Java绘画程序给出很多错误

Question

I want to make a simple painting program. So,when you drag your mouse a line will draw in a GUI. The problem is when the user drags the mouse, it will paint automatically, but my code doesn't work. Can someone please tell me how do this? Sorry for my English and if you don't understand my question, look at my code maybe you will than.

My main class:

import javax.swing.JFrame;
public class MainClass {
    public static void main(String args[]){
        tuna kip = new tuna();
        kip.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        kip.setSize(800,600);
        kip.setVisible(true);
    }
}

This is my other class:

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

public class tuna extends JFrame {
    JPanel jpanel = new JPanel();

    public tuna(){
        super("Painting Program");
        jpanel.setBackground(Color.WHITE);
        add(jpanel);

        hand handler = new hand();
        jpanel.addMouseListener(handler);
        jpanel.addMouseMotionListener(handler);
    }

    private class hand implements MouseListener ,MouseMotionListener { //THE ERRORS START TO APPEAR HERE
        public void mouseDragged(MouseEvent event){
            public void paintComponent(Graphics g){
                super.paintComponent(g);
                g.setColor(Color.BLACK);
                g.fillRect(event.getX(), event.getY(), 5, 5);
            }
        }
    }
}

Answer 1

When I try to run the code I get too much error messages:

You mean when you try to compile the code you get compile errors.

class hand implements MouseListener ,MouseMotionListener 

Your class does not implement all the methods in those listeners. You only implement one method.

Read the section from the Swing tutorial on How to Write a MouseMotionListener for a working example.

If you only care about the mouseDragged() method then you only need to implement the MouseMotionListener .

Or as a simpler solution you can extend MouseMotionAdapter . This class implements all the methods of the MouseMotionListener so you only need to override the methods you want to change. The tutorial also discusses adapters.

Finally class names SHOULD start with an upper case character. Look at the Java API and you will notice this. Follow the Java conventions and don't make up your own.

Answer 2

Yes, there are so many issues in your codes. I just edited it and make it runnable at least. Just study and experiment on it. I hope you will learn something out of this even if I don't explain all of the changes.

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

public class tuna extends JFrame {
    int x, y, w, h;

    MyPanel jpanel = new MyPanel();

    public tuna(){
        super("Painting Program");
        setLayout(new BorderLayout());
        jpanel.setBackground(Color.WHITE);
        add(jpanel);

        hand handler = new hand();
        jpanel.addMouseListener(handler);
        jpanel.addMouseMotionListener(handler);
    }

    private class hand implements MouseListener , MouseMotionListener { //THE ERRORS START TO APPEAR HERE
        public void mouseClicked(MouseEvent e) {
        }

        public void mousePressed(MouseEvent e) {
            x=e.getX();
            y=e.getY();
        }

        public void mouseReleased(MouseEvent e) {
        }

        public void mouseEntered(MouseEvent e) {
        }

        public void mouseExited(MouseEvent e) {
        }

        public void mouseDragged(MouseEvent e) {
            w = e.getX() - x;
            h = e.getY() - y;
            jpanel.repaint();
        }

        public void mouseMoved(MouseEvent e) {
        }
    }

    class MyPanel extends JPanel {
        public void paintComponent(Graphics g){
            super.paintComponent(g);
            g.setColor(Color.BLACK);
            g.fillRect(x, y, w, h);
        }
    }
}