Computer Science 120
Introduction to Programming

Spring 2012, Siena College

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DragABall Demo

A working demo of DragABall will appear below. Click inside the applet to interact with it.

DragABall BlueJ Project

Click here to download a BlueJ project for DragABall.

DragABall Source Code

The Java source code for DragABall is below. Click on a file name to download it.

DragABall.java

import objectdraw.*;
import java.awt.*;

/*
 * Example DragABall
 * Simple example to demonstrate dragging of a graphical object
 *
 * Jim Teresco, Siena College, CSIS 120, Spring 2011
 *
 * $Id: DragABall.java 1507 2011-01-27 02:20:59Z terescoj $
 */

public class DragABall extends WindowController {

    // named constants for the size and initial location of the ball
    private static final int BALL_DIAMETER = 50;
    private static final Location BALL_START = new Location(100, 100);

    // a ball
    private FilledOval ball;

    // the last previous known location of the mouse
    private Location lastMouse;
    
    // Is the ball currently being dragged around the screen?
    private boolean ballGrabbed;
    

    // place the ball on the canvas to start
    public void begin() {

        ball = new FilledOval(BALL_START,
                              BALL_DIAMETER, BALL_DIAMETER,
                              canvas);
    }
    
    // check if the mouse is pressed on the ball -- if so, set up for
    // dragging
    public void onMousePress(Location point) {
        
        if (ball.contains(point)) {
            // note that we've grabbed the ball and remember this point
            ballGrabbed = true;
            lastMouse = point;
        }
    }
    
    // complete the drag to the release point, stop the dragging
    public void onMouseRelease(Location point) {
        
        if (ballGrabbed) {
            ball.move(point.getX() - lastMouse.getX(),
                      point.getY() - lastMouse.getY());
            ballGrabbed = false;
        }
    }

    // if we are dragging, move the ball to follow the mouse,
    // update lastMouse location
    public void onMouseDrag(Location point) {
        
        if (ballGrabbed) {
            ball.move(point.getX() - lastMouse.getX(),
                      point.getY() - lastMouse.getY());
            lastMouse = point;
        }
    }
}