class.
• Add an Annotaion @Test to method testPrintMessage().
• Implement the test condition and check the condition using assertEquals API of JUnit.

Create a java class file name TestJunit.java in C:\>JUNIT_WORKSPACE.

import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class TestJunit {
   String message = "Hello World";
   MessageUtil messageUtil = new MessageUtil(message);

   @Test
   public void testPrintMessage() {
      assertEquals(message,messageUtil.printMessage());
   }
}

3. Create Test Runner Class :-

• Create a TestRunner java class.
• Use runClasses method of JUnitCore class of JUnit to run the test case of the above created test class.
• Get the result of test cases run in Result Object.
• Get failure(s) using the getFailures() method of Result object.
• Get Success result using the wasSuccessful() method of Result object.

Create a java class file named TestRunner.java in C:\>JUNIT_WORKSPACE to execute test case(s).

import org.junit.runner.JUnitCore;
import org.junit.runner.Result;
import org.junit.runner.notification.Failure;

public class TestRunner {

   public static void main(String[] args) {

      Result result = JUnitCore.runClasses(TestJunit.class);
      for (Failure failure : result.getFailures()) {
         System.out.println(failure.toString());
      }
      System.out.println(result.wasSuccessful());
   }
}

4. Compile the MessageUtil, Test case and Test Runner classes using javac.

C:\JUNIT_WORKSPACE>javac MessageUtil.java TestJunit.java TestRunner.java

5. Now run the Test Runner, which will run the test case defined in the provided Test Case class.

C:\JUNIT_WORKSPACE>java TestRunner

Verify the output.

Hello World

true

Now update TestJunit in C:\>JUNIT_WORKSPACE so that the test fails. Change the message string.

import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class TestJunit {
   String message = "Hello World";
   MessageUtil messageUtil = new MessageUtil(message);

   @Test
   public void testPrintMessage() {
      message = "New Word";
      assertEquals(message,messageUtil.printMessage());
   }
}

Let's keep the rest of the classes as is, and try to run the same Test Runner.

import org.junit.runner.JUnitCore;
import org.junit.runner.Result;
import org.junit.runner.notification.Failure;

public class TestRunner {

   public static void main(String[] args) {

      Result result = JUnitCore.runClasses(TestJunit.class);
      for (Failure failure : result.getFailures()) {
         System.out.println(failure.toString());
      }
      System.out.println(result.wasSuccessful());
   }
}

Now run the Test Runner, which will run the test case defined in the provided Test Case class.

C:\JUNIT_WORKSPACE>java TestRunner

Verify the output.

Hello World

testPrintMessage(TestJunit): expected:<[New Word]> but was:<[Hello World]>

false