尽管使用了 setText 函数，但在 JTextArea 中没有显示

Question

在这里，我想展示一个类的 MouseEvents 在另一个类中的使用...通过创建前一个类的对象...

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.lang.*;
public class MyPagalpanti extends JPanel implements MouseListener
{Point a,b;
boolean drawLin,drawOvl,drawOvl2;
MyPagalpanti(){a=new Point(0,0);b=new Point(0,0);drawLin=false;drawOvl=false,drawOvl2=false;;setSize(300,300);addMouseListener(this);}
public void mouseClicked(MouseEvent m)
{if(!drawOvl){a=new Point(m.getX(),m.getY());drawOvl=true;drawOvl2=false;}
else {b=new Point(m.getX(),m.getY());drawOvl=true;drawOvl2=true;repaint();}//record("clicked",m);}
}
public void mouseEntered(MouseEvent m)
{
}
public void mousePressed(MouseEvent m)
{a=new Point(m.getX(),m.getY());repaint();}
public void mouseReleased(MouseEvent m)
{b=new Point(m.getX(),m.getY());}
public void mouseExited(MouseEvent m)
{}
public void paintComponent(Graphics g)
{if(drawLin){g.setColor(Color.yellow);g.drawLine((int)a.getX(),(int)a.getY(),(int)b.getX(),(int)b.getY());}//d=0;}
else if(drawOvl&&drawOvl2){g.setColor(Color.red);int width=(int)Math.abs(b.getX()-a.getX());
int h=(int)Math.abs(b.getY()-a.getY());
g.drawOval((int)a.getX(),(int)a.getY(),width,h);
}}
}

形成 MyPagalpanti 对象的类：

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
//import java.swing.JPanel.*;

public class Mithu extends JFrame// implements MouseListener
{JTextArea jta1,jta2;
Mithu(String title)
{
super(title);
MyPagalpanti p1=new MyPagalpanti();
jta1=new JTextArea();
jta2=new JTextArea();
p1.setOpaque(true);
jta1.setEditable(false);
jta2.setEditable(false);
setForeground(Color.blue);
p1.setPreferredSize(new Dimension(600,600));
p1.setLayout(new GridLayout(3,1,0,0));
getContentPane().add(p1);
pack();
setVisible(true);
}
void record(String s,MouseEvent e)
{jta1.setText(jta1.getText()+"\n"+s+""+e.toString());}
public void mouseClicked(MouseEvent m)
{record("clicked",m);}
public void mouseEntered(MouseEvent m)
{record("entered",m);}
public void mousePressed(MouseEvent m)
{record("pressed",m);}
public void mouseReleased(MouseEvent m)
{ record("released",m);}
public void mouseExited(MouseEvent m)
{record("exited",m);}

public static void main(String args[])
{Mithu m=new Mithu("HI ! Karamvir");
m.setVisible(true);}
}

请不要逐字逐句...我知道我的代码还有许多其他错误...因为我仍在努力...但是对于那些说“鼠标事件将只交付给从它注册到 " 的组件中的 MouseListener ，我猜你说得对，但如果我们仔细观察，我只使用了这个概念......在这段代码中......

Answer 1

要添加它 JBNizet 的答案...

MouseListeners 只会传递给可显示且已注册通知的组件。创建一个侦听鼠标事件的组件并将其添加到您的组件不会神奇地将这些事件传递给您的组件。

然后，最重要的是，您将两个组件添加到由BorderLayout 管理的组件的默认 (CENTER) 位置...

getContentPane().add(panelTop);
//...
getContentPane().add(p1);

这实际上使第一个组件不可见，这意味着它不可显示...

• 您不应该调用 show，因为它已被弃用，请改用 setVisible。
• 在进行任何自定义绘画之前，您应该在 paintComponent 方法中调用 super.paintComponent，否则您将遇到无穷无尽的绘画问题
• 没有必要将paintComponent 设为public，因为您永远不希望任何人能够直接调用它...

更新

除非您注册有兴趣收到有关事件的通知，否则无论您创建什么对象或实现什么接口，您都不会收到通知。

这就像说“我对这部电影很感兴趣”，但除非你去电影院，否则你永远看不到它......

为了让您的Mithu 类接收鼠标事件通知，它必须针对它有兴趣监视的组件注册一个MouseListener。

以下示例在Mithu 类中针对MyPagalpanti 的实例注册一个MouseListener，这允许它接收由MyPagalpanti 的实例生成的鼠标事件

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.FontMetrics;
import java.awt.Graphics;
import java.awt.GridLayout;
import java.awt.Point;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextArea;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TestMouseListener {

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

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

                Mithu m = new Mithu("HI ! Karamvir");
                m.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                m.pack();
                m.setLocationRelativeTo(null);
                m.setVisible(true);
            }
        });
    }

    public class Mithu extends JFrame implements MouseListener
    {

        JTextArea jta1;

        Mithu(String title) {
            super(title);
            MyPagalpanti p1 = new MyPagalpanti();
            p1.addMouseListener(this);
            jta1 = new JTextArea();
            p1.setOpaque(true);
            jta1.setEditable(false);
            setForeground(Color.blue);
            setLayout(new GridLayout(2, 1));
            getContentPane().add(jta1);
            getContentPane().add(p1);
            pack();
            setVisible(true);
        }

        void record(String s, MouseEvent e) {
            jta1.append("\n" + s + "" + e.toString());
        }

        public void mouseClicked(MouseEvent m) {
            record("clicked", m);
        }

        public void mouseEntered(MouseEvent m) {
            record("entered", m);
        }

        public void mousePressed(MouseEvent m) {
            record("pressed", m);
        }

        public void mouseReleased(MouseEvent m) {
            record("released", m);
        }

        public void mouseExited(MouseEvent m) {
            record("exited", m);
        }
    }

    public class MyPagalpanti extends JPanel implements MouseListener {

        Point a, b;
        boolean drawLin, drawOvl, drawOvl2;

        MyPagalpanti() {
            a = new Point(0, 0);
            b = new Point(0, 0);
            drawLin = false;
            drawOvl = false;
            drawOvl2 = false;
            setSize(300, 300);
            addMouseListener(this);
        }

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

        public void mouseClicked(MouseEvent m) {
            if (!drawOvl) {
                a = new Point(m.getX(), m.getY());
                drawOvl = true;
                drawOvl2 = false;
            } else {
                b = new Point(m.getX(), m.getY());
                drawOvl = true;
                drawOvl2 = true;
                repaint();
            }//record("clicked",m);}
        }

        public void mouseEntered(MouseEvent m) {
        }

        public void mousePressed(MouseEvent m) {
            a = new Point(m.getX(), m.getY());
            repaint();
        }

        public void mouseReleased(MouseEvent m) {
            b = new Point(m.getX(), m.getY());
        }

        public void mouseExited(MouseEvent m) {
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.RED);
            FontMetrics fm = g.getFontMetrics();
            if (a != null && b != null) {
                g.drawString(a + "-" + b, 0, fm.getHeight());
            }
            if (drawLin) {
                g.setColor(Color.yellow);
                g.drawLine((int) a.getX(), (int) a.getY(), (int) b.getX(), (int) b.getY());
            }//d=0;}
            else if (drawOvl && drawOvl2) {
                g.setColor(Color.red);
                int width = (int) Math.abs(b.getX() - a.getX());
                int h = (int) Math.abs(b.getY() - a.getY());
                g.drawOval((int) a.getX(), (int) a.getY(), width, h);
            }
        }
    }

}

在你更进一步之前，你真的应该通读一遍......

• Performing Custom Painting
• Painting in AWT and Swing

如果您继续遵循当前的方法，您将会遇到一些非常令人讨厌的惊喜。

另外，通读How to Write a Mouse Listener也无妨

Answer 2

您的类有mouseClicked()、mouseReleased() 等方法，但它没有实现MouseListener。即使是这样，它也不会作为 MouseListener 添加到任何组件中。

所以，让你的类实现 MouseListener：

public class MouseEvtEx2 extends JFrame implements MouseListener {

并将自己作为鼠标侦听器添加到您要侦听的组件中：

panelTop.addMouseListener(this);

当然，为了帮助自己阅读和理解自己的代码，请正确缩进。

Answer 3

我进行了一些更改以使其正常工作。您需要在 MouseEvtEx2 框架上添加 implement MouseListener，还需要将其作为 MouseListener 添加到您的 panel p1

public class MouseEvtEx2 extends JFrame implements MouseListener {
    private JTextArea txtArea;

    public MouseEvtEx2(String title) {
        super(title);
        Panel p1 = new Panel();
        MyPanel panelTop = new MyPanel();
        panelTop.setBackground(new Color(0.98f, 0.97f, 0.85f));
        panelTop.setOpaque(true);
        panelTop.setPreferredSize(new Dimension(400, 200));
        panelTop.setBorder(BorderFactory.createRaisedBevelBorder());

        getContentPane().add(panelTop);
        txtArea = new JTextArea();
        txtArea.setEditable(false);
        JScrollPane pane = new JScrollPane(txtArea);
        pane.setPreferredSize(new Dimension(400, 200));
        p1.setLayout(new GridLayout(2, 1, 0, 0));
        getContentPane().add(p1);
        p1.addMouseListener(this);
        p1.add(pane);

        // txtArea.setText
        // revalidate();
        setSize(600, 600);
        show();
    }

    public void record(String st, MouseEvent et) {
        txtArea.setText("" + st + "" + et.toString());
    }// setVisible(true);}

    public void mouseClicked(MouseEvent evt) {
        record("Mouse clicked # of mouse clicks: " + evt.getClickCount(), evt);// +
                                                                                // " "
                                                                                // +

    }

    public void mouseEntered(MouseEvent evt) {
        record("Mouse entered ", evt);// .toString());
    }

    public void mouseExited(MouseEvent evt) {
        record("Mouse exited ", evt);// .toString());
    }

    public void mousePressed(MouseEvent evt) {
        record("Mouse pressed # of mouse clicks: ", evt);// .getClickCount() +
                                                            // " " +

    }

    public void mouseReleased(MouseEvent evt) {
        record("Mouse released ", evt);
    }

    public static void main(String[] args) {
        new MouseEvtEx2("Mouse events");
    }
}

希望这会有所帮助。