Change the StringBuffer.java class to add a method

You will need these Handy bash scripts before you do any of the below.

$ cd $HOME/sources/jdk8_tl/

or

$ cd $HOME/sources/jdk8/

or

$ cd $HOME/sources/jdk9/

$ gedit jdk/src/share/classes/java/lang/StringBuffer.java

or

$ gedit jdk/src/java.base/share/classes/java/lang/StringBuffer.java

Note: please ensure that the annotations at the top of all the test file are preserved, they are needed for the jtreg.

 /**
    * Mitia's greatest contribution ever!
    * 
    * @return boolean 
    */
 public boolean isEmpty() {
             return count==0;
 }

Write the below tests in IsEmptyTest.java in some folder

/* @test
 * @summary Test StringBuffer.isEmpty();
 * @run main IsEmptyTest
 */

public class IsEmptyTest {

    private static int failed =0;

    private static void testNew() {
        StringBuffer buffer = new StringBuffer();

        if (!buffer.isEmpty()) {
            System.out.println(
                    "StringBuffer isEmpty on new object failed");
            failed++;
        }
    }

    private static void testReNew() {
        StringBuffer buffer = new StringBuffer();
        buffer.append("a");
        buffer.delete(0, 1);

        if (!buffer.isEmpty()) {
            System.out.println(
            "StringBuffer isEmpty on appended and deleted element of object failed");
            failed++;
        }
    }

    private static void testNotEmpty() {
        StringBuffer buffer = new StringBuffer();
        buffer.append("aa");

        if (buffer.isEmpty()) {
            System.out.println(
            "StringBuffer isEmpty on changed object failed");
            failed++;
        }
    }

    public static void main(String[] args) {
        testNew();
        testNotEmpty();
        testReNew();

        if (failed>0){
            throw new RuntimeException("Failed: "+failed);
        }
    }
}

Write the below (TestNG) tests in the IsEmptyTestNG.java file

/* @test
 * @summary Test StringBuffer.isEmpty();
 * @run testng IsEmptyTestNG
 */

import static org.testng.Assert.assertFalse;
import static org.testng.Assert.assertTrue;

import java.lang.StringBuffer;

import org.testng.annotations.Test;

public class IsEmptyTestNG {

    @Test
    public void testNew() {
        StringBuffer buffer = new StringBuffer();

        assertTrue(buffer.isEmpty());
    }

    @Test
    public void testReNew() {
        StringBuffer buffer = new StringBuffer();
        buffer.append("aa");
        buffer.delete(0, 2);

        assertTrue(buffer.isEmpty());
    }

    @Test
    public void testNotEmpty() {
        StringBuffer buffer = new StringBuffer();
        buffer.append("aa");

        assertFalse(buffer.isEmpty());
    }
}

$ make images

or

$ make jdk

Finally run JTReg commands shown in How to use JTReg… - Java Regression Test Harness ? to ensure the tests run, and pass (see section below).

Change the implementation to something else and re-run the tests again to see them fail.