如何在Java中使用KeyListener移动多边形对象

Question

我正在作为学习项目来开发2D游戏，但遇到了麻烦。我无法弄清楚如何使用JPanel（添加到JFrame）中的KeyListener移动Polygon对象。我已经尝试过frog.translate（int
x，int y）方法，该方法不会更新位置。我也尝试过手动更改数组坐标。我的代码示例如下：

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

public class Board extends JPanel implements KeyListener {

    private Frog frog;

    public Board() {
        setBackground(Color.GREEN);
        addKeyListener(this);
        setFocusable(true);
        setFocusTraversalKeysEnabled(false);
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;

        frog = new Frog();

        // Frog graphics
        g2.setColor(Color.BLACK);
        g2.drawPolygon(frog);
        g2.setColor(new Color(0,150,15));
        g2.fillPolygon(frog);
    }

    @Override
    public void keyTyped(KeyEvent ke) {
    }

    @Override
    public void keyPressed(KeyEvent ke) {
        int c = ke.getKeyCode();
        if(c == KeyEvent.VK_LEFT){
            frog.moveFrogLeft(25);
            //frog.translate(-25,0);
        }

        if(c == KeyEvent.VK_RIGHT){
            frog.moveFrogRight(25);
            //frog.translate(25,0);
        }

        if(c == KeyEvent.VK_UP){
            frog.moveFrogUp(25);
            //frog.translate(0,-25);
        }

        if(c == KeyEvent.VK_DOWN){
            frog.moveFrogDown(25);
            //frog.translate(0,25);
        }
        repaint();
     }

     @Override
     public void keyReleased(KeyEvent ke) {
     } 
 }

///////////////////////

import java.awt.Polygon;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

public class Frog extends Polygon {

    private Integer[] xcoord;
    private Integer[] ycoord;

    public Frog(){

        xcoord = new Integer[] {5,10,10,15,15,20,
            20,30,30,35,35,40,40,
            45,45,40,40,30,30,40,
            40,45,45,40,40,35,35,
            30,30,20,20,15,15,10,
            10,5,5,10,10,20,20,
            10,10,5,5};

        ycoord = new Integer[] {10,10,5,5,20,20,
            10,10,20,20,5,5,10,10,
            15,15,25,25,30,30,35,35,
            40,40,45,45,35,35,40,40,
            35,35,45,45,40,40,35,35,
            30,30,25,25,15,15,10};

        for(int i = 0; i < xcoord.length; i++){
            this.addPoint(xcoord[i],ycoord[i]);
        }
    }

    public void moveFrogLeft(int x) {
        if(xcoord[0] - x < 0){
            //do nothing
        } else {
            for(int i = 0; i < xcoord.length; i++){
                xcoord[i] = xcoord[i] - x;
            }
        }
    }

    public void moveFrogRight(int x){
        if(xcoord[0] + x > 600){
            //do nothing
        } else {
            for(int i = 0; i < xcoord.length; i++){
                xcoord[i] = xcoord[i] + x;
            }
        }
    }

    public void moveFrogUp(int y){
        if(ycoord[0] - y < 0){
            //do nothing
        } else {
            for(int i = 0; i < ycoord.length; i++){
                ycoord[i] = ycoord[i] - y;
            }
        }
    }

    public void moveFrogDown(int y){
        if(ycoord[0] + y > 600){
            //do nothing
        } else {
            for(int i = 0; i < ycoord.length; i++){
                ycoord[i] = ycoord[i] + y;
            }
        }
    }
}

Answer 1

这段代码有一个简单的问题：

@Override
public void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D)g;

    frog = new Frog();// <-- !!!!!

    // Frog graphics
    g2.setColor(Color.BLACK);
    g2.drawPolygon(frog);
    g2.setColor(new Color(0,150,15));
    g2.fillPolygon(frog);
}

每次绘制青蛙时，标记线都会用新的实例覆盖青蛙，从而将其重置为原始点。除了明显的问题（这是导致异常行为的原因）外， 切勿
在paintComponent(...)-method中进行任何不必要的计算。任何预计算，对象生成等都应在paintComponent!!!
之外进行！