import javax.swing.*;
import java.awt.*;
import java.awt.image.BufferedImage;

public class InternalIcon {

    private JFrame frame;
    private JInternalFrame jif;

    public static void main(String[] args) throws Exception {
        final InternalIcon test = new InternalIcon();
        SwingUtilities.invokeAndWait(new Runnable() {
            @Override
            public void run() {
                test.setupUI();
            }
        });
    }

    private void setupUI() {
        frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JDesktopPane desktop = new JDesktopPane();
        frame.add(desktop);

        jif = new JInternalFrame("Internal Frame", true, true, true, true);
        jif.setBounds(20, 20, 200, 100);
        desktop.add(jif);

        BufferedImage bi = new BufferedImage(16, 16, BufferedImage.TYPE_INT_RGB);
        Graphics2D g = (Graphics2D) bi.getGraphics();

        g.setColor(Color.BLACK);
        g.fillRect(0, 0, bi.getWidth(), bi.getHeight());
        g.setColor(Color.YELLOW);
        g.drawLine(0, bi.getHeight(), bi.getWidth(), 0);
        jif.setFrameIcon(new ImageIcon(bi));

        jif.setVisible(true);

        Dimension screen = Toolkit.getDefaultToolkit().getScreenSize();
        frame.setBounds((screen.width - 400) / 2, (screen.height - 400) / 2, 400, 400);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

}