Question

我必须在HoltSoft的Ready to Program中使用Console类。我不应该使用挥杆，所以如果我不能挥杆，请不要这样做。

//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Console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，但屏幕上没有任何内容。为什么？帮助

如果我不应该使用try / catch来忽略它，我该怎么办？

我不允许在此程序中使用除Console（）之外的任何内容。这是课程作业。

Answer 1

看看这个：

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。它在这里声明：

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

在构造函数的某处，你应该这样做，因此它不再为null：

e = (something);

Answer 2

Swing是一个事件驱动的系统，是一个单线程系统。

这意味着您的应用程序“等待”事件发生（事件调度线程会照顾您），并且任何阻止EDT的人，如循环，长时间运行的进程或阻塞IO，都会阻止您接收这些事件的通知后，申请无法运行。

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，你不明白在Swing中如何生成事件，以及两个，EDT实际上是如何工作的。

与某些UI框架不同，您不需要实现事件循环，Swing会对此进行处理。像这样阻止EDT会阻止它处理事件

相反，请删除drawLineMethod，因为它对您来说绝对没有任何意义，并将您的主要方法替换为类似......

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Console类是什么或者做什么，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

enter image description here

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);

            }
        }
    }
}

我建议你花点时间阅读......

Creating a GUI with Swing了解Swing的整体基础知识
Performing Custom Painting了解自定义绘画如何在Swing中实际执行
Painting in AWT and Swing因为所有希望在Swing中执行自定义绘画的开发人员都需要了解其工作原理......

使用纯AWT版本进行更新

正如我所指出的那样OP是使用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);

            }
        }
    }
}

点击后为什么不显示圆圈？

2 个答案: