设置动画球的边界 [英] Setting boundaries of animated balls

问题描述

我有一个简单的类，可以创建一个框架，其中带有一些从侧面反弹的球。出于某种原因，这些球会从框架的北，西和东侧弹起，但在弹起之前会略微越过南侧。设置边界时，我会考虑球的大小，这在x轴上效果很好，但在y轴上效果很好。

I have a simple class that creates a frame with some balls in it that bounce off the sides. For some reason the balls bounce fine off of the north, west, and east sides of the frame but go slightly past the south side before bouncing off of it. I take the size of the ball into account when setting the boundaries and this works fine on the x axis but not the y.

import javax.swing.*;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.Shape;
import java.awt.event.*;
import java.awt.geom.Ellipse2D;
import java.util.ArrayList;

public class BallBounceFrame
{

public static void main(String[] args)
{
    new BallBounceFrame();
}

JFrame frame;   
static int WIDTH = 500;
static int HEIGHT = 500;

public BallBounceFrame()
{   
    frame = new JFrame("Ball Bounce Frame");
    frame.setSize(WIDTH, HEIGHT);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLocationRelativeTo(null);
    frame.setResizable(false);
    BallCanvas c = new BallCanvas(5);
    frame.add(c, BorderLayout.CENTER);
    frame.setVisible(true); 
    c.animate();
}

class BallCanvas extends JPanel
{

    private static final long serialVersionUID = 1L;

    ArrayList<Ball> balls = new ArrayList<Ball>();

    public BallCanvas(int ballNum)
    {
        for(int i = 0; i < ballNum; i++)
        {
            balls.add(new Ball(20));
        }
    }

    public void paint(Graphics g)
    {
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setColor(Color.RED);
        for(Ball b : balls)
        {
            b.move();
            g2.fill(b);
        }
    }

    public void animate()
    {
        while(true)
        {
            try
            {
                frame.repaint();
                Thread.sleep(10);
            }
            catch(Exception e)
            {
                System.out.println(e);
            }
        }
    }
}

class Ball extends Ellipse2D.Float
{
    private int xVel, yVel;
    private int size;
    private int WIDTH = BallBounceFrame.WIDTH;
    private int HEIGHT = BallBounceFrame.HEIGHT;

    public Ball(int size)
    {
        super((int) (Math.random() * (BallBounceFrame.WIDTH - 20) + 1), (int) (Math.random() * (BallBounceFrame.HEIGHT - 20) + 1), size, size);
        this.size = size;
        this.xVel = (int) (Math.random() * 5 + 1);
        this.yVel = (int) (Math.random() * 5 + 1);          
    }

    public void move()
    {
        if(super.x < 0 || super.x > WIDTH - size) xVel *= -1;
        if(super.y < 0 || super.y > HEIGHT - size ) yVel *= -1;
        super.x += xVel;
        super.y += yVel;
    }
}
}

推荐答案

问题是WIDTH和HEIGHT来自JFrame。特别是窗口标题的标题会降低面板的高度。可以将面板的边界/大小传递给球的移动。

The problem is that WIDTH and HEIGHT are from the JFrame. Especially the window title caption decreases the panel's height. One could pass the panel boundary/size to the ball's move.

@Override
public void paint(Graphics g)
{
    ...
        b.move(getSize());
    ...
}

public void move(Dimension panelSize)
{
    if (x < 0 || x > panelSize.getWidth() - size) xVel *= -1;
    if (y < 0 || y > panelSize.getHeight - size) yVel *= -1;
    x += xVel;
    y += yVel;
}

要使球保持在范围之内，您可以考虑：

To keep the ball inside the bounds you might consider:

public void move(Dimension panelSize)
{
    x += xVel;
    y += yVel;
    if (x < 0) {
        x *= -1;
        xVel *= -1;
    } else if (x > panelSize.getWidth() - size) {
        x -= 2 * (x - panelSize.getWidth() - size);
        xVel *= -1;
    }
    if (y < 0) {
        y *= -1;
        yVel *= -1;
    } else if (y > panelSize.getHeight() - size) {
        y -= 2 * (y - panelSize.getHeight() - size);
        yVel *= -1;
    }
}

根据

    \ |         (xVel == 5)
     \|
     /|\
    / | \
   /  |  \

（通常也称为马蹄莲 pack（）在 BallBounceFrame 的末尾进行布局计算。）

(One normally also calla pack() at the end of the BallBounceFrame to do the layout calculation.)

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