在围绕屏幕移动球的情况下，如何正确处理JComponent的按键和重绘？

Question

我以为我会尝试编写一个可以绘制球的程序，并在按下箭头键时将球沿着屏幕上的方向移动。首先，我开始尝试制作一个仅执行“向上”箭头键移动的程序。

我一直在寻找解决方案，但无法弄清楚这段代码有什么问题。我不知道这是我的输入和动作图出现问题（即程序识别按键的问题）还是JComponent和JFrame类如何在摆动中工作的问题。我以为也许这个问题也可能是重点。我真的不知道如何分辨组件何时具有焦点。我认为该键已设置为CNTRL
+ Y而不是暂时设置为上，只是因为在某些时候我认为这可能是因为我的字符串在输入映射器中指定了向上箭头。

在这一点上，我很沮丧，我只是想让该死的事情去 做某事 ，所以我使用的输入映射超出了必要。

代码如下，它很短，格式很糟糕（对不起）：

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

class BallMover
{
    public static void main(String[] args)
    {
        EventQueue.invokeLater(new Runnable()
        {
            public void run()
            {
                BallFrame frame = new BallFrame();
            }
        });

    }
}


class BallFrame extends JFrame
{
    private static final int DEFAULT_WIDTH = 500;
    private static final int DEFAULT_HEIGHT = 500;
    private BallComponent comp;

    public BallFrame()
    {
        super.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        super.setSize(this.DEFAULT_WIDTH, this.DEFAULT_HEIGHT);
        super.setResizable(false);
        super.add(new BallComponent());
        super.setVisible(true);
        super.setFocusable(true);
    }
}


class BallComponent extends JComponent
{
    private Ellipse2D.Double ellipse;
    private double x = 225;
    private double y = 225;
    private ActionPress actionPress;

    public BallComponent()
    {
        super();
        super.setFocusable(true);

        InputMap imap1 = this.getInputMap(JComponent.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT);
        imap1.put(KeyStroke.getKeyStroke("ctrl Y"), "keyUp1");
        InputMap imap2 = this.getInputMap(JComponent.WHEN_FOCUSED);
        imap1.put(KeyStroke.getKeyStroke("ctrl Y"), "keyUp2");
        InputMap imap3 = this.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW);   
        imap1.put(KeyStroke.getKeyStroke("ctrl Y"), "keyUp3");

        ActionMap amap = this.getActionMap();
        amap.put("keyUp1", actionPress);
        amap.put("keyUp2", actionPress);    
        amap.put("keyUp3", actionPress);
    }

    public void paintComponent(Graphics g)
    {
        super.repaint(); // clear component //
        Graphics2D g2d = (Graphics2D)g;
        this.ellipse = new Ellipse2D.Double(x, y, 50, 50);
        g2d.fill(this.ellipse);
    }

    private class ActionPress extends AbstractAction
    {
        public void actionPerformed(ActionEvent event)
        {
            y = y + 10;
            ellipse = new Ellipse2D.Double(x, y, 50, 50);
            repaint();
        }
    } 
}

Answer 1

似乎您从未初始化过actionPress-尝试将其添加到BallComponent构造函数中：

actionPress = new ActionPress();

即，您的构造函数将如下所示

public BallComponent()
{
    super();
    super.setFocusable(true);

    InputMap imap1 = this.getInputMap(JComponent.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT);
    imap1.put(KeyStroke.getKeyStroke("ctrl Y"), "keyUp1");
    InputMap imap2 = this.getInputMap(JComponent.WHEN_FOCUSED);
    imap1.put(KeyStroke.getKeyStroke("ctrl Y"), "keyUp2");
    InputMap imap3 = this.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW);   
    imap1.put(KeyStroke.getKeyStroke("ctrl Y"), "keyUp3");

    actionPress = new ActionPress();
    ActionMap amap = this.getActionMap();
    amap.put("keyUp1", actionPress);
    amap.put("keyUp2", actionPress);    
    amap.put("keyUp3", actionPress);
}