Java - Centrar formulario en la pantalla

JFrame frame = new JFrame();

frame.pack();

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();

int height = screenSize.height;

int width = screenSize.width;

frame.setSize(width/2, height/2);

frame.setLocationRelativeTo(null);

frame.setVisible(true);

package ejercicios;

import java.awt.BorderLayout;

import java.awt.EventQueue;

import javax.swing.JFrame;

import javax.swing.JPanel;

import javax.swing.border.EmptyBorder;

import javax.swing.JButton;

import javax.swing.JTextArea;

import java.awt.event.ActionListener;

import java.awt.event.ActionEvent;

import java.awt.event.MouseAdapter;

import java.awt.event.MouseEvent;

import java.awt.Window.Type;

public class Prueba01 extends JFrame {

	private JPanel contentPane;

	/**

	 * Launch the application.

	 */

	public static void main(String[] args) {

		EventQueue.invokeLater(new Runnable() {

			public void run() {

				try {

					Prueba01 frame = new Prueba01();

					frame.setVisible(true);

				} catch (Exception e) {

					e.printStackTrace();

				}

			}

		});

	}

	/**

	 * Create the frame.

	 */

	public Prueba01() {

		setTitle("T\u00EDtulo de la ventana");

		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

		setBounds(100, 100, 450, 300);

		contentPane = new JPanel();

		contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));

		setContentPane(contentPane);

		contentPane.setLayout(null);

		JTextArea textArea = new JTextArea();

		// Ventana centro del escritorio.

		// ###############################################################

		frame.pack();

		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

		Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();

		int height = screenSize.height;

		int width = screenSize.width;

		frame.setSize(width/2, height/2);

		frame.setLocationRelativeTo(null);

		frame.setVisible(true);

		// ###############################################################

		JButton btnMostrar = new JButton("Mostrar");

		btnMostrar.addMouseListener(new MouseAdapter() {

			@Override

			public void mouseClicked(MouseEvent arg0) {

			}

		});

		btnMostrar.addActionListener(new ActionListener() {

			public void actionPerformed(ActionEvent e) {

				// Variable tipo String.

				String variable = "Hola mundo.";

				// Mostramos el contenido de la variable en el JTextArea.

				textArea.append(variable + "\n");

			}

		});

		btnMostrar.setBounds(165, 11, 89, 23);

		contentPane.add(btnMostrar);

		textArea.setBounds(10, 55, 414, 195);

		contentPane.add(textArea);

	}

}

Dimension dVentana = new Dimension(500, 500);

Dimension dPantalla = Toolkit.getDefaultToolkit().getScreenSize();

int posX = (int) (dPantalla.getWidth() - dVentana.getWidth()) / 2;

int posY = (int) (dPantalla.getHeight() - dVentana.getHeight()) / 2;

this.setLocation(posX, posY);

this.setSize(dVentana);