如何在一个JPanel上绘制多个相同类的对象？

Question

I am a university student and I'm having trouble with my assignment. Normally, I would just go to the lab time and ask the TA, but he's been sick all week so we haven't had any lab time and this assignment is due on Monday!

the specific problem I have is related to creating a java application that displays a frame with a button allowing the user to create a ball that starts bouncing on the screen and rebounds off the frame boundaries.

One of the exercises from a previous assignment was to create a similar program but that when run, would immediately display ONE ball bouncing. (which I got to work) Now, we have to modify our code and incorporate the button which allows us to create multiple balls.

At first, I thought this would be an easy modification, but now I am getting confused about how to actually instantiated the Ball objects. My thought process is that I first have to make the ReboundPanel and button panel appear (which works), and then whenever the user presses the button, a new Ball object is instantiated and displayed onto the ReboundPanel. (which does not work at present)

thanks to all for the help!

Main program:

import java.awt.*;

public class Rebound {

public static void main(String[] args) {

    JFrame frame = new JFrame ("Rebound");
    frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().setLayout(new BoxLayout(frame.getContentPane(), BoxLayout.Y_AXIS));

    JPanel reboundPanel = new ReboundPanel();
    JPanel buttonPanel = new ButtonPanel();
    frame.getContentPane().add(reboundPanel);
    frame.getContentPane().add(buttonPanel);
    frame.pack();
    frame.setVisible(true);
}
}

Panel where ball should appear:

import java.awt.*;

public class ReboundPanel extends JPanel {

private final int WIDTH = 400, HEIGHT = 300;

public ReboundPanel() {

    setPreferredSize (new Dimension(WIDTH, HEIGHT));
    setBackground (Color.black);

}
}

Button Panel:

import java.awt.*;

public class ButtonPanel extends JPanel {

private final int WIDTH = 400, HEIGHT = 35;

public ButtonPanel() {

    setPreferredSize (new Dimension(WIDTH, HEIGHT));
    setBackground (Color.GRAY);

    JButton button = new JButton("New ball");
    add(button);

    button.addActionListener(new ActionListener() {

        public void actionPerformed(ActionEvent event) {

            new Ball(); 

        }
    });
}
}

Ball Class:

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

public class Ball extends JPanel {

private final int DELAY = 20, IMAGE_SIZE = 35;

private ImageIcon image;
private Timer timer;
private int x, y, moveX, moveY;


public Ball() {

    timer = new Timer(DELAY, new ReboundListener());
    image = new ImageIcon ("/src/pa1/images/earth.gif");
    x = 0;
    y = 40;
    moveX = moveY = 3;
    draw(null);
    timer.start();
}

public void draw(Graphics page) {

    super.paintComponent (page);
    image.paintIcon (new ReboundPanel(), page, x, y);
}

private class ReboundListener implements ActionListener {

    public void actionPerformed (ActionEvent event) {

        x += moveX;
        y += moveY;

        if (x <= 0 || x >= WIDTH-IMAGE_SIZE)
            moveX = moveX * -1;

        if (y <= 0 || x >= WIDTH-IMAGE_SIZE)
            moveY = moveY * -1;

        repaint();
    }
}
}

Answer 1

Think about the relationship of the elements of your application and let that help form your class and object design.

Description:

The application will have a window that contains a button and a container area to hold 0 or more balls. When the button is clicked, a new ball should be added to the container. A ball should move around its container at a fixed speed and bounce off the boundaries of the container.

Design:

This description tells us a lot about how we could structure our code. We have some nouns: (application), window, button, container, boundaries and ball. We have some verbs: (have, contains, hold, add), move[ball], bounce[ball], click[button].

The nouns hint at possible classes to implement. And the verbs at possible methods to be implemented in the associated classes.

Let's create a class to represent the window and call it Rebound, a class representing the container called BallPanel and a class representing a ball called Ball. In this context the window and the application can be considered to be the same. It turns out that the button can be implemented neatly without creating a separate class for it. And the boundaries are simple enough to be represented by integers.

Above I have just explained one approach to help clarify the problem and below I will provide one possible implementation. These are intended to offer some instructional hints to help your understanding. There are many ways you could analyse this problem or implement the solution, I hope you find these helpful.

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.ComponentAdapter;
import java.awt.event.ComponentEvent;
import java.util.ArrayList;
import java.util.List;

import javax.swing.AbstractAction;
import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class Rebound extends JFrame {
    /* Milliseconds between each time balls move */
    static final int MOVE_DELAY = 20;

    /* The JButton for adding a new ball. An AbstractAction
     * provides a neat way to specify the label and on-click
     * code for the button inline */
    JButton addBallButton = new JButton(new AbstractAction("Add ball") {
        public void actionPerformed(ActionEvent e) {
            ballContainer.addBall();
        }
    });

    /* The Panel for holding the balls. It will need to
     * keep tracks of each ball, so we'll make it a subclass
     * of JPanel with extra code for the ball management (see
     * the definition, after the end of the Rebound class) */
    BallPanel ballContainer = new BallPanel();

    public Rebound() {
        super("Rebound");
        setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
        getContentPane().setLayout(new BoxLayout(getContentPane(), BoxLayout.Y_AXIS));

        /* There was no neat way to specify the button size
         * when we declared it, so let's do that now */
        addBallButton.setPreferredSize(new Dimension(400, 35));

        /* Add the components to this window */
        getContentPane().add(addBallButton);
        getContentPane().add(ballContainer);

        pack();

        /* Create a timer that will send an ActionEvent
         * to our BallPanel every MOVE_DELAY milliseconds */
        new Timer(MOVE_DELAY, ballContainer).start();
    }

    /* The entry point for our program */
    public static void main(String[] args) {
        /* We use this utility to ensure that code
         * relating to Swing components is executed
         * on the correct thread (the Swing event 
         * dispatcher thread) */
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new Rebound().setVisible(true);
            }
        });
    }
}

/* Our subclass of JPanel that also manages a list of
 * balls. It implements ActionListener so that it can
 * act on the Timer event we set up in the Rebound class */
class BallPanel extends JPanel implements ActionListener {
    /* An automatically expanding list structure that can
     * contain 0 or more Ball objects. We'll create a Ball
     * class to manage the position, movement and draw code
     * for each ball. */
    List<Ball> balls = new ArrayList<Ball>();
    /* Let's add some code that will be run
     * when the panel is resized (which will happen
     * if its window is resized.) We need to make sure
     * that each Ball is told about the new bounds
     * of the component, so it knows that the place
     * where it should bounce has changed */
    public BallPanel() {
        super();
        setPreferredSize(new Dimension(400,300));
        addComponentListener(new ComponentAdapter() {
            public void componentResized(ComponentEvent e) {
                if (BallPanel.this == e.getComponent()) {
                    for (Ball ball : balls) {
                        ball.setBounds(getWidth(), getHeight());
                    }
                }
            }
        });
    }

    /* This method is part of the JPanel class we are subclassing.
     * Here we change the implementation of the method, ensuring
     * we call the original implementation so that we are only
     * adding to what it does. */
    public void paintComponent(Graphics g) {
        /* Call the original implementation of this method */
        super.paintComponent(g);

        /* Lets draw a black border around the bounds of the component
         * to make it clear where the balls should rebound from */
        g.drawRect(0,0,getWidth(),getHeight());

        /* Now lets draw all the balls we currently have stored in
         * our list. */
        for (Ball ball : balls) {
            ball.draw(g);
        }
    }
    /* This method will add a new Ball into our list. Remember
     * from earlier that we call this when our button is clicked. */
    public void addBall() { 
        balls.add(new Ball(this,10,10,getWidth(),getHeight())); 
    }
    /* This method will receive the event from Timer we set up in
     * the Rebound class. We want it to cause all the ball to
     * move to their next position. */
    public void actionPerformed(ActionEvent e) {
        for(Ball ball : balls) {
            ball.move();
        }
        /* Request that Swing repaints this JPanel. This should
         * cause the paintComponent() method we implemented
         * above to be called soon after. */
        repaint();
    }
}
/* This is our class for keeping track of an individual ball
 * and it's position, movement and how it is drawn. */
class Ball {
    /* Let's say all balls will have the same diameter of 35.
     * The static modifier says that this is a value
     * that is shared by all instances of Ball. */ 
    static final int SIZE = 35;
    /* Let's say all balls will have a speed in both the X and Y
     * axes of 3. The static modifier says that this is a value
     * that is shared by all instances of Ball. */ 
    static final int SPEED = 3;
    /* Each ball needs to know its position, which we will store
     * as x and y coordinates in 2D space */
    int x, y; 
    /* Each ball needs to know the bounds in which it lives, so
     * it knows when to bounce. We'll be assuming the minimum
     * bound is 0,0 in 2D space. The maximum bound will be
     * maxX,mayY in 2D space. We could have made these static
     * and shared by all balls, but that means we would have
     * to remember to change them to not be static if in the
     * future we wanted Ball to be used on more than one JPanel.
     * If we didn't remember, then we'd see some buggy behaviour. */
    int maxX, maxY;
    /* Each ball needs to know its current speed in the X and Y 
     * directions. We can use positive and negative values to
     * keep track of the direction of the ball's movement. */
    int speedX = SPEED, speedY = SPEED;
    /* Each ball needs to know which panel it is being drawn to
     * (this is needed by ImageIcon#drawImage()). */
    JPanel panel;
    public Ball(JPanel panel, int x, int y, int maxX, int maxY) { 
        this.x = x; this.y = y;
        this.maxX = maxX; this.maxY = maxY;
        this.panel = panel;
    }
    public void setBounds(int maxX, int maxY) {
        this.maxX = maxX; this.maxY = maxY;
    }
    /* This method updates the position of this ball, using
     * the current speed and bounds to work out what the new
     * position should be.
     * This should be called by our BallPanel#actionPerformed()
     * method in response to the Timer we set up in the Rebound
     * class. */
    public void move() {
        x += speedX;
        y += speedY;
        // Approx bounce, okay for small speed
        if (x<0) { speedX=-speedX; x=0; }
        if (y<0) { speedY=-speedY; y=0; }
        if (x+SIZE>maxX) { speedX=-speedX; x=maxX-SIZE; }
        if (y+SIZE>maxY) { speedY=-speedY; y=maxY-SIZE; }
    }
    /* This method is responsible for drawing this ball on
     * the provided graphics context (which should come from
     * the JPanel associated with the ball). We also have
     * the panel, should we need it (ImageIcon#drawImage() needs 
     * this, but Graphics#drawOval() does not.)
     */
    public void draw(Graphics g) {
        //image.paintIcon(panel, g, x, y); - commented out because I don't have an ImageIcon
        g.drawOval(x, y, SIZE, SIZE);
    }
}

Answer 2

I made the ReboundPanel responsible for telling the balls to move and paint – it has a timer and an ArrayList of all the balls. I marked the changes to Rebound.java and ButtonPanel.java. The other two changed so much there wasn't any point.

Rebound.java

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

public class Rebound {

public static void main(String[] args) {

    JFrame frame = new JFrame ("Rebound");
    frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().setLayout(new BoxLayout(frame.getContentPane(), BoxLayout.Y_AXIS));

    ReboundPanel reboundPanel = new ReboundPanel(); /****/
    JPanel buttonPanel = new ButtonPanel(reboundPanel); /****/
    frame.getContentPane().add(reboundPanel);
    frame.getContentPane().add(buttonPanel);
    frame.pack();
    frame.setVisible(true);
}
}

ReboundPanel.java

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

public class ReboundPanel extends JPanel {

private final int WIDTH = 400, HEIGHT = 300;
private final int DELAY = 20;
private ArrayList<Ball> balls;
private Timer timer;

public ReboundPanel() {

    balls = new ArrayList<Ball>();
    timer = new Timer(DELAY, new ReboundListener());
    setPreferredSize (new Dimension(WIDTH, HEIGHT));
    setBackground (Color.black);
    timer.start();

}

public void addBall(Ball b) {

    balls.add(b);
}

protected void paintComponent(Graphics g) {

    super.paintComponent(g);

    for (Ball b : balls) {

        b.paint(g);
    }
}

private class ReboundListener implements ActionListener {

    public void actionPerformed (ActionEvent event) {

        for (Ball b : balls) {

            b.move(getWidth(), getHeight());
        }
        repaint();
    }
}
}

ButtonPanel.java

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

public class ButtonPanel extends JPanel {

private final int WIDTH = 400, HEIGHT = 35;

public ButtonPanel(final ReboundPanel panel) { /****/

    setPreferredSize (new Dimension(WIDTH, HEIGHT));
    setBackground (Color.GRAY);

    JButton button = new JButton("New ball");
    add(button);

    button.addActionListener(new ActionListener() {

        public void actionPerformed(ActionEvent event) {

            panel.addBall(new Ball()); /****/

        }
    });
}
}

Ball.java

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

public class Ball {

private final int IMAGE_SIZE = 15;
private int x, y, moveX, moveY;
private ImageIcon image;

public Ball() {

    x = 0;
    y = 40;
    moveX = moveY = 3;
    image = new ImageIcon("/src/pa1/images/earth.gif");
}

public void move(int width, int height) {

    x += moveX;
    y += moveY;

    if (x <= 0 || x >= width - IMAGE_SIZE)
        moveX = moveX * -1;

    if (y <= 0 || y >= height - IMAGE_SIZE)
        moveY = moveY * -1;
}
public void paint(Graphics g) {

    image.paintIcon(null, g, x, y);
}
}