如何在随机坐标上绘制一个随机大小的圆，使圆完全可见？

Question

代码：

Random rand = new Random();
JPanel mainPanel;

int randomSize = 0; 
int randomPositionX = 0;
int randomPositionY = 0;

final static int FRAME_HEIGHT = 500;
final static int FRAME_WIDTH  = 500;

final static int TITLE_BAR    = 30 ;

final static int MAX_SIZE     = 100;
final static int MIN_SIZE     = 10 ;

/* All the below code is put into a method */

mainPanel = new JPanel(){   
    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.RED);
        g.fillOval(randomPositionY, randomPositionX, randomSize, randomSize);
    }
};

do{
    randomSize = rand.nextInt(MAX_SIZE) + 1;
}while(randomSize < MIN_SIZE);

do{
    randomPositionX = rand.nextInt(FRAME_WIDTH);
    randomPositionY = rand.nextInt(FRAME_HEIGHT);
}while((randomPositionX + randomSize > FRAME_WIDTH) || (randomPositionY + randomSize > FRAME_HEIGHT - TITLE_BAR));

repaint();

我想要的是圆的大小是随机的，最小为 10，最大为 100。圆也应该在随机坐标上绘制，这样圆是 在 JPanel mainPanel.

中完全可见

请注意，mainPanel 将添加到使用 setSize(FRAME_WIDTH, FRAME_HEIGHT); 设置大小的 JFrame。

但问题是有时候，圆的一部分在 JPanel 的外面一半里面：

我哪里错了？

Answer 1

当框架包含可变的高标题栏和框架插图时，您在计算圆的位置时尝试使用框架大小
您尝试使用不相关的框架大小，它是您应该使用的组件大小，因为它在您要绘制的 mainPanel 的坐标上下文中

So, how do I fix the issues?

丢弃所有帧 "padding"/"offsets"。 mainPanel 有它自己的坐标上下文（它的左上角将是 0x0）并且它将在它的 parent 的坐标上下文中，所以你不需要关心框架或其插图。

简化您的计算，例如...

int randomSize = MIN_SIZE + (rand.nextInt(MAX_SIZE) + 1);

int randomPositionX = rand.nextInt(getWidth() - randomSize);
int randomPositionY = rand.nextInt(getHeight() - randomSize);

应该允许您生成不会落在可视区域之外的结果，因为尺寸的最大范围是容器的当前大小减去所需大小，请注意，如果可用大小更小然后 MAX_SIZE，你会遇到问题 ;)

示例每秒生成一个新圆圈

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Test {

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

    public Test() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public static class TestPane extends JPanel {

        public final static int MAX_SIZE = 100;
        public final static int MIN_SIZE = 10;
        private Rectangle bounds;
        private Random rand;

        public TestPane() {
            rand = new Random();
            Timer timer = new Timer(1000, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {

                    int randomSize = MIN_SIZE + (rand.nextInt(MAX_SIZE) + 1);

                    int randomPositionX = rand.nextInt(getWidth() - randomSize);
                    int randomPositionY = rand.nextInt(getHeight() - randomSize);

                    bounds = new Rectangle(randomPositionX, randomPositionY, randomSize, randomSize);
                    repaint();
                }
            });
            timer.start();
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            if (bounds != null) {
                g2d.setColor(Color.RED);
                g2d.fillOval(bounds.x, bounds.y, bounds.width, bounds.height);
            }
            g2d.dispose();
        }

    }

}

如何在随机坐标上绘制一个随机大小的圆，使圆完全可见？

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