Cannot create multiple polygons in Java - only one

问题

Given the following code :

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import java.util.ArrayList;
import java.util.Iterator;
import javax.swing.JFrame;
import javax.swing.JPanel;


/**
 * 
 * @author X2
 *
 */
public class PolygonnerJframe
{
    public static void main (String[] args)
    {
     JFrame frame = new JFrame("Draw polygons");
     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     frame.setContentPane(new DrawingPanel());
     frame.pack();
     frame.setVisible(true);
 }
}




/**
 * Main class
 * @author X2
 *
 */
class DrawingPanel extends JPanel implements MouseListener, MouseMotionListener
{
    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    private static final Dimension MIN_DIM = new Dimension(300, 300);
    private static final Dimension PREF_DIM = new Dimension(500, 500);
    private boolean polygonDone = false;
    private final Point trackPoint = new Point(); // The 'dummy' point tracking the mouse
    private ArrayList<Point> points = new ArrayList<Point>(); // The list of points making up a polygon
    private ArrayList<Point> helper = new ArrayList<Point>(); // The list of points making up a polygon


    public ArrayList<Point> copyCreate(ArrayList<Point> input , ArrayList<Point> output)
    {
        int i = 0;
        if (output == null)
            output = new ArrayList<Point>();
        while (i < input.size())
        {
            output.add(input.get(i));
            i++;
        }
        return output;
    }




    /**
     * Setting the dimensions of the windows
     */
    public Dimension getMinimumSize() { return MIN_DIM; }

    public Dimension getPreferredSize() { return PREF_DIM; }



    /**
     *  The only constructor needed for this class
     */
    DrawingPanel()
    {
        super();
        addMouseListener(this);
        addMouseMotionListener(this);
    }



    /**
     *  The drawing itself 
     */
    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);

        int numPoints = points.size();
        if (numPoints == 0)
            return; // nothing to draw


        Point prevPoint = (Point) points.get(0);

        // draw polygon
        Iterator<Point> it = points.iterator();
        while (it.hasNext())
        {
            Point curPoint = (Point) it.next();
            draw(g, prevPoint, curPoint);           
            prevPoint = curPoint;
        }

        // now draw tracking line or complete the polygon
        if (polygonDone == true)
        {
            draw(g, prevPoint, (Point) points.get(0));
        }

        else  // polygonDone == false
            draw(g, prevPoint, trackPoint); 

    }





    /**
     * MouseListener interface 
     */
    public void mouseClicked(MouseEvent evt)
    {
        int x = evt.getX();
        int y = evt.getY();

        switch (evt.getClickCount())
        {
            case 1: // single-click
                if (polygonDone == true)
                {
                    this.helper = this.copyCreate(this.points, this.helper);  // copy the new coordinates into the helper 
                    points.clear();
                    polygonDone = false;
                }
                points.add(new Point(x, y));
                repaint();
                break;

            case 2: // double-click
                polygonDone = true;
                points.add(new Point(x, y));
                // repaint();
                break;

            default: // ignore anything else
                break;
        }
    }






    /**
     * MouseMotionListener interface 
     */
    public void mouseMoved(MouseEvent evt)
    {
        trackPoint.x = evt.getX();
        trackPoint.y = evt.getY();
        repaint();
    }



    /**
     * draw points and lines 
     * @param g
     * @param p1
     * @param p2
     */
    private void draw(Graphics g, Point p1, Point p2)
    {
        int x1 = p1.x;
        int y1 = p1.y;

        int x2 = p2.x;
        int y2 = p2.y;

        // draw the line first so that the points
        // appear on top of the line ends, not below
        g.setColor(Color.green.darker());
        g.drawLine(x1 + 3, y1 + 3, x2 + 3, y2 + 3);
        g.drawLine(x1 + 4, y1 + 4, x2 + 4, y2 + 4);
        g.drawLine(x1 + 5, y1 + 5, x2 + 5, y2 + 5);

        g.setColor(Color.green);
        g.fillOval(x1, y1, 8, 8);

        g.setColor(Color.black);
        g.fillOval(x2, y2, 8, 8);
    }





    public void mouseDragged(MouseEvent evt) { /* EMPTY */ }

    public void mousePressed(MouseEvent evt) { /* EMPTY */ }

    public void mouseReleased(MouseEvent evt) { /* EMPTY */ }

    public void mouseEntered(MouseEvent evt) { /* EMPTY */ }

    public void mouseExited(MouseEvent evt) { /* EMPTY */ }
}

I can only draw one polygon each time , meaning - when I try to start a new polygon the "old" polygon vanishes , but I don't understand why .

So how can I draw multiple polygons ?

What causes the old polygon to vanish ? I thought maybe due to repaint() , but I tried without it but it didn't help .

I'd appreciate your help

回答1:

The polygons are indeed being erased by calling points.clear(). To counter this, you could to maintain co-ordinate information using the Polygon class about previous polygons in a separate List which could be painted along with the "in-progress" polygon. This is outlined in Custom Painting Approaches.

来源：https://stackoverflow.com/questions/15649228/cannot-create-multiple-polygons-in-java-only-one