Question

How to move a triangle to a new location using mouse drag (which was previous drawn using mouse drag)?

...
java.util.List<Polygon> triangles = new LinkedList<Polygon>();
Point startDrag, endDrag, midPoint;
Polygon triangle;
...
public PaintSurface() {     
  this.addMouseListener(new MouseAdapter() {
  public void mousePressed(MouseEvent e) {
    startDrag = new Point(e.getX(), e.getY());
    endDrag = startDrag;
    repaint();
  }//end mousePressed   

public void mouseReleased(MouseEvent e) {
...
  int[] xs = { startDrag.x, endDrag.x, midPoint.x };
  int[] ys = { startDrag.y, startDrag.y, midPoint.y };      
  triangles.add( new Polygon(xs, ys,3));                    
  startDrag = null;
  endDrag   = null;
  repaint();
 }//end mouseReleased   
...


 });//end addMouseListener

  this.addMouseMotionListener(new MouseMotionAdapter() {

/* I dont know how to move (drag) the whole triangle to new location and later delete the previous drawn triangle. The mouseDragged method only draw a new triangle using mouse drag 🙁
*/

    public void mouseDragged(MouseEvent e) {
        endDrag = new Point(e.getX(), e.getY());
        repaint();
     }//end mouseDragged
        }//end paintSurface       

         //Draw triangles
         public void paint(Graphics g) {
           Graphics2D g2 = (Graphics2D) g;
           g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

           //draw the thickness of the line
           g2.setStroke(new BasicStroke(1));
           g2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 1.00f));        
           g2.setPaint(Color.black);//set the triangle color 
           for (Polygon triangle : triangles)  g2.drawPolygon(triangle);
             if (startDrag != null && endDrag != null) {
                g2.setPaint(Color.red);
                g2.drawPolygon(triangle);   
             }   
          }//end paint       

              }//end private class PaintSurface

Answer 1

when you start to drag you have to detect if your current mouse location is on one of the existing Polygons, also mark the starting location

When it is you dont add a new polygon, but you add the amount moved to the different points of the existing polygon and repaint