1

我必须使用 HoltSoft 的 Ready to Program 中的 Console 类。我不应该使用swing，所以如果我不能没有swing，请忽略这个。

//imports
import java.awt.*; 
import java.awt.event.*;
import hsa.*;

public class DrawLines extends Panel implements MouseListener, MouseMotionListener
{
    Console c;
    int startX, startY, prevX, prevY; //mouse coordinates
    private boolean dragging; //whether or not the mouse is being dragged
    MouseEvent e;
    public DrawLines ()
    {
        c = new Console (); //creates console window
        addMouseListener (this); //detects press/release
        addMouseMotionListener (this);//detects dragging
    }


    public void mousePressed (MouseEvent e)
    {
        while (!dragging)
        {
            try
            {
                startX = e.getX ();//get the
                startY = e.getY ();//original co-ordinates
                dragging = true;
            }
            catch (NullPointerException q) //because I kept getting this error
            {
            }
        }
    }


    public void mouseDragged (MouseEvent e)
    {
        while (dragging)
        {
            try
            {
                int x = e.getX (); //gets and
                int y = e.getY (); //updates
                prevX = x;         //the mouse
                prevY = y;         //coordinates
            }
            catch (NullPointerException q)//because I kept getting this error
            {
            }
        }
    }


    public void mouseReleased (MouseEvent e)
    {
        dragging = false; //stopped dragging
    }


    public void drawTheLine ()
    {
        mousePressed (e);
        mouseDragged (e);
        c.setColor (Color.black);
        c.fillOval (prevX, prevY, 50, 50); //draws a circle where the mouse is 
        mouseReleased (e);
    }


    public void mouseMoved (MouseEvent e){}
    public void mouseEntered (MouseEvent e){}
    public void mouseExited (MouseEvent e){}
    public void mouseClicked (MouseEvent e){}

    public static void main (String[] args)
    {
        DrawLines a = new DrawLines ();
        a.drawTheLine ();
    }
}

我一直在尝试在控制台中使用 MouseListener 和 MouseMotionListener。起初，程序一直给我错误，所以我添加了 try/catch 结构。现在它没有崩溃，但屏幕上什么也没有出现。为什么？帮助？

如果我不应该使用 try/catch 来忽略它，我该怎么办？

我不允许在这个程序中使用除 Console() 之外的任何东西。这是一个课程作业。

4

2 回答 2

2

看这个：

public void drawTheLine ()
{
    while (true)
    {
        mousePressed (e);
        mouseDragged (e);
        c.setColor (Color.black);
        c.fillOval (prevX, prevY, 50, 50); //draws a circle where the mouse is 
        mouseReleased (e);
    }
}

您传递的参数“e”为空。在这里声明：

public class DrawLines extends Panel 
    implements MouseListener, MouseMotionListener
{
    MouseEvent e; // IT IS NEVER SET TO ANYTHING! IT IS NULL!!!

在你的构造函数中的某个地方你应该这样做，所以它不再为空：

e = (something);
于 2013-01-03T17:19:31.467 回答
2

Swing 是一个事件驱动系统，是一个单线程系统。

这意味着您的应用程序“等待”事件发生（由事件调度线程处理）并且任何阻塞 EDT 的人，例如循环、长时间运行的进程或阻塞 IO，都会阻止您的应用程序接收这些事件的通知，使您的应用程序无法运行。

所以，如果我们看看这个...

    while (true)
    {
        mousePressed (e);
        mouseDragged (e);
        c.setColor (Color.black);
        c.fillOval (prevX, prevY, 50, 50);
        mouseReleased (e);
    }
}

它表明...一，你不了解事件是如何在 Swing 中生成的，二，EDT 是如何实际工作的。

与某些 UI 框架不同，您不需要实现事件循环，这由 Swing 处理。像这样阻塞 EDT，将阻止它处理事件

相反，请删除它，drawLineMethod因为它对您没有任何作用，并将您的 main 方法替换为...

public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        @Override
        public void run() {
            try {
                UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
            } catch (Exception ex) {
            }

            JFrame frame = new JFrame("Test");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setLayout(new BorderLayout());
            frame.add(new DrawLines());
            // I prefer pack, but you've not specified a preferred size for your panel...
            //frame.pack();
            frame.setSize(400, 400);
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
        }
    });
}

现在。我不知道这个Console类是什么或做什么，但是在你的鼠标事件方法中，你需要更新它，以便它可以更新它的输出......

更新了示例

在此处输入图像描述

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 (Exception ex) {
                }

                JFrame frame = new JFrame("Test");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new DrawPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class DrawPane extends JPanel {

        private Point center;
        private int radius;

        public DrawPane() {
            MouseAdapter handler = new MouseAdapter() {

                @Override
                public void mousePressed(MouseEvent e) {
                    center = e.getPoint();
                    radius = 0;
                    repaint();
                }

                @Override
                public void mouseDragged(MouseEvent e) {
                    int width = Math.max(e.getX(), center.x) - Math.min(e.getX(), center.x);
                    int height = Math.max(e.getY(), center.y) - Math.min(e.getY(), center.y);
                    radius = Math.max(width, height);
                    repaint();
                }

            };
            addMouseListener(handler);
            addMouseMotionListener(handler);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(400, 400);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            if (center != null) {
                g.setColor(Color.RED);
                g.fillOval(center.x - 2, center.y - 2, 4, 4);

                g.drawOval(center.x - (radius / 2), center.y - (radius / 2), radius, radius);

            }
        }
    }
}

建议你花点时间读一读...

使用纯 AWT 版本更新

正如有人向我指出的那样，OP 使用的是 AWT 而不是 Swing，为什么，因为他们似乎能够......

public class DrawCircleAWT {

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

    public DrawCircleAWT() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                Frame frame = new Frame("Testing");
                frame.addWindowListener(new WindowAdapter() {
                    @Override
                    public void windowClosing(WindowEvent e) {
                        System.exit(0);
                    }
                });
                frame.setLayout(new BorderLayout());
                frame.add(new DrawPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class DrawPane extends Panel {

        private Point center;
        private int radius;

        public DrawPane() {
            MouseAdapter handler = new MouseAdapter() {
                @Override
                public void mousePressed(MouseEvent e) {
                    center = e.getPoint();
                    radius = 0;
                    repaint();
                }

                @Override
                public void mouseDragged(MouseEvent e) {
                    int width = Math.max(e.getX(), center.x) - Math.min(e.getX(), center.x);
                    int height = Math.max(e.getY(), center.y) - Math.min(e.getY(), center.y);
                    radius = Math.max(width, height);
                    repaint();
                }
            };
            addMouseListener(handler);
            addMouseMotionListener(handler);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(400, 400);
        }

        @Override
        public void paint(Graphics g) {
            super.paint(g);

            if (center != null) {
                g.setColor(Color.RED);
                g.fillOval(center.x - 2, center.y - 2, 4, 4);

                g.drawOval(center.x - (radius / 2), center.y - (radius / 2), radius, radius);

            }
        }
    }
}
于 2013-01-03T21:31:38.723 回答