How do I make this mouselistener only care about one object?

Question

I'm working on these online Stanford lessons on Java, and we just made the leap to events, and I'm having difficulty wrapping my head around it. I'm playing around with a program that is in the "Art and Science of Java" book. This program will move a rectangle and oval around on the canvas if you click on them.

I modified the run method to try and get the listener to only work on the rectangle, but I was surprised to see even with my changes, both objects are being listened to...why?

Original run method:

public void run() {
    GRect rect = new GRect(100, 100, 150, 100);
    rect.setFilled(true);
    rect.setColor(Color.RED);
    add(rect);
    GOval oval = new GOval(300, 115, 100, 70);
    oval.setFilled(true);
    oval.setColor(Color.GREEN);
    add(oval);
    addMouseListeners();
}

My changed program (with the MouseListener in the private createRectangle method):

import java.awt.*;
import java.awt.event.*;
import acm.graphics.*;
import acm.program.*;

/** This class displays a mouse-draggable rectangle and oval */

public class DragObjects extends GraphicsProgram {


    public void run() {
        createRectangle();
        createOval();

    }

    private void createOval(){

        GOval oval = new GOval(300, 115, 100, 70);
        oval.setFilled(true);
        oval.setColor(Color.GREEN);
        add(oval);

    }

    private void createRectangle(){

        GRect rect = new GRect(100, 100, 150, 100);
        rect.setFilled(true);
        rect.setColor(Color.RED);
        add(rect);
        addMouseListeners();
    }

/** Called on mouse press to record the coordinates of the click */
    public void mousePressed(MouseEvent e) {
        lastX = e.getX();
        lastY = e.getY();
        gobj = getElementAt(lastX, lastY);
    }

/** Called on mouse drag to reposition the object */
    public void mouseDragged(MouseEvent e) {
        if (gobj != null) {
            gobj.move(e.getX() - lastX, e.getY() - lastY);
            lastX = e.getX();
            lastY = e.getY();
        }
    }

/** Called on mouse click to move this object to the front */
    public void mouseClicked(MouseEvent e) {
        if (gobj != null) gobj.sendToFront();
    }

/* Instance variables */
private GObject gobj;   /* The object being dragged */
private double lastX;   /* The last mouse X position */
private double lastY;   /* The last mouse Y position */
}

score 2 · Answer 1 · answered Feb 28 '10 at 03:42

It would be helpful if you would point out that the method addMouseListeners() is in the superclass, GraphicsProgram. What it does is adds the listener to the canvas, not just to the individual shape. From there, you'll need to somehow determine whether the mouseclick occurred in the rectangle or in the oval.

Or there might be a way to add the listener just to a single shape. Check the Javadoc for the GRect and GOval classes. I'm assuming those are also in one of the acm.* packages, which means they're not built in to the Java language. (This is why I recommend using an IDE like Eclipse that can automatically import each class automatically, instead of importing an entire package.)

It might also be helpful to post a link to the online lessons you're following.

score 1 · Answer 2 · answered Feb 28 '10 at 03:52

I haven't looked at the source other than what you have posted. But you will need to other modify gOval and gRect or a superclass to accept a mouseListener, or in your listener do something like the following.

in the MouseClicked, MouseMoved, etc. methods. Get the point of the event, and then go through your Objects and query them to see if the point exists withing their bounds.

You would need a list of objects to loop through and then call something like gRect.containsPoint(myPoint) in this method check to see if the point exists in the shape. You will still have issues where shapes overlap, so you will need some concept of a z-axis or depth to determine which shape is on top.

Rather than modifying the GOval and GRect classes (which may not be possible anyway), I would probably create subclasses with the added behavior if they don't have that behavior built-in. Check the Javadoc for those classes, maybe what you want already exists. — Tyler, Feb 28 '10 at 04:53

score 0 · Answer 3 · edited May 23 '17 at 12:18

0

You need to post the source for the addMouseListeners.

If you take a look at this post over here you might get some ideas on how listeners can work (if you post the addMouseListener source we can help with your specific question of course!)

edited May 23 '17 at 12:18

Community

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answered Feb 28 '10 at 03:24

TofuBeer

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http://java.sun.com/j2se/1.3/docs/api/java/awt/Component.html#addMouseListener%28java.awt.event.MouseListener%29 – Joel Feb 28 '10 at 03:27
No, he/she means your call to `addMouseListeners();` at the end of `run()` and at the end of `createRectangle()`. It looks like that method is probably calling `Component.addMouseListener()` on multiple components. – Tyler Feb 28 '10 at 03:30
Please read the question closely...the first bit if code is what it was (with addMouseListeners in the run method) and the second bit is what I changed it to, and yet both seem to do the same thing for some reason. – Joel Feb 28 '10 at 03:34
addMouseListeners is not a JDK method - it is your own. I can see where you are calling it, but without knowing what it actually does it is impossible to tell you want it wrong with the code. Odds are addMouseListeners is calling addMouseListener to both components instead of just one of them. – TofuBeer Feb 28 '10 at 04:38
http://www.ifs.tuwien.ac.at/ifs/lehre/eprog/javadoc/jdk/docs/api/java/awt/event/package-summary.html – Joel Feb 28 '10 at 05:25
1

I do not understand why you are pointing us to the event package javadocs? There is no method called addMouseListeners in the standard Java API. Given the comments from other people it is on the parent class of the one you have modified. Without knowing what that method does it is pretty hard to give you a definitive answer. If you want people to look at that method you should provide a link to the original code or paste it in your question so we can look at it. I found this but not sure if it is it: http://jtf.acm.org/javadoc/student/acm/program/GraphicsProgram.html – TofuBeer Feb 28 '10 at 07:51

How do I make this mouselistener only care about one object?

3 Answers3