Java - JFrame con bordes redondeados

public class JframeDialogo extends javax.swing.JFrame {

    public JframeDialogo() {

//este es el codigo que me permite redondear los bordes.

       this.setUndecorated(true);

        Shape forma = new RoundRectangle2D.Double(0, 0, this.getBounds().width, this.getBounds().height, 30, 30);

        AWTUtilities.setWindowShape(this, forma);

 inicializarComponentes();   // inicializamos los atributos o componentes

    }

}

this.setUndecorated(true);

this.setSize(400, 300);

Shape forma = new RoundRectangle2D.Double(0, 0, this.getBounds().width, this.getBounds().height, 30, 30);

AWTUtilities.setWindowShape(this, forma);

JFrame frame = new JFrame();

frame.setUndecorated(true);

frame.setSize(400, 300);

Shape forma = new RoundRectangle2D.Double(0, 0, frame.getBounds().width, frame.getBounds().height, 30, 30);

frame.setShape(forma);

import java.awt.FlowLayout;

import java.awt.Shape;

import java.awt.event.ActionEvent;

import java.awt.event.ActionListener;

import java.awt.geom.RoundRectangle2D;

import javax.swing.JButton;

import javax.swing.JFrame;

public class MarcoRedondeado{

	private JButton btCerrar;

	public MarcoRedondeado() {

		btCerrar = new JButton("Cerrar");

		btCerrar.addActionListener(new ActionListener() {

			@Override

			public void actionPerformed(ActionEvent e) {

				System.exit(0);

			}

		});

		JFrame marco = new JFrame();

		marco.setLayout(new FlowLayout());

		marco.add(btCerrar);

		marco.setUndecorated(true);

		marco.setSize(400, 300);

		Shape forma = new RoundRectangle2D.Double(0, 0, marco.getBounds().width, marco.getBounds().height, 30, 30);

		marco.setShape(forma);

		marco.setLocationRelativeTo(null);

		marco.setVisible(true);

	}

	public static void main(String[] args) {

		new MarcoRedondeado();

	}

}