Java自定义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).