Going crazy with these mouseEvent methods in Java

Question

A week into it, and I am just banging my head up against this problem. It should be a simple excersise:

I have a rectangle. I click and drag the rectangle on an x axis. The rectangle should only move within a set bounding area (the canvas), so if the canvas is 200 px wide, then the x coordinate should only go from 0 to getWidth()-RECTANGLE_WIDTH.

Simple enough, but I just can not get the darn thing to work.

Below is my unwieldy code.

I have attached two GLabels to view coordinates. I noticed that the mouseMoved label will only display coordinates within the canvas (what I want), but the mouseDragged label will go negative, and will also show coordinates beyond the canvas-this is also where the rectangle object movement is being controlled. Im'm not sure why those two behaviors are different.

Right now, my statement below:

if ((gobj.getX()) > 0 && (gobj.getX()) < (APPLICATION_WIDTH - PADDLE_WIDTH)){
                gobj.move(e.getX() - lastX, 0);

               } 

what that statement does so far is get the rectangle to the edge, but then it just sticks there and won't return. I'm pulling my hair out on this thing...

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

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

public class DragObject extends GraphicsProgram {

private static final int PADDLE_WIDTH = 150;
public static final int APPLICATION_WIDTH = 700;

    public void run() {

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

        label2 = new GLabel ("");
        add(label2, 300, 400);


        label = new GLabel ("");
        add(label, 300, 300);
        addMouseListeners();


    }

    /* these coordinates are never going beyond the canvas-even if the 
     * mouse does...this is good...i want this   */

    public void mouseMoved(MouseEvent e){

        label2.setLabel("Coordinates:" + e.getX() + ", " + e.getY());

    }

/** 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.getX()) > 0 && (gobj.getX()) < (APPLICATION_WIDTH - PADDLE_WIDTH)){
            gobj.move(e.getX() - lastX, 0);

           } 

            lastX = e.getX();


            /* This label is active when i click the object, and does go into negative 
             * numbers and still counts even when off the canvas...*/
            label.setLabel("Coordinates:" + e.getX() + ", " + e.getY());
    }

/* Instance variables */

private GLabel label2;
private GLabel label;
private GObject gobj;   /* The object being dragged */
private double lastX;   /* The last mouse X position */
private double lastY;   /* The last mouse Y position */
}

Answer 1

You can drag out of a component, even out of a window, quite by design in order to support drag and drop.

For drag events you just have to constrain the event location to within it's container:

static public final int MAX_X=(CANVAS_WIDTH - PADDLE_WIDTH);

...

public void mouseDragged(MouseEvent e) { 
    int px=Math.max(0,Math.min(e.getX(),MAX_X));
    int dx=(px-lastX);

    if(dx!=0) { 
        gobj.move(dx); 
        lastX=px;
        }
    }

---

After sleeping on it, and re-reading the comments, I have an idea where the problem might lie - in the code provided GObject is not shown, nor is how it's created/initialized. So it's hard to guess at the context for the code in the question.

I would wager that the paddle is created to be initially in the middle (perhaps middle/bottom) of the canvas. The code above, and in the question, only constrains a delta to prevent the delta from exceeding the bounds of the canvas - it does not account for the initial position of the paddle. That is, if the paddle starts out at x=350, and the delta allows for a total of 700-150=550, then the constrained delta will allow the paddle to move to 350-550=-200, the paddle will move to 200 pixels beyond the left edge.

So you need to pass the move method the upper bounds and constrain the resulting position to 0 <= px <= UPPER... something like:

    ...
    if(dx!=0) { 
        gobj.moveX(dx,MAX_X); 
        lastX=px;
        }
    ...

GObject:

public void moveX(int dlt, int max) {
    positionX=Util.limitInt((positionX+dlt),0,max);
    }

Util:

/**
 * Limit the range of a number to the specified values.
 */
static public int limitInt(int val, int low, int hgh) {
    return ((val<low) ? low : ((val<hgh) ? val : hgh));
    }

And I would repeat all the code for Y, and manipulate this axis separately. Otherwise you will need to extend your move method to:

public void move(int dltX, int maxX, int dltY, int maxY) {
    positionX=Util.limitInt((positionX+dltX),0,maxX);
    positionY=Util.limitInt((positionX+dltY),0,maxY);
    }