重绘问题（）

Question

我在使用重绘时遇到了一些麻烦。代码应该在光标后面画一条线。

package javaapplication6;
import java.awt.*;
import javax.swing.JFrame;
import java.awt.event.*;

class Test extends Canvas implements MouseListener, MouseMotionListener {
int x[]=new int[1024];
int y[]=new int[1024];
int size=0;

public void MouseDemo(){
    addMouseListener(this);
    addMouseMotionListener(this);
}

public void paint (Graphics g){
    g.setColor(Color.red);
    for (int i=1;i<size;i++) g.drawLine(x[i],y[i],x[i-1],y[i-1]);

}

public void mousePressed(MouseEvent e){
x[size]=e.getX();
y[size]=e.getY();
size++;

}

public void mouseDragged(MouseEvent e){
    y[size]=e.getY();
    x[size]=e.getX();
    size++;
    repaint();
}

public void mouseEntered(MouseEvent e){
    size=0;
    y[size]=e.getY();
    x[size]=e.getX();
    repaint();
}

public void mouseExited (MouseEvent e){
    size=0;
    repaint();

}

public void mouseReleased (MouseEvent e){}

public void mouseClicked (MouseEvent e){

}

public void mouseMoved (MouseEvent e){}
}

public class JavaApplication6 {


public static void main(String[] args) {

   JFrame win= new JFrame ("Target");
win.setSize(600,500);
win.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
win.add(new Test());
win.setVisible(true);

  }}

这个相同的代码在学校的计算机上工作，但当我回到家并尝试做另一个例子并且它没有真正起作用时，我决定测试这个并且它也不起作用。我真的很感激一些帮助和解释至于为什么重画不会触发。

Answer 1

你的主要问题是......

public void MouseDemo() {
    addMouseListener(this);
    addMouseMotionListener(this);
}

这是一个方法，而不是构造函数，因此它永远不会注册您的侦听器。相反，将其更改为......

public MouseDemo() {
    addMouseListener(this);
    addMouseMotionListener(this);
}

或者，基于你实际的示例代码......

public Test() {
    addMouseListener(this);
    addMouseMotionListener(this);
}

java.awt.Canvas是一个重量级的组件，AWT组件并不总是与Swing很好用，因为它们没有Swing实现它的z排序概念。

从JPanel开始你会更好。

按照惯例，你应该覆盖paintComponent方法并在其中执行自定义绘画，记住先调用super.paintComponent

因为框架边框插入到框架的可见区域，所以“可用”区域将小于setSize指定的区域，相反，您应该覆盖getPreferredSize的JPanel方法并指定您喜欢的“可用”区域。从那里，你可以pack围绕它的框架。

为避免线程出现任何问题，您应该使用EventQueue.invokeLater启动UI

例如...

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Test extends JPanel implements MouseListener, MouseMotionListener {

    int x[] = new int[1024];
    int y[] = new int[1024];
    int size = 0;

    public Test() {
        addMouseListener(this);
        addMouseMotionListener(this);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(600, 500);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.red);
        for (int i = 1; i < size; i++) {
            g.drawLine(x[i], y[i], x[i - 1], y[i - 1]);
        }

    }

    @Override
    public void mousePressed(MouseEvent e) {
        x[size] = e.getX();
        y[size] = e.getY();
        size++;

    }

    public void mouseDragged(MouseEvent e) {
        y[size] = e.getY();
        x[size] = e.getX();
        size++;
        repaint();
    }

    public void mouseEntered(MouseEvent e) {
        System.out.println("Entered");
        size = 0;
        y[size] = e.getY();
        x[size] = e.getX();
        repaint();
    }

    public void mouseExited(MouseEvent e) {
        size = 0;
        repaint();

    }

    public void mouseReleased(MouseEvent e) {
    }

    public void mouseClicked(MouseEvent e) {

    }

    public void mouseMoved(MouseEvent e) {
    }

    public static void main(String[] args) {
        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 Test());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }
}