Sierpinski Triangle with animation

GUI.java

import javax.swing.*;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;
import java.awt.*;

/**
 * Draw a recursive 2D graphics
 *
 * @version Fall 2015
 * @author Hans Dulimarta
 */
public class GUI implements ChangeListener {

    private JFrame top;
    private JSlider sld;
    private SierpinskiTriangle g;

    public GUI() {
        top = new JFrame();
        g = new SierpinskiTriangle();
        top.add(g, BorderLayout.CENTER);
        sld = new JSlider(1, 7);
        sld.setValue(1);
        sld.setPaintTicks(true);
        sld.setMajorTickSpacing(1);
        sld.setSnapToTicks(true);
        sld.addChangeListener(this);

        g.changeDetails(1);

        top.add (sld, BorderLayout.SOUTH);
        top.pack();
        top.setVisible(true);
        top.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

    }

    @Override
    public void stateChanged(ChangeEvent e) {
        g.changeDetails(sld.getValue());
    }

    public static void main(String[] args) {
        new GUI();
    }

}

SierpinskiTriangle.java

import javax.swing.*;
import javax.swing.Timer;
import javax.swing.border.BevelBorder;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.*;
import java.util.List;

/**
 * A simple panel for drawing the Sierpinski Triangle
 *
 * @version Fall 2015
 * @author Hans Dulimarta
 */
public class SierpinskiTriangle extends JPanel implements ActionListener {
    private static final int SIZE = 500;
    private static final int MARGIN = 40;
    private int height, side, subdivide;
    private Timer ticker;
    private ArrayList<int[]> x_coordQueue, y_coordQueue;
    private ArrayList<Color> colorQueue;
    int x1, y1, x2, y2, x3, y3, lastItemToDraw;

    public SierpinskiTriangle() {
        setPreferredSize(new Dimension(SIZE, SIZE));
        setBorder(BorderFactory.createBevelBorder(BevelBorder.LOWERED));
        height = SIZE - 2 * MARGIN;
        side = (int) (height / Math.sin(Math.toRadians(60.0)));
        x1 = SIZE / 2;
        y1 = MARGIN;
        x2 = (SIZE/2) - (side/2);
        y2 = SIZE - MARGIN;
        x3 = (SIZE/2) + (side/2);
        y3 = SIZE - MARGIN;
        subdivide = 1; /* use this variable for recursive subdivision */
        ticker = new Timer(200, this);
        x_coordQueue = new ArrayList<int[]>();
        y_coordQueue = new ArrayList<int[]>();
        colorQueue = new ArrayList<Color>();
    }

    public void changeDetails (int N) {
        subdivide = N;
        x_coordQueue.clear();
        y_coordQueue.clear();
        colorQueue.clear();
        drawTriangle(x1, y1, x2, y2, x3, y3, Color.ORANGE);
        lastItemToDraw = 0;
        ticker.setDelay(100/N);
        ticker.start();
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        for (int k = 0; k < lastItemToDraw; k++) {
            g.setColor(colorQueue.get(k));
            g.fillPolygon(x_coordQueue.get(k), y_coordQueue.get(k), 3);
        }
    }

    private void drawTriangle (int x1, int y1,
                               int x2, int y2,
                               int x3, int y3, Color c)
    {
        /* push the drawing information to a list */
        x_coordQueue.add(new int[]{x1, x2, x3});
        y_coordQueue.add(new int[]{y1, y2, y3});
        colorQueue.add(c);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (lastItemToDraw < x_coordQueue.size()) {
            repaint(); /* redraw using the new subdivision level */
            lastItemToDraw++;
        }
        else
            ticker.stop();
    }
}