Deixar Frame Transparente

package interfaces;

import com.sun.awt.AWTUtilities;
import java.awt.Color;
import java.awt.Container;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JRootPane;

public class teste extends JFrame {

    private JButton fechar, mostrar;
    private boolean x;

    public teste() {
        final Container tela = getContentPane();
        tela.setLayout(null);
        this.setResizable(false);
        this.setTitle("Teste");
        this.setBounds(0, 0, 400, 400);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        this.getContentPane().setLayout(null);
        this.setUndecorated(true);
        this.getRootPane().setWindowDecorationStyle(JRootPane.FRAME);
        this.setLocationRelativeTo(null);

        fechar = new JButton();
        fechar.setOpaque(false);
        fechar.setBounds(0, 0, 200, 200);
        fechar.setText("Fechar");
        fechar.setFont(new Font("Arial", Font.BOLD, 12));
        fechar.setBackground(new Color(190, 190, 190));
        this.add(fechar);

        mostrar = new JButton();
        mostrar.setOpaque(false);
        mostrar.setBounds(200, 0, 200, 200);
        mostrar.setText("Ocultar");
        mostrar.setFont(new Font("Arial", Font.BOLD, 12));
        mostrar.setBackground(new Color(190, 190, 190));
        this.add(mostrar);


        mostrar.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                mostrar();
            }
        });
        fechar.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                fechar();
            }
        });

    }

    public void fechar() {
        System.exit(0);
    }

    public void mostrar() {

        if (x == false) {
            AWTUtilities.setWindowOpacity(this, 1f);//Deixa 100% visível
            mostrar.setText("Ocultar");
            x = true;
        } else {
            AWTUtilities.setWindowOpacity(this, 0.5f);   //deixa 50% transparente
            //AWTUtilities.setWindowOpacity(this, 0.01f);// Deixa 99% transparente
            mostrar.setText("Mostrar");
            x = false;
        }
    }
}

AWTUtilities.setWindowOpacity(this, 0.5f);