Dessiner plusieurs graphes dans une interface

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Point;
import java.util.Random;
import javax.swing.JFrame;

public class Fourmi extends Thread
{
    private final Graphics gph;
    private final Dimension dim;
    private final Color clr;
    private Point pos;
    
    private static final Random random = new Random();

    public Fourmi(JFrame frame, Color color)
    {
        dim = frame.getSize();
        gph = frame.getGraphics();
        clr = color;
        pos = new Point(random.nextInt(dim.width), random.nextInt(dim.height));
    }
    
    @Override
    public void run()
    {
        while (true)
        {
            Point next;
            
            switch (random.nextInt(4))
            {
            case 0: // à gauche
                if (pos.x==0)
                    pos.x=dim.width;
                next = new Point(pos.x-1,pos.y);
                break;
                
            case 1: // à droite
                if (pos.x==dim.width)
                    pos.x=0;
                next = new Point(pos.x+1,pos.y);
                break;
                
            case 2: // en bas
                if (pos.y==0)
                    pos.y=dim.height;
                next = new Point(pos.x,pos.y-1);
                break;
                
            case 3: // en haut
                if (pos.y==dim.height)
                    pos.y=0;
                next = new Point(pos.x,pos.y+1);
                break;
            default:
                next=pos;
            }
            
            synchronized (gph)
            {
                gph.setColor(clr);
                gph.drawLine(pos.x,pos.y, next.x, next.y);
            }
            
            pos = next;
        }
    }

    public static void main(String[] args)
    {
        JFrame frame = new JFrame();
        frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
        frame.setVisible(true);
        frame.setResizable(false);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        new Fourmi(frame,Color.RED).start();
        new Fourmi(frame,Color.GREEN).start();
        new Fourmi(frame,Color.BLUE).start();
        new Fourmi(frame,Color.BLACK).start();
    }
}