Java custom Path2D

Question

I have created a custom Path2D class to draw an H-shaped "calliper" on screen, for a project I am doing. I want to drag and eventually resize the calliper on screen. I have managed to get the Path2D set up so I can draw the calliper, and the code looks like this:

1. Declaration and Constructor:

   public class Calliper extends Path2D.Double
   {
       // X and Y coordinates of all six points on Calliper
       double cX1, cX2, cX3, cX4, cX5, cX6;
       double cY1, cY2, cY3, cY4, cY5, cY6;
   
       // Width and Height
       double cWidth;
       double cHeight;
   
   public Calliper(double x, double y, double w, double h)
   {
   
       cWidth = w;
       cHeight = h;
       cX1 = x;
       cY1 = y;
       cX2 = x;
       cY2 = y + (h/2);
       cX3 = x;
       cY3 = y + h;
       cX4 = x + w;
       cY4 = y;
       cX5 = cX4;
       cY5 = cY4 + (h /2);
       cX6 = cX4;
       cY6 = cY4 + h;
   
       build();
   
   }
   

2. build() method (used to draw the path) and setCalliper() method, used to redefine the coordinates, or width, height:

   private void build()
   {
   
       // Draw the path for the calliper
   
       moveTo(cX1, cY1);
       lineTo(cX2, cY2);
       lineTo(cX3, cY3);
       moveTo(cX2, cY2);
       lineTo(cX5, cY5);
       moveTo(cX4, cY4);
       lineTo(cX6, cY6);
   
   }
   
   public void setCalliper(double x, double y, double w, double h)
   {
       // Rebuild the calliper using different x,y coordinates, or 
       // different width/height
   
       cWidth = w;
       cHeight = h;
       cX1 = x;
       cY1 = y;
       cX2 = x;
       cY2 = y + (h/2);
       cX3 = x;
       cY3 = y + h;
       cX4 = x + w;
       cY4 = y;
       cX5 = cX4;
       cY5 = cY4 + (h /2);
       cX6 = cX4;
       cY6 = cY4 + h;
   
       build();
   }
   

I have created a class to draw this calliper on the screen, which it will do, however if I try to drag the calliper around the screen, it doesn't erase the original shape as I drag, so I get a long trail of shapes left behind. I thought I had omitted super.paintComponent(g) from my paintComponent(Graphics g) method, but even with it in there the code still does not work.

My drag method looks like this:

@Override
public void mouseDragged(MouseEvent ev) 
{

    double mx = ev.getX();
    double my = ev.getY();

    if (dragging)
    {
        calX = mx - offsetX;
        calY = my - offsetY;

        cal = setCalliper(calX, calY, calW, calH);
        repaint();
    }

}

If I change the line cal = setCalliper(calX, calY, calW, calH); above to read cal = new Calliper(calX, calY, calW, calH); then it works, but I have been told I shouldn't do it this way.

Any ideas why it doesn't work as expected?

Answer 1

The setCalliper() directly calls the build method, a method which appends new points to all the previous points added to the Path2D - so each time mouseDragged is called more points are added to the Path. Try calling reset() before calling build() (or call reset in the build method before the moveTo/lineTo calls).