在Java中无法创建多个多边形 - 只有一个 [英] Cannot create multiple polygons in Java - only one

问题描述

由于以下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 */ }
}

我只能得出每次一个多边形，这意味着 - 当我尝试启动一个新的多边形
老字号多边形消失，但我不明白为什么。

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 .

那么，怎样才能我画多个多边形？

So how can I draw multiple polygons ?

是什么原因导致旧的多边形消失？我想也许是由于重绘（），但我想没有它，但它并没有帮助。

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

我想AP preciate你的帮助

I'd appreciate your help

推荐答案

多边形确实正在通过调用 points.clear删除（）。为了解决这个问题，你可以使用的多边形了解在一个单独的列表这可能与正在进行的多边形一起画previous多边形类。这在风俗画途径概述。

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.

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