有没有更好的方法来设置JPanel图形的初始位置？） [英] Is there a better way to set an initial position for a JPanel graphic?)

查看：168 发布时间：2017/8/29 20:06:33 java swing event-handling jpanel mouseevent

本文介绍了有没有更好的方法来设置JPanel图形的初始位置？）的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

在Liang的Java编程简介（第7版）的第15章中，他介绍了一个在JPanel上制作（2-D）球的程序，并在点击放大/缩小按钮。我修改了程序，它也可以1）放大/缩小球，如果用户点击/选项+点击，2）允许你通过按一下按钮选择球的颜色，3）允许你移动通过用鼠标拖动圈子。

最后一个修改是什么让我麻烦了一段时间，因为我想把球放在一开始，然后允许用户用鼠标移动球。我想出的解决方案是使paintComponent方法仅在第一次绘制时设置相对于getWidth（）和getHeight（）的球的x坐标和y坐标。为此，我向BallCanvas类添加了一个paintCount变量，并创建了一个if语句，以便它只会在第一次执行。当我试图弄清楚如何做到这一点，我看到其他解决方案，如这里给出的：为什么我无法访问我的面板的getWidth（）和getHeight（）函数？，但我发现我的解决方案更简单。

所以问题是：我是否认为坏的编码风格？专业程序员会否嘲笑这个解决方案？还是可以吗？

更重要的是，有没有一个更好的（但也是相对简单的）这样做，不涉及设置一个计数器？

以下是相关的代码段：

BallCanvas的开始： p>

  public static class BallCanvas extends JPanel {
 
 private int radius = 20; 
私人颜色color = Color.BLACK; 
 private int ballX; 
 private int ballY; 
 private int paintCount = 0; 
 
 ...

移动方法（响应MouseDragged事件）：

  public void move（MouseEvent e）{
 
 ballX = e.getX（）半径; 
 ballY = e.getY（） -  radius; 
 repaint（）; 
 
}

paintComponent方法：

  protected void paintComponent（Graphics g）{
 
 super.paintComponent（g）; 
 g.setColor（color）; 
 if（paintCount< 1）{
 ballX = getWidth（）/ 2  -  radius; 
 ballY = getHeight（）/ 2  -  radius; 
} 
 g.fillOval（ballX，ballY，2 * radius，2 * radius）; 
 paintCount ++; 
 
}

完整程序：

  //参考：Liang的Java编程简介
 
 import javax.swing。*; 
 import java.awt。*; 
 import java.awt.event。*; 
 
 public class ControlBall extends JFrame {
 
 private JButton jbtRed = new JButton（Red）; 
 private JButton jbtGreen = new JButton（Green）; 
 private JButton jbtBlue = new JButton（Blue）; 
 private JButton jbtBlack = new JButton（Black）; 
 private BallCanvas canvas = new BallCanvas（）; 
 private JMenuBar menuBar = new JMenuBar（）; 
 private JMenu menu = new JMenu（Edit）; 
 private JMenuItem miEnlarge = new JMenuItem（Enlarge）; 
 private JMenuItem miShrink = new JMenuItem（Shrink）; 
 
 public ControlBall（）{
 
 menuBar.add（menu）; 
 menu.add（miEnlarge）; 
 menu.add（miShrink）; 
 
 JPanel panel = new JPanel（）; 
 panel.add（jbtRed）; 
 panel.add（jbtGreen）; 
 panel.add（jbtBlue）; 
 panel.add（jbtBlack）; 
 
 this.add（canvas，BorderLayout.CENTER）; 
 this.add（panel，BorderLayout.SOUTH）; 
 this.add（menuBar，BorderLayout.NORTH）; 
 
 jbtRed.addActionListener（new ActionListener（）{
 public void actionPerformed（ActionEvent e）{
 canvas.setColor（Color.RED）; 
} 
}）; 
 
 jbtGreen.addActionListener（new ActionListener（）{
 public void actionPerformed（ActionEvent e）{
 canvas.setColor（Color.GREEN）; 
} 
}）; 
 
 jbtBlue.addActionListener（new ActionListener（）{
 public void actionPerformed（ActionEvent e）{
 canvas.setColor（Color.BLUE）; 
} 
}）; 
 
 jbtBlack.addActionListener（new ActionListener（）{
 public void actionPerformed（ActionEvent e）{
 canvas.setColor（Color.BLACK）; 
} 
}）; 
 
 miEnlarge.addActionListener（new ActionListener（）{
 public void actionPerformed（ActionEvent e）{
 canvas.enlarge（）; 
} 
}） ; 
 
 miShrink.addActionListener（new ActionListener（）{
 public void actionPerformed（ActionEvent e）{
 canvas.shrink（）; 
} 
}） ; 
 
 canvas.addMouseListener（new MouseListener（）{
 public void mouseClicked（MouseEvent e）{
 canvas.changeSize（e）; 
} 
 public void mousePressed（MouseEvent e）{} 
 public void mouseReleased（MouseEvent e）{} 
 public void mouseEntered（MouseEvent e）{} 
 public void mouseExited（MouseEvent e）{} 
}）; 
 
 canvas.addMouseMotionListener（new MouseMotionAdapter（）{
 
 public void mouseDragged（MouseEvent e）{
 
 canvas.move（e）; 
 
} 
}）; 
 
} 
 
 public static void main（String [] args）{
 
 JFrame frame = new ControlBall（）; 
 frame.setTitle（ControlBall）; 
 frame.setLocationRelativeTo（null）; 
 frame.setDefaultCloseOperation（JFrame.EXIT_ON_CLOSE）; 
 frame.setSize（400，200）; 
 frame.setVisible（true）; 
 
} 
 
 public static class BallCanvas扩展JPanel {
 
 private int radius = 20; 
私人颜色color = Color.BLACK; 
 private int ballX; 
 private int ballY; 
 private int paintCount = 0; 
 
 public BallCanvas（）{
 
 System.out.println（getWidth（）++ getHeight（））; 
 
} 
 
 public BallCanvas（int initialRadius）{
 
 radius = initialRadius; 
 
} 
 
 public void setColor（Color color）{
 
 this.color = color; 
 repaint（）; 
 
} 
 
 public void changeSize（MouseEvent e）{
 
 int numClicks = e.getClickCount（）; 
 
 if（e.isAltDown（））{
 if（radius> = 6）{
 this.radius  -  = 5 * numClicks; 
} else {
 // do nothing 
} 
} else {
 
 this.radius + = 5 * numClicks; 
} 
 
 repaint（）; 
 
} 
 
 public void enlarge（）{
 
 this.radius + = 5; 
 repaint（）; 
 
} 
 
 public void shrink（）{
 
 if（radius> = 10）{
 this.radius  -  = 5 ; 
} 
 repaint（）; 
} 
 
 public void move（MouseEvent e）{
 
 ballX = e.getX（） -  radius; 
 ballY = e.getY（） -  radius; 
 repaint（）; 
 
} 
 
 protected void paintComponent（Graphics g）{
 
 super.paintComponent（g）; 
 g.setColor（color）; 
 if（paintCount< 1）{
 ballX = getWidth（）/ 2  -  radius; 
 ballY = getHeight（）/ 2  -  radius; 
} 
 g.fillOval（ballX，ballY，2 * radius，2 * radius）; 
 paintCount ++; 
 
} 
 
} 
 
}

解决方案

值得注意的几件事：

覆盖 getPreferredSize（） 以建立面板的初始几何。

调用 pack（） 和然后设置位置&

使用 Action 来封装菜单和控件共享的代码。

使用适配器。

使用初始线程正确。

看到这个 Q& A ，从几个角度审视了一个相关的例子。

< img src =https://i.stack.imgur.com/iwscr.pngalt =image>

  import javax.swing。*; 
 import java.awt。*; 
 import java.awt.event。*; 
 
 public class ControlBall extends JFrame {
 
 private JButton jbtRed = new JButton（Red）; 
 private JButton jbtGreen = new JButton（Green）; 
 private JButton jbtBlue = new JButton（Blue）; 
 private JButton jbtBlack = new JButton（Black）; 
 private BallCanvas canvas = new BallCanvas（）; 
 private JMenuBar menuBar = new JMenuBar（）; 
 private JMenu menu = new JMenu（Edit）; 
 private JMenuItem miEnlarge = new JMenuItem（Enlarge）; 
 private JMenuItem miShrink = new JMenuItem（Shrink）; 
 
 public ControlBall（）{
 
 menuBar.add（menu）; 
 menu.add（miEnlarge）; 
 menu.add（miShrink）; 
 
 JPanel panel = new JPanel（）; 
 panel.add（jbtRed）; 
 panel.add（jbtGreen）; 
 panel.add（jbtBlue）; 
 panel.add（jbtBlack）; 
 
 this.add（canvas，BorderLayout.CENTER）; 
 this.add（panel，BorderLayout.SOUTH）; 
 this.add（menuBar，BorderLayout.NORTH）; 
 
 jbtRed.addActionListener（new ActionListener（）{
 @Override 
 public void actionPerformed（ActionEvent e）{
 canvas.setColor（Color.RED）; 
} 
}）; 
 
 jbtGreen.addActionListener（new ActionListener（）{
 @Override 
 public void actionPerformed（ActionEvent e）{
 canvas.setColor（Color.GREEN）; 
} 
}）; 
 
 jbtBlue.addActionListener（new ActionListener（）{
 @Override 
 public void actionPerformed（ActionEvent e）{
 canvas.setColor（Color.BLUE）; 
} 
}）; 
 
 jbtBlack.addActionListener（new ActionListener（）{
 @Override 
 public void actionPerformed（ActionEvent e）{
 canvas.setColor（Color.BLACK）; 
} 
}）; 
 
 miEnlarge.addActionListener（new ActionListener（）{
 @Override 
 public void actionPerformed（ActionEvent e）{
 canvas.enlarge（）; 
} 
}）; 
 
 miShrink.addActionListener（new ActionListener（）{
 @Override 
 public void actionPerformed（ActionEvent e）{
 canvas.shrink（）; 
} 
}）; 
 
 canvas.addMouseListener（new MouseAdapter（）{
 @Override 
 public void mouseClicked（MouseEvent e）{
 canvas.changeSize（e）; 
 } 
 
 @Override 
 public void mouseDragged（MouseEvent e）{
 canvas.move（e）; 
} 
}）; 
} 
 
 public static void main（String [] args）{
 
 EventQueue.invokeLater（new Runnable（）{
 @Override 
 public void run（）{
 JFrame frame = new ControlBall（）; 
 frame.setTitle（ControlBall）; 
 frame.setDefaultCloseOperation（JFrame.EXIT_ON_CLOSE）; 
框架.pack（）; 
 frame.setLocationRelativeTo（null）; 
 frame.setVisible（true）; 
} 
}）; 
 
} 
 
 public static class BallCanvas extends JPanel {
 
 private static final int SIZE = 400; 
 private int radius = 20; 
私人颜色color = Color.BLACK; 
 private int ballX = SIZE / 2  -  radius; 
 private int ballY = SIZE / 2  -  radius; 
 
 public BallCanvas（）{
 System.out.println（getWidth（）++ getHeight（））; 
} 
 
 public BallCanvas（int initialRadius）{
 radius = initialRadius; 
} 
 
 public void setColor（Color color）{
 this.color = color; 
 repaint（）; 
} 
 
 public void changeSize（MouseEvent e）{
 
 int numClicks = e.getClickCount（）; 
 
 if（e.isAltDown（））{
 if（radius> = 6）{
 this.radius  -  = 5 * numClicks; 
} else {
 // do nothing 
} 
} else {
 
 this.radius + = 5 * numClicks; 
} 
 
 repaint（）; 
 
} 
 
 public void enlarge（）{
 
 this.radius + = 5; 
 repaint（）; 
 
} 
 
 public void shrink（）{
 
 if（radius> = 10）{
 this.radius  -  = 5 ; 
} 
 repaint（）; 
} 
 
 public void move（MouseEvent e）{
 
 ballX = e.getX（） -  radius; 
 ballY = e.getY（） -  radius; 
 repaint（）; 
 
} 
 
 @Override 
 protected void paintComponent（Graphics g）{
 
 super.paintComponent（g）; 
 g.setColor（color）; 
 g.fillOval（ballX，ballY，2 *半径，2 *半径）; 
 
} 
 
 @Override 
 public Dimension getPreferredSize（）{
 return new Dimension（SIZE，SIZE）; 
} 
} 
}

In Chapter 15 of Liang's Intro to Java Programming (7th ed.), he introduces a program to make a (2-D) ball on a JPanel and enlarge it upon clicking enlarge/shrink buttons. I've modified the program so that it also 1) enlarges/shrinks the ball if the user clicks/option+clicks, 2) allows you to pick the color of the ball by pressing a button, and 3) allows you to move the circle by dragging it with your mouse.

The last modification is what was giving me trouble for a while, because I wanted to center the ball at the beginning, but then allow the user to move the ball around with the mouse. The solution I came up with was to have the paintComponent method only set the x- and y-coordinates of the ball relative to getWidth() and getHeight() the first time it paints. To do that, I added a paintCount variable to the BallCanvas class and made an if statement so that it would only execute the first time around. When I was trying to figure out how to do this initially, I saw other solutions, like the ones given here: Why can't I access my panel's getWidth() and getHeight() functions? , but I find my solution much simpler.

So the question is: is what I did considered bad coding style? Would a professional programmer scoff at this solution? Or is it OK?

More importantly, is there a better (but also, relatively simple) way to do this that doesn't involve setting up a counter?

Here are the relevant bits of code:

The beginning of BallCanvas:

public static class BallCanvas extends JPanel {

    private int radius = 20;
    private Color color = Color.BLACK;
    private int ballX;
    private int ballY;
    private int paintCount = 0;

    ...

The move method (which responds to a MouseDragged event):

public void move(MouseEvent e){

        ballX = e.getX() - radius;
        ballY = e.getY() - radius;
        repaint();

}

The paintComponent method:

protected void paintComponent(Graphics g){

        super.paintComponent(g);
        g.setColor(color);
        if(paintCount < 1){
            ballX = getWidth()/2 - radius;
            ballY = getHeight()/2 - radius;
        }
        g.fillOval(ballX, ballY, 2*radius, 2*radius);
        paintCount++;

}

Full program:

// Reference: Liang's Intro to Java Programming

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

public class ControlBall extends JFrame{

    private JButton jbtRed = new JButton("Red");
    private JButton jbtGreen = new JButton("Green");
    private JButton jbtBlue = new JButton("Blue");
    private JButton jbtBlack = new JButton("Black");
    private BallCanvas canvas = new BallCanvas();
    private JMenuBar menuBar = new JMenuBar();
    private JMenu menu = new JMenu("Edit");
    private JMenuItem miEnlarge = new JMenuItem("Enlarge");
    private JMenuItem miShrink = new JMenuItem("Shrink");

    public ControlBall(){

        menuBar.add(menu);
        menu.add(miEnlarge);
        menu.add(miShrink);

        JPanel panel = new JPanel();
        panel.add(jbtRed);
        panel.add(jbtGreen);
        panel.add(jbtBlue);
        panel.add(jbtBlack);

        this.add(canvas, BorderLayout.CENTER);
        this.add(panel, BorderLayout.SOUTH);
        this.add(menuBar, BorderLayout.NORTH);

        jbtRed.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e){
                canvas.setColor(Color.RED);
            }
        });

        jbtGreen.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e){
                canvas.setColor(Color.GREEN);
            }
        });

        jbtBlue.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e){
                canvas.setColor(Color.BLUE);
            }
        });

        jbtBlack.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e){
                canvas.setColor(Color.BLACK);
            }
        });

        miEnlarge.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e){ 
                canvas.enlarge();
            }
        });

        miShrink.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e){ 
                canvas.shrink();
            }
        });

        canvas.addMouseListener(new MouseListener() {
            public void mouseClicked(MouseEvent e){
                canvas.changeSize(e);
            }
            public void mousePressed(MouseEvent e){}
            public void mouseReleased(MouseEvent e){}
            public void mouseEntered(MouseEvent e){}
            public void mouseExited(MouseEvent e){}
        });

        canvas.addMouseMotionListener(new MouseMotionAdapter() {

            public void mouseDragged(MouseEvent e) {

                canvas.move(e);

            }
        });

    }

    public static void main(String[] args){

        JFrame frame = new ControlBall();
        frame.setTitle("ControlBall");
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 200);
        frame.setVisible(true);

    }

    public static class BallCanvas extends JPanel {

        private int radius = 20;
        private Color color = Color.BLACK;
        private int ballX;
        private int ballY;
        private int paintCount = 0;

        public BallCanvas(){

            System.out.println(getWidth() + " " + getHeight());

        }

        public BallCanvas(int initialRadius){

            radius = initialRadius;

        }

        public void setColor(Color color){

            this.color = color;
            repaint();

        }

        public void changeSize(MouseEvent e){

            int numClicks = e.getClickCount();

            if(e.isAltDown()){
                if(radius >= 6){
                    this.radius -= 5*numClicks;
                } else{
                    // do nothing
                }
            } else{

                this.radius += 5*numClicks;
            }

            repaint();

        }

        public void enlarge(){

            this.radius += 5;
            repaint();

        }

        public void shrink(){

            if(radius >= 10){
                this.radius -= 5;
            }
            repaint();
        } 

        public void move(MouseEvent e){

            ballX = e.getX() - radius;
            ballY = e.getY() - radius;
            repaint();

        }

        protected void paintComponent(Graphics g){

            super.paintComponent(g);
            g.setColor(color);
            if(paintCount < 1){
                ballX = getWidth()/2 - radius;
                ballY = getHeight()/2 - radius;
            }
            g.fillOval(ballX, ballY, 2*radius, 2*radius);
            paintCount++;

        }

    }

}

解决方案

Several things merit attention:

Override getPreferredSize() to establish the panel's initial geometry.
Use that geometry to establish the ball's initial position.
Invoke pack() and then set the location & visibility.
Use Action to encapsulate code shared by menus and controls.
Use adapters consistently.
Use initial threads correctly.
See this Q&A, which examines a related example from several perspectives.

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

public class ControlBall extends JFrame {

    private JButton jbtRed = new JButton("Red");
    private JButton jbtGreen = new JButton("Green");
    private JButton jbtBlue = new JButton("Blue");
    private JButton jbtBlack = new JButton("Black");
    private BallCanvas canvas = new BallCanvas();
    private JMenuBar menuBar = new JMenuBar();
    private JMenu menu = new JMenu("Edit");
    private JMenuItem miEnlarge = new JMenuItem("Enlarge");
    private JMenuItem miShrink = new JMenuItem("Shrink");

    public ControlBall() {

        menuBar.add(menu);
        menu.add(miEnlarge);
        menu.add(miShrink);

        JPanel panel = new JPanel();
        panel.add(jbtRed);
        panel.add(jbtGreen);
        panel.add(jbtBlue);
        panel.add(jbtBlack);

        this.add(canvas, BorderLayout.CENTER);
        this.add(panel, BorderLayout.SOUTH);
        this.add(menuBar, BorderLayout.NORTH);

        jbtRed.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                canvas.setColor(Color.RED);
            }
        });

        jbtGreen.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                canvas.setColor(Color.GREEN);
            }
        });

        jbtBlue.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                canvas.setColor(Color.BLUE);
            }
        });

        jbtBlack.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                canvas.setColor(Color.BLACK);
            }
        });

        miEnlarge.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                canvas.enlarge();
            }
        });

        miShrink.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                canvas.shrink();
            }
        });

        canvas.addMouseListener(new MouseAdapter() {
            @Override
            public void mouseClicked(MouseEvent e) {
                canvas.changeSize(e);
            }

            @Override
            public void mouseDragged(MouseEvent e) {
                canvas.move(e);
            }
        });
    }

    public static void main(String[] args) {

        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new ControlBall();
                frame.setTitle("ControlBall");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });

    }

    public static class BallCanvas extends JPanel {

        private static final int SIZE = 400;
        private int radius = 20;
        private Color color = Color.BLACK;
        private int ballX = SIZE / 2 - radius;
        private int ballY = SIZE / 2 - radius;

        public BallCanvas() {
            System.out.println(getWidth() + " " + getHeight());
        }

        public BallCanvas(int initialRadius) {
            radius = initialRadius;
        }

        public void setColor(Color color) {
            this.color = color;
            repaint();
        }

        public void changeSize(MouseEvent e) {

            int numClicks = e.getClickCount();

            if (e.isAltDown()) {
                if (radius >= 6) {
                    this.radius -= 5 * numClicks;
                } else {
                    // do nothing
                }
            } else {

                this.radius += 5 * numClicks;
            }

            repaint();

        }

        public void enlarge() {

            this.radius += 5;
            repaint();

        }

        public void shrink() {

            if (radius >= 10) {
                this.radius -= 5;
            }
            repaint();
        }

        public void move(MouseEvent e) {

            ballX = e.getX() - radius;
            ballY = e.getY() - radius;
            repaint();

        }

        @Override
        protected void paintComponent(Graphics g) {

            super.paintComponent(g);
            g.setColor(color);
            g.fillOval(ballX, ballY, 2 * radius, 2 * radius);

        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(SIZE, SIZE);
        }
    }
}

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有没有更好的方法来设置JPanel图形的初始位置？） [英] Is there a better way to set an initial position for a JPanel graphic?)

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