我想在小程序中添加计时器

Question

I made CountDown.java file and try to add in my Word-trouble.java file (which is main applet) as CountDown ct = new CountDown(); but it is not showing timer in main applet.

Here is coding:

package pack.urdu;
import java.awt.*; //windows toolkit

import java.applet.*; //applet support

public class CountDown extends Applet implements Runnable{

int counter; Thread cd;

public void start() { // create thread

counter = 60; cd = new Thread(this); cd.start();

}

public void stop() { cd = null;}

public void run() {  // executed by Thread

while (counter>0 && cd!=null) {

try{Thread.sleep(1000);} catch (InterruptedException e){}

--counter; repaint(); //update screen

}

}

public void paint(Graphics g) {

g.drawString(String.valueOf(counter),25,75);

}

}

Answer 1

You are making a mistake that I see a lot of programmers make: you are mixing up the calculation of elapsed time, with the calculation of the refresh time. If the duration of sleep takes long than a second because of thread contention, your timer will drift.

Instead of tracking a counter that increments every second, just record the start time:

long startTime = System.currentTimeMillis();

Then later, your paint method becomes:

public void paint(Graphics g) {
    int elapsedSeconds = (int)(System.currentTimeMillis()-startTime)/1000
    g.drawString(String.valueOf(elapsedSeconds),25,75);
}

This method can be called as often, and as many times as you like, and it will always display the correct elapsed seconds. There is no need to increment anything at any specified time.

The only other thing you have to do is to arrange that the screen gets refreshed. (I like to say that you only have to refresh the screen when the user looks at it :-) but since we don't know that we need to refresh more often). The mechanism for this may depend upon the graphic library. One lazy idea is to refresh ten times a second and the screen will be right most of the time.

If you do want to have a thread that sends repaint events, you should have those events sent just at the time that timer clicks over to a new value, and thereby send only one per second. This is done with:

while (stillRunning) {
    long elapsedTime = System.currentTimeMillis() - startTime;
    long timeTillNextDisplayChange = 1000 - (elapsedTime % 1000);
    Thread.sleep(timeTillNextDisplayChange);
    repaint();
}

Note that you do not sleep 1000ms! If your system is performing well, this will be very close to 1000ms, but slightly less than that to account for (1) the thread startup delay, possibly caused by thread contention, and (2) the processing time for this loop (which is quite small). In any case, calculating the sleep in this way will prevent timer drift, and assure that your display updates just as the seconds value changes.

See an extended discussion of Common Misunderstandings of Timers on my website.