Question

The problem is that this ball after it is dragged and exited click, it is supposed to repaint according to the new y component given. This is calculated from the gravity final and added to the velocity which is added to the existing y component in a loop.

I have debugged many times and I just cant hit it on the head.

Its supposed to..
move to where you drag it >>> when you let go it is supposed to fall until it hits the ground.

Thank you ahead of time.

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

public class DragBallPanel extends JPanel implements MouseListener, MouseMotionListener
{
     private static final int BALL_DIAMETER = 40;
     private int screen_size_x = 300;
     private int screen_size_y = 300;
     private int ground_lvl = screen_size_y - 15;

     private int _ballX     = ground_lvl/2;
     private int _ballY     = ground_lvl - BALL_DIAMETER;
     private final double GRAVITY = -9.8;
     private double velocity;
     private static final double TERM_VEL = -100;

     private int _dragFromX = 0;
     private int _dragFromY = 0;

     private boolean _canDrag  = false;


     public DragBallPanel() throws InterruptedException
     {
        setPreferredSize(new Dimension(screen_size_x, screen_size_y));
        setBackground(Color.darkGray);
        setForeground(Color.darkGray);

        this.addMouseListener(this); 
        this.addMouseMotionListener(this);
    }

    public void paintComponent(Graphics g)
     {
        super.paintComponent(g);   // Required for background.
          g.setColor (Color.green);
          g.fillRect (0, 280, 400, 50 );
          g.setColor (Color.black);
        g.fillOval(_ballX, _ballY, BALL_DIAMETER, BALL_DIAMETER);

    }

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

        if (x >= _ballX && x <= (_ballX + BALL_DIAMETER)
                && y >= _ballY && y <= (_ballY + BALL_DIAMETER))\
          {
            _canDrag = true;
            _dragFromX = x - _ballX;
            _dragFromY = y - _ballY;
        } else
          {
            _canDrag = false;
        }
    }

    //===== mouseDragged ======
    /** Set x,y  to mouse position and repaint. */
    public void mouseDragged(MouseEvent e)
    {
        if (_canDrag)
          {   // True only if button was pressed inside ball.
            //--- Ball pos from mouse and original click displacement
            _ballX = e.getX() - _dragFromX;
            _ballY = e.getY() - _dragFromY;

            //--- Don't move the ball off the screen sides
            _ballX = Math.max(_ballX, 0);
            _ballX = Math.min(_ballX, getWidth() - BALL_DIAMETER);

            //--- Don't move the ball off top or bottom
            _ballY = Math.max(_ballY, 0);
            _ballY = Math.min(_ballY, getHeight() - BALL_DIAMETER);

            this.repaint();
        }
    }

    public void mouseExited(MouseEvent e)
    {
          while(_ballY < ground_lvl)
          {
                 simulateGravity();
          }   
    }

     public void simulateGravity()
     {
         if(_canDrag)
         {
             try{
                 velocity = velocity + GRAVITY;

               if (velocity < TERM_VEL)
                 {
                    velocity = TERM_VEL;
                 }

                if (_ballY >= ground_lvl - BALL_DIAMETER)
                {
                   velocity = velocity/4; 
                } 
                _ballY += velocity;
                 Thread.sleep(400);
                 this.repaint();//**problem occurs here**

              }catch(InterruptedException ie)
              {
              }
         }
     }

    public void mouseMoved   (MouseEvent e){}
    public void mouseEntered (MouseEvent e){}
    public void mouseClicked (MouseEvent e){}
    public void mouseReleased(MouseEvent e){}
}

main() class

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


public class DragDemo extends JApplet
{
    public static void main(String[] args) throws InterruptedException
    {
        JFrame window = new JFrame();
        window.setTitle("Drag Demo");
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          //window.add(new DragBallPanel());
        window.setContentPane(new DragBallPanel());
          window.setResizable(false);
        window.pack();
        window.show();
    }

    public DragDemo() throws InterruptedException
    {
        new DragBallPanel();
    }
}

Answer 1

This SSCCE begins to show the problems in the code.

1. Compile the code.
2. Run it.
3. Drag the ball upwards.
4. Release the ball.
5. Remove the mouse from the drawing area, to see..
6. The ball fall upwards!

You seem to have gotten the Y values upside down. They start at top of screen, and go downwards. Also, the code was blocking the EDT in an infinite loop. To solve that, run the animation using a Swing Timer.

Please read the document on the SSCCE & ask if there is anything in it you do not understand. I am well placed to explain. 🙂

---

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

public class DragBallPanel extends JPanel implements MouseListener, MouseMotionListener
{

    private static final int BALL_DIAMETER = 40; // Diameter of ball

    private int screen_size_x = 300;
    private int screen_size_y = 300;
    private int ground_lvl = screen_size_y - 15;

    private int _ballX     = ground_lvl/2;
    private int _ballY     = ground_lvl - BALL_DIAMETER;
    private final double GRAVITY = -9.8;
    private double velocity;
    private static final double TERM_VEL = 100;

    private int _dragFromX = 0;    // pressed this far inside ball's
    private int _dragFromY = 0;    // bounding box.

    /** true means mouse was pressed in ball and still in panel.*/
    private boolean _canDrag  = false;

    public DragBallPanel()
    {
        setPreferredSize(new Dimension(screen_size_x, screen_size_y));
        setBackground(Color.darkGray);
        setForeground(Color.darkGray);

        this.addMouseListener(this);
        this.addMouseMotionListener(this);
    }

    public void paintComponent(Graphics g)
     {
        super.paintComponent(g);   // Required for background.
          g.setColor (Color.green);
          g.fillRect (0, 280, 400, 50 );
          g.setColor (Color.black);
        g.fillOval(_ballX, _ballY, BALL_DIAMETER, BALL_DIAMETER);

    }

    public void mousePressed(MouseEvent e)
    {
        int x = e.getX();   // Save the x coord of the click
        int y = e.getY();   // Save the y coord of the click

        if (x >= _ballX && x <= (_ballX + BALL_DIAMETER)
                && y >= _ballY && y <= (_ballY + BALL_DIAMETER)) {
            _canDrag = true;
            _dragFromX = x - _ballX;  // how far from left
            _dragFromY = y - _ballY;  // how far from top
        } else {
            _canDrag = false;
        }
    }

    //========= mouseDragged =================
    /** Set x,y  to mouse position and repaint. */
    public void mouseDragged(MouseEvent e)
    {
        if (_canDrag) {   // True only if button was pressed inside ball.
            //--- Ball pos from mouse and original click displacement
            _ballX = e.getX() - _dragFromX;
            _ballY = e.getY() - _dragFromY;

            //--- Don't move the ball off the screen sides
            _ballX = Math.max(_ballX, 0);
            _ballX = Math.min(_ballX, getWidth() - BALL_DIAMETER);

            //--- Don't move the ball off top or bottom
            _ballY = Math.max(_ballY, 0);
            _ballY = Math.min(_ballY, getHeight() - BALL_DIAMETER);

            this.repaint(); // Repaint because position changed.
        }
    }

    //====================================================== method mouseExited
    /** Turn off dragging if mouse exits panel. */
    public void mouseExited(MouseEvent e)
     {
         System.out.println("Exited: " + e);
        //_canDrag = false;
        runGravity();
        /*  while(_ballY < ground_lvl)
          {
             simulateGravity();
          }*/
    }

    Timer timer;
    ActionListener animate;

    public void runGravity() {
        if (animate==null) {
            animate = new ActionListener() {
                public void actionPerformed(ActionEvent ae) {
                    System.out.println("Ground: " + (_ballY-ground_lvl));
                    if (_ballY > ground_lvl) {
                        timer.stop();
                    } else {
                        simulateGravity();
                    }
                }
            };
            timer = new Timer(100,animate);
        }
        timer.start();
    }

     public void simulateGravity()
     {
         System.out.println("_canDrag: " + _canDrag);
         if(_canDrag)
         {

             velocity = velocity + GRAVITY;

           if (velocity > TERM_VEL)
             {
                velocity = TERM_VEL;
             }

            if (_ballY >= ground_lvl - BALL_DIAMETER)
            {
                //We have hit the "ground", so bounce back up. Reverse
                //the speed and divide by 4 to make it slower on bouncing.
                //Just change 4 to 2 or something to make it faster.
               velocity = velocity/4;
            }
            _ballY += velocity;
             //this.revalidate();
             this.repaint();
         }
    }

    public void mouseMoved   (MouseEvent e){}
    public void mouseEntered (MouseEvent e){}
    public void mouseClicked (MouseEvent e){}
    public void mouseReleased(MouseEvent e){}

    public static void main(String[] args) {
        SwingUtilities.invokeLater( new Runnable() {
            public void run() {
                DragBallPanel dbp = new DragBallPanel();
                JOptionPane.showMessageDialog(null, dbp);
            }
        });
    }
}