[Executável] - Contador de caracteres(Java Version)

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JTextField;

public class HelloWorld {

    public static void programStarter()
    {
        JFrame window = new JFrame("Aplicativo Willian");
        final JTextField txb = new JTextField("Digite seu texto aqui");
        JButton btn = new JButton("Continuar");

        MouseListener evento_txb = new MouseListener() {

            @Override
            public void mouseReleased(MouseEvent e) {
            }

            @Override
            public void mousePressed(MouseEvent e) {
            }

            @Override
            public void mouseExited(MouseEvent e) {
                if(txb.getText().isEmpty())
                {
                    txb.setText("Digite seu texto aqui");
                }
            }

            @Override
            public void mouseEntered(MouseEvent e) {
            }

            @Override
            public void mouseClicked(MouseEvent e) {
                if(txb.getText().contains("Digite seu texto aqui"))
                {
                    txb.setText(null);
                }
                else if(!txb.getText().isEmpty())
                {
                    txb.selectAll();
                }
            }
        };
        ActionListener evento_btn = new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                if(!txb.getText().isEmpty())
                {
                    JOptionPane.showMessageDialog(new JFrame(), txb.getText().length());
                }
                else
                {
                    JOptionPane.showMessageDialog(new JFrame(), "Digite algo no campo de texto!");
                }

            }
        };

        btn.addActionListener(evento_btn);
        txb.addMouseListener(evento_txb);

        //-------------------------------------------------

        window.add(txb,BorderLayout.CENTER);
        window.add(btn,BorderLayout.EAST);
        window.setBounds(1,1,300,80);
        window.setVisible(true);
    }
    public static void main(String[] args) {
        programStarter();
    }
}