java - 图形未显示在可点击界面上

Question

我有一个程序，应该根据用户单击的按钮在屏幕上放置咖啡杯的图像，但是我的图像并不总是显示，而且当显示时，它也不会移动。有人可以帮我找出我的代码哪里不正确吗？谢谢!

import java.awt.*;
import java.awt.event.*;
import java.applet.*;

public class MoveIt extends Applet implements ActionListener

{
    //Declare Variables
    private Image cup;
    private Panel keyPad;
    public int top = 15;
    public int left = 15;
    private boolean foundKey;

    public void init()
    {
        cup = getImage(getDocumentBase(), "cup.gif");

        Canvas myCanvas = new Canvas();

        keyPad = new Panel();

        keysArray = new Button[5];


        //Create Buttons
        keysArray[0] = new Button("Up");
        keysArray[1] = new Button("Left");
        keysArray[2] = new Button("Right");
        keysArray[3] = new Button("Down");
        keysArray[4] = new Button("Center");

        keysArray[0].setActionCommand("Up");
        keysArray[1].setActionCommand("Left");
        keysArray[2].setActionCommand("Right");
        keysArray[3].setActionCommand("Down");
        keysArray[4].setActionCommand("Center");

        setBackground(Color.blue);

        //Sets the layout manager for the frame
        setLayout(new BorderLayout());

        //Sets the layout manager for the Panel
        keyPad.setLayout(new BorderLayout());

        //Add Buttons to keypad panel
        keyPad.add(keysArray[0], BorderLayout.NORTH);
        keyPad.add(keysArray[1], BorderLayout.WEST);
        keyPad.add(keysArray[2], BorderLayout.EAST);
        keyPad.add(keysArray[3], BorderLayout.SOUTH);
        keyPad.add(keysArray[4], BorderLayout.CENTER);

        add(myCanvas, BorderLayout.NORTH);
        add(keyPad, BorderLayout.SOUTH);

        //adds actionlistener to the buttons
        for(int i = 0; i < keysArray.length; i++)
           keysArray[i].addActionListener(this);
    }

    //puts image on the canvas
    public void paint( Graphics g )
    {
       g.drawImage( cup, left, top, this );
    }

    public void actionPerformed(ActionEvent e)
    {
        String arg = e.getActionCommand();


        if(arg == "Up") top = top - 15;
        if(arg == "Left") left = left - 15;
        if(arg == "Right") left = left + 15;
        if(arg == "Down") top = top + 15;
        if(arg == "Center")
        {
            top = 60;
            left = 125;
        }
        repaint();
    }
}

Answer 1

这有很多问题，但让我们从根本原因开始......

这个 if (arg == "Up") { 并不是 Java 中 String 比较的工作方式。相反，您应该使用 "Up".equals(arg) 或者如果您不关心大小写 "Up".equalsIgnoreCase(arg)

用户一次按下多个按钮的可能性也很小，因此，而不是

if ("Up".equals(arg)) {...}
if ("Left".equals(arg)) {...}

您应该使用 if-else 语句

if ("Up".equals(arg)) {
    //...
} else if ("Left".equals(arg)) {
    //..
} ...

已更新

我要推荐的一些内容已经由 rendon 开始讨论，所以给他 +1。

• (个人)避免使用 AWT 并使用 Swing。大约 13 年前，AWT 被 Swing 取代
• 避免直接绘制到顶层容器。这些容器的顶部通常有很多层，直接在它们上绘画可能会弄乱它们。相反，创建一个容器(如 JPanel 并在其上构建您的应用程序)。这将使其在未来更加便携和可重复使用
• 您必须调用 super.paintXxx，除非您有非常非常充分的理由不调用，并且准备好处理这些方法所做的所有工作。
• 避免“神奇”数字。相反，依赖绝对值。在本例中，是要绘制的容器的宽度和高度以及图像的大小。这将确保“center”实际上是“center”

以下示例演示了其中一些想法。基本上，我创建了一个 ControlPane Pane ，其中包含按钮和 CupPane。 CupPane 是独立的，负责绘制和定位杯子

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JApplet;
import javax.swing.JButton;
import javax.swing.JPanel;

public class MoveIt extends JApplet {

    public void init() {
        setLayout(new BorderLayout());
        add(new ControlPane());
    }

    public class CupPane extends JPanel {

        private Image cup;
        public int top = 15;
        public int left = 15;

        public CupPane() {
            cup = getImage(getDocumentBase(), "cup.png");
            setBackground(Color.BLUE);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.drawImage(cup, left, top, this);
        }

        public void moveUp() {
            top -= 15;
            repaint();
        }

        public void moveLeft() {
            left -= 15;
            repaint();
        }

        public void moveRight() {
            left += 15;
            repaint();
        }

        public void moveDown() {
            top += 15;
            repaint();
        }

        public void center() {
            int width = getWidth();
            int height = getHeight();
            top = (height - cup.getHeight(this)) / 2;
            left = (width - cup.getWidth(this)) / 2;
        }
    }

    public class ControlPane extends JPanel {

        private JPanel keyPad;
        public int top = 15;
        public int left = 15;
        private boolean foundKey;
        private JButton[] keysArray;
        private CupPane cupPane;

        public ControlPane() {
            keyPad = new JPanel();
            keysArray = new JButton[5];
            cupPane = new CupPane();

            //Create Buttons
            keysArray[0] = new JButton("Up");
            keysArray[1] = new JButton("Left");
            keysArray[2] = new JButton("Right");
            keysArray[3] = new JButton("Down");
            keysArray[4] = new JButton("Center");

            keysArray[0].setActionCommand("Up");
            keysArray[1].setActionCommand("Left");
            keysArray[2].setActionCommand("Right");
            keysArray[3].setActionCommand("Down");
            keysArray[4].setActionCommand("Center");

            setBackground(Color.blue);

            //Sets the layout manager for the frame
            setLayout(new BorderLayout());

            //Sets the layout manager for the Panel
            keyPad.setLayout(new BorderLayout());

            //Add Buttons to keypad panel
            keyPad.add(keysArray[0], BorderLayout.NORTH);
            keyPad.add(keysArray[1], BorderLayout.WEST);
            keyPad.add(keysArray[2], BorderLayout.EAST);
            keyPad.add(keysArray[3], BorderLayout.SOUTH);
            keyPad.add(keysArray[4], BorderLayout.CENTER);

            add(cupPane, BorderLayout.CENTER);
            add(keyPad, BorderLayout.SOUTH);

            ActionHandler handler = new ActionHandler();
            //adds actionlistener to the buttons
            for (int i = 0; i < keysArray.length; i++) {
                keysArray[i].addActionListener(handler);
            }

        }

        protected class ActionHandler implements ActionListener {

            @Override
            public void actionPerformed(ActionEvent e) {
                String arg = e.getActionCommand();

                if ("Up".equals(arg)) {
                    cupPane.moveUp();
                } else if ("Left".equals(arg)) {
                    cupPane.moveLeft();
                } else if ("Right".equals(arg)) {
                    cupPane.moveRight();
                } else if ("Down".equals(arg)) {
                    cupPane.moveDown();
                } else if ("Center".equals(arg)) {
                    cupPane.center();
                }
                repaint();
            }
        }
    }
}

您可能想看看Performing Custom Painting和 Painting in AWT and Swing了解更多详情