Java Code:
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Container;
import java.awt.Graphics;
import java.awt.Polygon;

import javax.swing.Icon;
import javax.swing.JButton;
import javax.swing.JFrame;

public class ChangingButtonIconTest {
  static class TriangleIcon implements Icon {
    String name;

    static class State {
      public static final int NORMAL = 0;

      public static final int PRESSED = 1;

      public static final int ROLLOVER = 2;

      private State() {
      }
    }

    int state;

    Color color;

    public TriangleIcon(Color c, int state) {
      color = c;
      this.state = state;
    }

    public int getIconWidth() {
      return 20;
    }

    public int getIconHeight() {
      return 20;
    }

    public void paintIcon(Component c, Graphics g, int x, int y) {
      g.setColor(color);
      Polygon p = new Polygon();
      if (state == State.NORMAL) {
        p.addPoint(x + (getIconWidth() / 2), y);
        p.addPoint(x, y + getIconHeight() - 1);
        p.addPoint(x + getIconWidth() - 1, y + getIconHeight() - 1);
      } else if (state == State.PRESSED) {

        p.addPoint(x, y);
        p.addPoint(x + getIconWidth() - 1, y);
        p.addPoint(x + (getIconWidth() / 2), y + getIconHeight() - 1);
      } else {
        p.addPoint(x, y);
        p.addPoint(x, y + getIconHeight() - 1);
        p.addPoint(x + getIconWidth() - 1, y + (getIconHeight() / 2));
      }
      g.fillPolygon(p);
    }  }

  public static void main(String args[]) {
    JFrame frame = new JFrame();
    Container contentPane = frame.getContentPane();
    Icon normalIcon = new TriangleIcon(Color.red, TriangleIcon.State.NORMAL);
    Icon pressedIcon = new TriangleIcon(Color.red,
        TriangleIcon.State.PRESSED);
    Icon rolloverIcon = new TriangleIcon(Color.red,
        TriangleIcon.State.ROLLOVER);
    JButton b = new JButton("Button", normalIcon);
    b.setPressedIcon(pressedIcon);
    b.setRolloverIcon(rolloverIcon);
    b.setRolloverEnabled(true);
    contentPane.add(b, BorderLayout.NORTH);
    frame.setSize(300, 100);
    frame.show();
  }
}