Thread: How to drag the shape to move?

Hi guys
Below code can press to draw a random polygon whenever the mouse is pressed. I also want to drag any polygon to anywhere I want, so I have add something in the methods of mousePressed, mouseReleased, mouseDragged. but got no luck.
Does anyone know what the problem is?
public class Octagon extends Polygon {....} and public class Main {...} sre omitted here, see my OctagonPanel class below:

public class OctagonPanel extends JPanel
{
    private Octagon octagon;
    private Octagon selectedOctagon = null;
    private ArrayList<Octagon> lstOctagon = new ArrayList<Octagon>();
 
    static Random generator = new Random();
    int oldx, oldy, newx, newy;
    boolean dragging = false;
 
    public OctagonPanel()
    {
      addMouseListener (new OctagonsListener());
    }
 
   public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        for(Octagon o : lstOctagon)
        {
            o.draw(g);
        }
    }
 
     private class OctagonsListener implements MouseListener, MouseMotionListener
     {
         public void mousePressed (MouseEvent e)
         {
            oldx = e.getX();
            oldy = e.getY();
 
            for(Octagon o : lstOctagon)
            {
               if(o.contains(oldx, oldy))
               {
                   selectedOctagon = o;
                   dragging = true;
               }
            }
            if(selectedOctagon == null)
            {
                int n = generator.nextInt(80);
                int[] newx = {oldx, oldx + n, oldx + n, oldx, oldx - 2*n, oldx - 3*n, oldx - 3*n, oldx - 2*n};
                int[] newy = {oldy, oldy + n, oldy + 2*n, oldy + 3*n, oldy + 3*n, oldy + 2*n, oldy + n, oldy};
                octagon = new Octagon(newx, newy, 8);
 
                lstOctagon.add(octagon);
                repaint();
            }
          }
 
         public void mouseReleased(MouseEvent e)
         {
             if(dragging = true)
             {
                 repaint();
             }
             selectedOctagon = null;
         }
         public void mouseClicked(MouseEvent e)
         {
         }
         public void mouseMoved(MouseEvent e) {}
         public void mouseDragged(MouseEvent e)
         {
            newx = e.getX();
            newy = e.getY();
            int xtrans = newx - oldx;
            int ytrans = newy - oldy;
            selectedOctagon.translate(xtrans,ytrans);
            repaint();
 
         }
         public void mouseEntered(MouseEvent e) {}
         public void mouseExited(MouseEvent e) {}
     }
 
 }

Thanks heaps

Can you define what you mean by 'no luck'? At what point does it 'break' (dragging, drawing)? Add some println statements (or use a debugger) to make sure the mousePressed and mouseDragged are doing what you wish.

Hi copeg,

I mean I want to drag the shape to the place where I release the mouse, but the shape doesn't move at all. I thnk I have specified what to do in the methods of mousePressed, mouseReleased, mouseDragged, obviously I didn't specify them correctly.

Is there any way to poiint out the mistakes from code of mousePressed, mouseReleased, mouseDragged apart from using debug? (At moment I am not good enough to be able to use debugger to figure out things)
Thanks heaps

Try adding the OctagonsListener as a mouseMotionlistener to your JPanel

addMouseMotionListener(new OctagonsListener());

Hi copeg
Thank you for your advice, I have changed it to what you mean (in red), then it can't even draw any polygon when I press the mouse?

Any more thought?

 public OctagonPanel()
    {
      [COLOR="red"]addMouseMotionListener[/COLOR](new OctagonsListener());
    }

And

private class OctagonsListener implements MouseListener,[COLOR="Red"] MouseMotionListener[/COLOR]

Hard to tell from your post...did you add the mouseListener AND mouseMotionListener. The two are different, and must be added using the appropriate method.

OctagonsListeners listener = new OctaginsListener();
addMouseListener(listener);
addMouseMotionListener(listener);

Ah, I see what you mean - shoudl add both. Now I add both, then it comes back to the first place, which is I only can press to draw a polygon, but I still can't drag to move them.

public OctagonPanel()
{
addMouseListener(new OctagonsListener());
addMouseMotionListener(new OctagonsListener());
}

Any more thoughts?
Thanks heaps

As I mentioned in a post above, add some System.out.println statements in your code. This will allow you to see what the values are, whether certain methods or control statements are entered, etc...this is one of the first rules in debugging code (the second rule, step through code using a debugger ).

How could System.out.println help it? It just simply print text out, I still don't know what is going on. As for debugger, I am hopeless on that.
Is there other ways to tell whic part of code is wrong? I bellieve the mistake must be inside either the methods of mousePressed, mouseReleased, mouseDragged , how to manipulate these 3 confusing things to drag the shape? (I didn't even use mouseMoved, is it ok?)

Originally Posted by ice

How could System.out.println help it?

System.out.println doesn't help it, System.out.println helps you. In this case, printing out values can let you know the coordinates of Shape (eg Octagon), whether the Shape contains method is working properly, whether any particular methods in you code are being entered, etc...in situations such as this, printing out values is invaluable.

Thanks copeg, but it didn't work out for me, I still don't know how to fix it.
Is there any way yo point out the error inside the code?
Thanks heaps

Hi guys, don't give up here yet, the problem is still not solved yet.

When you say above the println statements 'didn't fix it', what do you mean? When you add the println statements, can you tell from what the command line prints if the code enter the mousePressed and/or mouseDragged functions? If so, are the conditional if statements entered under the conditions you expect?

In fact I don't know what exact I should print to analyse; then I tried to put System.out.println(xtrans + " " + ytrans);inside mousedragged method, is that right thing to print? but it print out nothing.

Please tell me what is going on?

public void mouseDragged(MouseEvent e)
         {
            newx = e.getX();
            newy = e.getY();
            int xtrans = newx - oldx;
            int ytrans = newy - oldy;
            selectedOctagon.translate(xtrans,ytrans);
            repaint();
            System.out.println(xtrans + " " + ytrans);
         }

Thanks heaps

If it prints nothing, that tells you an enormous amount of information...it tells you that line of code is never reached. Since it is in the mouseDragged statement of a MouseMotionListener, what could be the cause? If you move that prinln statement up in the mouseDragged method, does it print anything? If you move the println statement to mouseMoved method, does it print anything? Is your MouseMotionListener added to the component?

I feel the error lies in your Octagon class, so I recommend you post that code.

great, that souds promising, here is Octagon Class:

import java.awt.*;
import java.util.Random;
 
public class Octagon extends Polygon
{ 
    private int[] x; 
    private int[] y; 
    private Color color;
    static Random generator = new Random();
 
    public Octagon(int[]x, int[]y, int n)
    {
        this.x = x;
        this.y = y;
        color = new Color(Math.abs(generator.nextInt())% 16777216);
    }
 
    void draw(Graphics g)
    {
        g.setColor(color);
        g.fillPolygon(x, y, 8);
    }
 
}

Wow, it does have some magic! Now mouseClick part works, selectedOctagon can get selected. But something odd happened: As soon as I start to drag selectedOctagon, it moves very very quickly to somewhere else and disappears from the whole screen. (It doesn’t follow the mouse’s drag).

I guess is that because I didn’t use translate method in mouseDrag correctly? What do you think?

Thanks heaps

Once again, println's will help you see the values of how much the shape is translated. Bottom line, every time the mouse is dragged the shape is dragged by the current mouse location minus the mouse pressed location (hint, reset the old values after translating in the mouse dragged)

oh, my god! It finally did it!
Thank u a lot copeg!!!