dynamic Drawing -->Vector ?

[Raschin Ghanad]

I have a basic question about painting.
When I want to add dynamically lines,
is it possible to append everytime one line to my Graphic Context?

I tried so, but everytime it repaints(), all previouse lines got lost.
So I stored them all in a Vector, and each time the Panel repaints,
it gets the whole data from the Vector to draw.

The Problem is now,that I sometimes get a NullPointer Exception, but I don't know exactly why.
So, I think it's not a good solution for my Problem.

I have to cast each Element of my Vector to a Basic type, that I've defined in as an extra Interface. This Basic-Type just has the method paint().

Otherwise it was'nt possible to draw the elements of my Vector.
The elements were just Line-Objects (I also wrote a class Line, that implements Basic)

I did it like this:

 
    for (Iterator iter = elements.iterator(); iter.hasNext()   :wink:    
                ((Basic) iter.next()).paint( g );    //Error : Compiler stops here!
 
 

and got java.lang.NullPointerException

This appears only sometimes, I don'tknow exactly why...It seems there is a rerference to an object that does not exists?


The Code how I build this Vector:
(The function gets a Vector filled with Point-Objects)

  public void addCurve(Vector v) {
    int[] x = new int[v.size()+1];
    int[] y = new int[v.size()+1];
    int i = 0;
    
    for (Iterator iter = v.iterator(); iter.hasNext()  :wink:   {
          Point p = (Point) iter.next();
          x[i] = (int) p.getX();
          y[i] = (int) p.getY();                                    
          i++;
    }   
    for(int u=0;u<i;u++) {
      System.out.println("X:" + x[u]);
      elements.add(new Line(x[u],y[u],x[u+1],y[u+1]));      
    }    
        
  }

and for completion, here is the "Line" Class I've defined:

 public class Line implements Basic {
 
  private int x1, y1, x2, y2;
  
  public Line(int x1, int y1, int x2, int y2) {
        this.x1 = x1; this.y1 = y1;
        this.x2 = x2; this.y2 = y2;
  }
  
  public void paint( Graphics g) {      
      g.drawLine(x1, y1, x2, y2);
  }
  
}
 

[ July 29, 2004: Message edited by: Raschin Ghanad ]
[ July 29, 2004: Message edited by: Raschin Ghanad ]

[Craig Wood]

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.util.*;
import java.util.List;
import javax.swing.*;
 
public class LineState
{
    public static void main(String[] args)
    {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(new LinePanel());
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}
 
class LinePanel extends JPanel
{
    List lineList;
    Random seed;
 
    public LinePanel()
    {
        lineList = new ArrayList();
        seed = new Random();
        addMouseListener(new LineWrangler(this));
        setBackground(Color.white);
    }
 
    protected void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        for(int j = 0; j < lineList.size(); j++)
            g2.draw((Line2D)lineList.get(j));
    }
 
    public void addLine()
    {
        int w = getWidth();
        int h = getHeight();
        int x1 = seed.nextInt(w);
        int y1 = seed.nextInt(h);
        int x2 = seed.nextInt(w);
        int y2 = seed.nextInt(h);
        Line2D line = new Line2D.Double(x1, y1, x2, y2);
        lineList.add(line);
        repaint();
    }
}
 
class LineWrangler extends MouseAdapter
{
    LinePanel linePanel;
 
    public LineWrangler(LinePanel lp)
    {
        linePanel = lp;
    }
 
    public void mousePressed(MouseEvent e)
    {
        linePanel.addLine();
    }
}

[ July 29, 2004: Message edited by: Craig Wood ]