java - Properly Initializing Graphics (or maybe some other thing, I am not sure) in Java

Question

I am back with a new question about the same code as last time. I took a lot of the suggestions that were given, and changed a few things. The program compiles, but no rectangle appears with any input. People in the last thread said that the code would work with the issues fixed, but this doesn't seem to work. I don't know if it a problem with the graphics (I think someone else suggested to do what I did in the code I have now) or with the way I am accepting key inputs.

Anyways, the program below is supposed to have a rectangle drawn, and for that rectangle be moved one coordinate to the right or left depending on if you press the <- or -> key on your keyboard. However, no rectangle appears with any input.

import java.awt.*;
import java.net.*;
import java.util.*;
import java.applet.Applet;


public class game extends Applet
{
    Thread loopThread;
    boolean left  = false;
    boolean right = false;
    int platPos = 50;


    public void run() 
    { 
        Graphics g = null;

        int i, j;
        long startTime;

        if (loopThread == null) 
        {
            loopThread = new Thread();
            loopThread.start();
        }

        Thread.currentThread().setPriority(Thread.MIN_PRIORITY);
        startTime = System.currentTimeMillis();
        while(Thread.currentThread() == loopThread) 
        {
            updatePlatter(g);
        }

    }


     public void updatePlatter(Graphics g) 
    {

         if(left)
         {
             g.setColor(new Color(255,255,255)); //sets color to white
             g.fillRect(50+platPos, 200, 100, 20); //draws over whatever existing rectangle there is with white
             platPos--; 
             g.setColor(new Color(100,100,100)); //sets new color
             g.fillRect(50+platPos,200, 100,20); //draws new rectangle 
             left = false; 
         }

         if(right)
         {
             g.setColor(new Color(255,255,255));
             g.fillRect(50+platPos,200,100,20);
             platPos++;
             g.setColor(new Color(100,100,100));
             g.fillRect(50+platPos,200,100,20);
             right = false;
         }

    }

    public boolean keyDown(Event e, int key) 
    {
            if (key == Event.LEFT)
            left = true;

            if (key == Event.RIGHT)
            right = true;

            return true;
     }
}

Also I want to use applet, not JFrame. Just a personal preference. Thanks for any help! ^.^

标签： javainputgraphicsappletawt

Answer 1

首先将图形提供给您的小程序。它是一个抽象类，当您使用小程序时，Java 会为您提供正确的实现。你不能只是实例化它。有一些方法可以处理/重新绘制小程序。所以我建议使用paint方法对矩形进行初始绘制，然后在循环中调用repaint。顺便说一句，使用线程进行这样的循环是不可读的代码（而且它不起作用）。您甚至不需要 run() 方法；) 当您创建小程序时，它开始工作。

所以我们可以这样做：

import java.applet.Applet;
import java.awt.Color;
import java.awt.Event;
import java.awt.Graphics;


public class Game extends Applet
{
    boolean left  = false;
    boolean right = false;
    int platPos = 0;


    @Override
     public void paint(Graphics g) 
    {
         g.fillRect(50+platPos, 0, 100, 20); 
         System.out.println("repaint");
         if(left)
         {

             g.setColor(new Color(255,255,255)); //sets color to white
             g.fillRect(50+platPos, 0, 100, 20); //draws over whatever existing rectangle there is with white
             platPos--; 
             g.setColor(new Color(100,100,100)); //sets new color
             g.fillRect(50+platPos,0, 100,20); //draws new rectangle 
             left = false; 
         }

         if(right)
         {
             g.setColor(new Color(255,255,255));
             g.fillRect(50+platPos,200,100,20);
             platPos++;
             g.setColor(new Color(100,100,100));
             g.fillRect(50+platPos,200,100,20);
             right = false;
         }

    }
    @Override
    public boolean keyDown(Event e, int key) 
    {
             if (key == Event.LEFT) {
                left = true;
             }

            if (key == Event.RIGHT) {
                right = true;
            }

            repaint();
            return true;
     }
}