计时器启动/停止参数

Question

I've made leaps and bounds in skill and progress since joining this community. You all are a huge help. I'm having trouble with giving a timer that I've implemented certain parameters for when it starts and stops.

I either get errors saying "the local variable timer may not have been initialized" or I get no errors, but nothing happens. Maybe I have the timer in the wrong place?

If I put timer.start(); in the constructor too everything works fine, but then the timer has to start when the program is initialized. I would really like to have the timer not start until a certain parameter is met. Say, for instance, until the int p1Laps=1; but if I place timer.start(); into an if-statement in the constructor (ie if(p1Laps>=1) { timer.start(); } the timer doesn't ever start.

I've tried placing timer.start(); in various places and have either gotten no response or generated an error about the lack of local variable timer .

A second, somewhat related problem I have is the inability to put any parameters in place to call on timer.stop(); without getting the aforementioned "local variable timer may not have been initialized" error. I've left timer.stop(); where I think it needs to be in the code, but it receives that error.

So in short, I want to be able to tell the timer to start when a parameter is met, namely when a player has completed a lap. And I want to be able to tell the timer to stop when it reaches a value.

Thanks in advance for the great advice I'm sure I'll receive. Note: this is not the whole code, just relevant information.

import javax.swing.*;
import javax.swing.event.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import java.awt.geom.*;

public class RacerDoom extends JFrame {
    int counter = 0;
    int p1Laps = 0;
public RacerDoom() {
        //create JFrame
        super("Racer Doom Squared");
        setSize(WIDTH,HEIGHT);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
        //set up Timer
        final Timer timer=new Timer(1000, new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                if(counter>=10) {
                    timer.stop(); //error here reads "local variable timer may
                                    //not have been initialized"
                }
                else{
                   counter++;
                   }
                System.out.println(counter);
            }
        });
        //inner class threads
        Move1 m1 = new Move1();
        m1.start();
        //start timer
        if(p1Laps>=1) {
            timer.start(); //error here is that timer will not start when
                            //p1Laps>=1
        }
    }
    private class Move1 extends Thread implements KeyListener {
        public void run() {
            addKeyListener(this);
            while(true) {
                try {
                    repaint();
                    //collisions
                    if(p1.intersects(finishtop)&&p1Direction==UP&&p1cross!=true){
                        p1cross=true;
                        p1Laps++;
                        p1Boost++;
                        counter=0;
                        System.out.println(p1Laps);
                    }
                    if(p1.intersects(finishtop)==false) {
                        p1cross=false;
                    }
    public static void main (String [] args) {

        new RacerDoom();
    }
}

Answer 1

As you want to start and stop the timer at different places in the code you should make it member variable. This will fix the problem where you are trying to stop the timer inside the action listener.

The variable p1Laps will not change in the constructor (after you have initialized it to 0) so you need to start the timer where you change the value of plLaps. I am not sure if it is safe to call timer.start() from another thread (Move1). So it may be safer to start timer with SwingUtilities.invokeLater().

Answer 2

Quick fix:

Rather than

timer.stop();

Do

((Timer)e.getSource()).stop();

The ActionEvent's getSource method will return a reference to the object that calls the actioPerformed method (the Timer), so this should work.

There may be other issues with your code including your background thread without a Thread.sleep(...), your use of KeyListeners rather than Key Binding, your adding a KeyListener in a background thread,...