使用MouseListener Java小程序拖动对象

Question

I'm trying to create an applet that draws a circle (defined as an object) to the screen then this circle can be dragged across the screen using the mouse. So far when the mouse is pressed the object is drawn and can be dragged, but what I want it to do is draw the object when the applet is started then allow the user to click on the object and drag it. Any help or clues would be much appreciated. here is the code:

import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;

public class sheepDog extends Applet implements ActionListener, MouseListener, MouseMotionListener
{
    manAndDog dog;
    int xposR;
    int yposR;

    public void init()
    {
        addMouseListener(this);
        addMouseMotionListener(this);

    }
    public void paint(Graphics g)
    {
        dog.display(g);

    }
    public void actionPerformed(ActionEvent ev)
    {}
    public void mousePressed(MouseEvent e)
    {

    }
    public void mouseReleased(MouseEvent e)
    {


    }
    public void mouseEntered(MouseEvent e)
    {}
    public void mouseExited(MouseEvent e)
    {}
    public void mouseMoved(MouseEvent e)
    {
    }
    public void mouseClicked(MouseEvent e)
    {}
    public void mouseDragged(MouseEvent e)
    {
        dog = new manAndDog(xposR, yposR);
        xposR = e.getX();
        yposR = e.getY();
        repaint();

    }
}

class manAndDog implements MouseListener, MouseMotionListener
{
    int xpos;
    int ypos;
    int circleWidth = 30;
    int circleHeight = 30;
    Boolean mouseClick;

    public manAndDog(int x, int y)
    {
        xpos = x;
        ypos = y;
        mouseClick = true;
        if (!mouseClick){
            xpos = 50;
            ypos = 50;
        }

    }

    public void display(Graphics g)
    {
        g.setColor(Color.blue);
        g.fillOval(xpos, ypos, circleWidth, circleHeight);
    }

    public void mousePressed(MouseEvent e)
    {
        mouseClick = true;
    }
    public void mouseReleased(MouseEvent e)
    {

    }
    public void mouseEntered(MouseEvent e)
    {}
    public void mouseExited(MouseEvent e)
    {}
    public void mouseMoved(MouseEvent e)
    {}
    public void mouseClicked(MouseEvent e)
    {}
    public void mouseDragged(MouseEvent e)
    {
        if (mouseClick){
            xpos = e.getX();
            ypos = e.getY();
        }


    }
}

Thanks

Answer 1

In the start method of your applet, assign a location for the manAndDog object and call repaint

Reimeus is more correct, the init method is a better place to initalise the manAndDog.

Hope you don't mind some feedback ;)

1. You should be calling super.paint(g) in your paint method. In fact, I'd encourage you to use JApplet and override paintComponent , but that's just me
2. I don't see the need to continuously recreate the manAndDog object.

For example. If you added a method setLocation , you could simply call 'setLocation` when the mouse is dragged.

public void mouseDragged(MouseEvent e) {
    dog.setLocation(xposR, yposR);
    xposR = e.getX();
    yposR = e.getY();
    repaint();
}

This is more efficient as it's not continuously creating short lived objects. It also means you can do more with the manAndDog object, such as apply animation. IMHO

Answer 2

最简单的方法是在init()方法中创建ManAndDog对象，如下所示：

dog = new ManAndDog(0, 0);