Creating a Java Timer and a TimerTask?

问题:

I'm new to Java and I'm trying to set a simple timer, I'm familiar with set_interval, because of experience with JavaScript and ActionScript,

I'm not so familiar with classes yet so I get confused easily, I understand that I need to set a new Timer, and then set a TimerTask, but I don't get how exactly to do it even though I'm looking at the documentation right now..

So I created an Applet, and that's my init method:

public void init() {
    TimerTask myTask;
    Timer myTimer;
    myTimer.schedule(myTask,5000);
}

How do I actually set the task code? I wanted it to do something like

g.drawString("Display some text with changing variables here",10,10);

回答1:

As told by many a wonderful Stackoverflow users, the right idea in here is to use the javax.swing.TImer. Here is one small code snippet for your help. Do ask any question, that is beyond your grasp, I will try to provide information likewise.

import java.awt.*;
import java.awt.event.*;
import java.util.Random;
import javax.swing.*;

public class DrawStringWithTimer
{
    private final int WIDTH = 400;
    private final int HEIGHT = 300;
    private Timer timer;
    private int x;
    private int y;
    private int counter;
    private Random random;
    private String[] messages = {
                                    "Bingo, I am ON",
                                    "Can you believe this !!",
                                    "What else you thinking about ?",
                                    "Ahha, seems like you are confused now !!",
                                    "Lets Roll and Conquer :-)"
                                };
    private CustomPanel contentPane;

    private ActionListener timerAction = new ActionListener()
    {
        @Override
        public void actionPerformed(ActionEvent ae)
        {
            if (counter == 5)
                counter = 0;
            /*
             * Creating random numbers where 
             * x will be equal to zero or 
             * less than WIDTH and y will be
             * equal to zero or less than HEIGHT.
             * And we getting the value of the
             * messages Array with counter variable
             * and passing this to setValues function,
             * so that it can trigger a repaint()
             * request, since the state of the 
             * object has changed now.
             */ 
            x = random.nextInt(WIDTH);
            y = random.nextInt(HEIGHT);
            contentPane.setValues(x, y, messages[counter]);
            counter++;
        }
    };

    public DrawStringWithTimer()
    {
        counter = 0;
        x = y = 10;
        random = new Random();
    }

    private void displayGUI()
    {
        JFrame frame = new JFrame("Drawing String Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        contentPane = new CustomPanel(WIDTH, HEIGHT);

        frame.setContentPane(contentPane);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
        /*
         * javax.swing.Timer is what you need
         * when dealing with Timer related
         * task when using Swing.
         * For more info visit the link
         * as specified by @trashgod, in the
         * comments.
         * Two arguments to the constructor
         * specify as the delay and the 
         * ActionListener associated with 
         * this Timer Object.
         */
        timer = new Timer(2000, timerAction);
        timer.start();
    }

    public static void main(String... args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            @Override
            public void run()
            {
                new DrawStringWithTimer().displayGUI();
            }
        });
    }
}

class CustomPanel extends JPanel
{
    private final int GAP = 10;
    private int width;
    private int height;
    private int x;
    private int y;
    private String message = "";

    public CustomPanel(int w, int h)
    {
        width = w;
        height = h;

        setOpaque(true);
        setBackground(Color.WHITE);
        setBorder(BorderFactory.createEmptyBorder(GAP, GAP, GAP, GAP));
    }

    public void setValues(int x, int y, String msg)
    {
        this.x = x;
        this.y = y;
        message = msg;

        /*
         * As the state of the variables will change,
         * repaint() will call the paintComponent()
         * method indirectly by Swing itself. 
         */
        repaint();
    }

    @Override
    public Dimension getPreferredSize()
    {
        return (new Dimension(width, height));
    }

    @Override
    protected void paintComponent(Graphics g)
    {
        /*
         * Below line is used to draw the JPanel
         * in the usual Java way first.
         */
        super.paintComponent(g);
        /*
         * This line is used to draw the dynamic
         * String at the given location.
         */
        g.drawString(message, x, y);
    }
}

回答2:

Whatever you want to perfom i.e. drawing or smwhat, just define task and implement the code inside it.

import javax.swing.*;
import java.awt.*;
import java.util.Timer;
import java.util.TimerTask;

public class TimerApplet extends JApplet {

  String someText;
  int count = 0;

  public TimerApplet() {
    Timer time = new Timer();
    Сalculate calculate = new Сalculate();
    time.schedule(calculate, 1 * 1000, 1000);
  }

  class Сalculate extends TimerTask {

    @Override
    public void run() {
      count++;
      System.out.println("working.. "+count);
      someText = "Display some text with changing variables here.." +count;
      repaint();

    }
  }

  //This is how do you actually code.
  @Override
  public void paint(Graphics g)//Paint method to display our message
  {
//    super.paint(g);   flickering
    Graphics2D g2d = (Graphics2D)g;
    g2d.setColor(Color.WHITE);
    g2d.fillRect(0, 0, getWidth(), getHeight());
    if (someText != null) {
      g2d.setColor(Color.BLACK);
      g2d.drawString(someText,10,10);
    }

    //.....
  }
}

回答3:

I've figured out the same problem at my small app, I know my solution is not the best, but it's simple. just use two self-changing parameters in the custom inner class. here is my code.

final static Random random = new Random();
 ....//other codes
static class MyTimerTask extends TimerTask {

        int index_x = random.nextInt(50);
        int index_y = random.nextInt(50);

        @Override
        public void run() {
            System.out.println("Display some text with changing variables here: " + index_x + "," + index_y);
            index_x = random.nextInt(50);
            index_y = random.nextInt(50);
            System.gc();
        }

        public static MyTimerTask getInstance() {
            return new MyTimerTask();
        }

    }

回答4:

Example as you expected in this link

Timer will be running for ever in our application untill application is closed or When there is no more job will be available to assign or Schedule.

TimerTask -- this is the task which has some functionality which is to be run based on time or duration.

In Timer we will assgin TimerTask to run for particular duration or to start its run at particular duration.

Please Understand how it works then apply with applet or any others

1,The GCTask class extends the TimerTask class and implements the run() method.

2,Within the TimerDemo program, a Timer object and a GCTask object are instantiated.

3, Using the Timer object, the task object is scheduled using the schedule() method of the Timer class to execute after a 5-second delay and then continue to execute every 5 seconds.

4,The infinite while loop within main() instantiates objects of type SimpleObject (whose definition follows) that are immediately available for garbage collection.

import java.util.TimerTask;

public class GCTask extends TimerTask
{
    public void run()
    {
        System.out.println("Running the scheduled task...");
        System.gc();
    }
}

import java.util.Timer;
public class TimerDemo
{
    public static void main(String [] args)
    {
        Timer timer = new Timer();
        GCTask task = new GCTask();
        timer.schedule(task, 5000, 5000);
        int counter = 1;
        while(true)
        {
            new SimpleObject("Object" + counter++);
            try
            {
                Thread.sleep(500);
            }
            catch(InterruptedException e) {}
        }
    }
}

public class SimpleObject
{
    private String name;
    public SimpleObject(String n)
    {
        System.out.println("Instantiating " + n);
        name = n;
    }
    public void finalize()
    {
        System.out.println("*** " + name + " is getting garbage collected ***");
    }
}