Ola Mundo !

package Teste;

// Bibliotecas
import java.awt.Color;

public class A extends javax.swing.JFrame
{
    // Construtor
    public A()
    {
        initComponents();// Inicializando componentes
        config();// Configurações
    }

    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {

        lblMensagem = new javax.swing.JLabel();
        btnClick = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("OLÁ");
        setResizable(false);

        lblMensagem.setFont(new java.awt.Font("Tahoma", 0, 18)); // NOI18N
        lblMensagem.setForeground(new java.awt.Color(0, 0, 153));
        lblMensagem.setText("Aqui vai aparecer a mensagem");
        lblMensagem.setCursor(new java.awt.Cursor(java.awt.Cursor.DEFAULT_CURSOR));

        btnClick.setForeground(new java.awt.Color(255, 0, 0));
        btnClick.setText("Clique em Mim");
        btnClick.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnClickActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addContainerGap()
                        .addComponent(lblMensagem, javax.swing.GroupLayout.PREFERRED_SIZE, 256, javax.swing.GroupLayout.PREFERRED_SIZE))
                    .addGroup(layout.createSequentialGroup()
                        .addGap(75, 75, 75)
                        .addComponent(btnClick)))
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addContainerGap(20, Short.MAX_VALUE)
                .addComponent(lblMensagem, javax.swing.GroupLayout.PREFERRED_SIZE, 33, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(18, 18, 18)
                .addComponent(btnClick, javax.swing.GroupLayout.PREFERRED_SIZE, 32, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(23, 23, 23))
        );

        pack();
    }// </editor-fold>

    private void btnClickActionPerformed(java.awt.event.ActionEvent evt) {

        // Mudança de texto no Lable
        lblMensagem.setText(String.format("Olá, Mundo !!!"));
    }

    // Método Principal
    public static void main(String args[])
    {
        try
        {
            for(javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels())
            {
                if("Nimbus".equals(info.getName()))
                {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        }

        catch(ClassNotFoundException | InstantiationException | IllegalAccessException | javax.swing.UnsupportedLookAndFeelException ex)
        {
            java.util.logging.Logger.getLogger(A.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }

        java.awt.EventQueue.invokeLater(() -> {
            new A().setVisible(true);
        });
    }
    // Variables declaration - do not modify
    private javax.swing.JButton btnClick;
    private javax.swing.JLabel lblMensagem;
    // End of variables declaration

    // Método: Configurações
    private void config()
    {
        // Frame
        this.setTitle("Olá");// Título
        this.setLocationRelativeTo(null);// Centralizado
        this.getContentPane().setBackground(Color.green);// Tela com fundo verde
    }
}