Find out how many threads in a Java program

Question

Why do I get ID 20, ID 21, ID 22 by runing the following code? I would like to know if i acturely create a thread then stop it probably. somehow i need to call it two more times in my program.
start() and interrrupt() are default thread methods. when I call start() it goes to my run() method. I am worry about if the thread 20 and 21 are still doing something after creating thread 22
main = new Thread(this);
        System.out.println("ID " + main.getId());
        main.start();
        main.interrupt();
        main = null;
        main = new Thread(this);
        System.out.println("ID " + main.getId());
        main.start();
        main.interrupt();
        main = null;
        main = new Thread(this);
        System.out.println("ID " + main.getId());
        main.start();
        main.interrupt();
        main = null;

Answer 1

Here is my code, I would like to know if I implement my threads nicely here, I am worrying those threads are still alive, they should be interrupted. but why I got different thread ID, when i resume it?
// Two player board game play in turn. Human plays first as red, the computer plays
// second as blue. I used thread here for repainting the applet, because the computer
// evaulation function is quite slow, the gui won't draw human player's move until it
// finishs calculating computer's move. the solution i use is interrupting the current
// thread, let the gui repaint before calculating the computer move.
// Board.java
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Board extends Canvas implements Runnable, MouseListener {
    private Thread main;
    private int state = 0;
    private double aMove = 0;
    public void init() {
        addMouseListener(this);
    public void run() {
        removeMouseListener(this);
        main.interrupt();
        main = null;
        repaint();
        // calculate computer's move, then making move
        // this is a slow evalution function goes here
        for (int i = 0; i <10000000; i++) {
            aMove = Math.pow(0.1,1000000000);
        state = 1;
        repaint();
        System.out.println("Blue makes a move on postion " + aMove);
        state = -1;
        addMouseListener(this);
    public void paint(Graphics g) {
        System.out.println("paint");
        if (state == 1) {
            g.setColor(Color.blue);
            g.fillOval(0, 0, 20, 20);
        if (state == -1) {
            g.setColor(Color.red);
            g.fillOval(0, 0, 20, 20);
    public void mouseReleased(MouseEvent e) {
        // human player makes a move on postion 0;
        aMove = 0;
        System.out.println("Red makes a move on postion " + aMove);
        state = -1;
        repaint();
        // Why the following gives me different thread here every time?
        main = new Thread(this);
        System.out.println("Thread ID: "+ main.getId());
        main.start();
    public void mousePressed(MouseEvent e) {
    public void mouseEntered(MouseEvent e) {
    public void mouseExited(MouseEvent e) {
    public void mouseClicked(MouseEvent e) {
// appletdemo.java
import java.applet.*;
import java.awt.*;
public class appletdemo extends Applet{
    Board b = new Board();
    public void init() {
        System.out.println("start ");
        setLayout(new BorderLayout());
        this.add(b);
        b.init();
        setVisible(true);
}