总程序:
/**
@version 1.32 2004-07-27
@author Cay Horstmann
*/import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.util.*;
import javax.swing.*;/**
Shows an animated bouncing ball.
*/
public class Bounce
{
public static void main(String[] args)
{
JFrame frame = new BounceFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
}/**
A ball that moves and bounces off the edges of a
rectangle
*/
class Ball
{
/**
Moves the ball to the next position, reversing direction
if it hits one of the edges
*/
public void move(Rectangle2D bounds)
{
x += dx;
y += dy;
if (x < bounds.getMinX())
{
x = bounds.getMinX();
dx = -dx;
}
if (x + XSIZE >= bounds.getMaxX())
{
x = bounds.getMaxX() - XSIZE;
dx = -dx;
}
if (y < bounds.getMinY())
{
y = bounds.getMinY();
dy = -dy;
}
if (y + YSIZE >= bounds.getMaxY())
{
y = bounds.getMaxY() - YSIZE;
dy = -dy;
}
} /**
Gets the shape of the ball at its current position.
*/
public Ellipse2D getShape()
{
return new Ellipse2D.Double(x, y, XSIZE, YSIZE);
} private static final int XSIZE = 15;
private static final int YSIZE = 15;
private double x = 0;
private double y = 0;
private double dx = 1;
private double dy = 1;
}/**
The panel that draws the balls.
*/
class BallPanel extends JPanel
{
/**
Add a ball to the panel.
@param b the ball to add
*/
public void add(Ball b)
{
balls.add(b);
} public void paintComponent(Graphics g)
{
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;
for (Ball b : balls)
{
g2.fill(b.getShape());
}
} private ArrayList<Ball> balls = new ArrayList<Ball>();
}/**
The frame with panel and buttons.
*/
class BounceFrame extends JFrame
{
/**
Constructs the frame with the panel for showing the
bouncing ball and Start and Close buttons
*/
public BounceFrame()
{
setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);
setTitle("Bounce"); panel = new BallPanel();
add(panel, BorderLayout.CENTER);
JPanel buttonPanel = new JPanel();
addButton(buttonPanel, "Start",
new ActionListener()
{
public void actionPerformed(ActionEvent event)
{
addBall();
}
});
addButton(buttonPanel, "Close",
new ActionListener()
{
public void actionPerformed(ActionEvent event)
{
System.exit(0);
}
});
add(buttonPanel, BorderLayout.SOUTH);
} /**
Adds a button to a container.
@param c the container
@param title the button title
@param listener the action listener for the button
*/
public void addButton(Container c, String title, ActionListener listener)
{
JButton button = new JButton(title);
c.add(button);
button.addActionListener(listener);
} /**
Adds a bouncing ball to the panel and makes
it bounce 1,000 times.
*/
public void addBall()
{
try
{
Ball ball = new Ball();
panel.add(ball); for (int i = 1; i <= STEPS; i++)
{
ball.move(panel.getBounds());
panel.paint(panel.getGraphics());
Thread.sleep(DELAY);
}
}
catch (InterruptedException e)
{
}
} private BallPanel panel;
public static final int DEFAULT_WIDTH = 450;
public static final int DEFAULT_HEIGHT = 350;
public static final int STEPS = 1000;
public static final int DELAY = 3;
}其中不明白的是移动球的代码:
public void move(Rectangle2D bounds)
{
x += dx;
y += dy;
if (x < bounds.getMinX())
{
x = bounds.getMinX();
dx = -dx;
}
if (x + XSIZE >= bounds.getMaxX())
{
x = bounds.getMaxX() - XSIZE;
dx = -dx;
}
if (y < bounds.getMinY())
{
y = bounds.getMinY();
dy = -dy;
}
if (y + YSIZE >= bounds.getMaxY())
{
y = bounds.getMaxY() - YSIZE;
dy = -dy;
}
}
这个代码的原理是什么啊?我实在想不出他是怎么实现移动球的,特别是为什么每次都dx=-dx和dy=-dy啊?
/**
@version 1.32 2004-07-27
@author Cay Horstmann
*/import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.util.*;
import javax.swing.*;/**
Shows an animated bouncing ball.
*/
public class Bounce
{
public static void main(String[] args)
{
JFrame frame = new BounceFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
}/**
A ball that moves and bounces off the edges of a
rectangle
*/
class Ball
{
/**
Moves the ball to the next position, reversing direction
if it hits one of the edges
*/
public void move(Rectangle2D bounds)
{
x += dx;
y += dy;
if (x < bounds.getMinX())
{
x = bounds.getMinX();
dx = -dx;
}
if (x + XSIZE >= bounds.getMaxX())
{
x = bounds.getMaxX() - XSIZE;
dx = -dx;
}
if (y < bounds.getMinY())
{
y = bounds.getMinY();
dy = -dy;
}
if (y + YSIZE >= bounds.getMaxY())
{
y = bounds.getMaxY() - YSIZE;
dy = -dy;
}
} /**
Gets the shape of the ball at its current position.
*/
public Ellipse2D getShape()
{
return new Ellipse2D.Double(x, y, XSIZE, YSIZE);
} private static final int XSIZE = 15;
private static final int YSIZE = 15;
private double x = 0;
private double y = 0;
private double dx = 1;
private double dy = 1;
}/**
The panel that draws the balls.
*/
class BallPanel extends JPanel
{
/**
Add a ball to the panel.
@param b the ball to add
*/
public void add(Ball b)
{
balls.add(b);
} public void paintComponent(Graphics g)
{
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;
for (Ball b : balls)
{
g2.fill(b.getShape());
}
} private ArrayList<Ball> balls = new ArrayList<Ball>();
}/**
The frame with panel and buttons.
*/
class BounceFrame extends JFrame
{
/**
Constructs the frame with the panel for showing the
bouncing ball and Start and Close buttons
*/
public BounceFrame()
{
setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);
setTitle("Bounce"); panel = new BallPanel();
add(panel, BorderLayout.CENTER);
JPanel buttonPanel = new JPanel();
addButton(buttonPanel, "Start",
new ActionListener()
{
public void actionPerformed(ActionEvent event)
{
addBall();
}
});
addButton(buttonPanel, "Close",
new ActionListener()
{
public void actionPerformed(ActionEvent event)
{
System.exit(0);
}
});
add(buttonPanel, BorderLayout.SOUTH);
} /**
Adds a button to a container.
@param c the container
@param title the button title
@param listener the action listener for the button
*/
public void addButton(Container c, String title, ActionListener listener)
{
JButton button = new JButton(title);
c.add(button);
button.addActionListener(listener);
} /**
Adds a bouncing ball to the panel and makes
it bounce 1,000 times.
*/
public void addBall()
{
try
{
Ball ball = new Ball();
panel.add(ball); for (int i = 1; i <= STEPS; i++)
{
ball.move(panel.getBounds());
panel.paint(panel.getGraphics());
Thread.sleep(DELAY);
}
}
catch (InterruptedException e)
{
}
} private BallPanel panel;
public static final int DEFAULT_WIDTH = 450;
public static final int DEFAULT_HEIGHT = 350;
public static final int STEPS = 1000;
public static final int DELAY = 3;
}其中不明白的是移动球的代码:
public void move(Rectangle2D bounds)
{
x += dx;
y += dy;
if (x < bounds.getMinX())
{
x = bounds.getMinX();
dx = -dx;
}
if (x + XSIZE >= bounds.getMaxX())
{
x = bounds.getMaxX() - XSIZE;
dx = -dx;
}
if (y < bounds.getMinY())
{
y = bounds.getMinY();
dy = -dy;
}
if (y + YSIZE >= bounds.getMaxY())
{
y = bounds.getMaxY() - YSIZE;
dy = -dy;
}
}
这个代码的原理是什么啊?我实在想不出他是怎么实现移动球的,特别是为什么每次都dx=-dx和dy=-dy啊?
每次调用move都会执行
x += dx;
y += dy;
下面的几个if, 是为了判断球是否到边上了,也就是说,只有到了边上,才会执行几个if里的内容
到边上了,移动方将就变了,才会有dx=-dx和dy=-dy不知道我说明白了没