Is there a better way to set an initial position f

2020-04-16 17:39发布

站内文章 / Java
52 0 0

问题:

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++;

        }

    }

}

回答1:

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);
        }
    }
}


标签： java swing event-handling jpanel mouseevent
举报

收藏的人(0)

Ta的文章 更多文章
登录 后发表评论
0条评论
还没有人评论过~