Exemplo JInternalFrame

public class NewClass extends javax.swing.JInternalFrame {
    
    /** Creates a new instance of NewClass */
    public NewClass() {
        setClosable(true);
        setIconifiable(true);
        setMaximizable(true);
        setResizable(true);
        setVisible(true);
        org.jdesktop.layout.GroupLayout jInternalFrame1Layout = new org.jdesktop.layout.GroupLayout(getContentPane());
        getContentPane().setLayout(jInternalFrame1Layout);
        jInternalFrame1Layout.setHorizontalGroup(
            jInternalFrame1Layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
            .add(0, 870, Short.MAX_VALUE)
        );
        jInternalFrame1Layout.setVerticalGroup(
            jInternalFrame1Layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
            .add(0, 521, Short.MAX_VALUE)
        );
    }
    
}

public class Gui_Principal extends javax.swing.JFrame {
    
    /** Creates new form Gui_Principal */
    public Gui_Principal() {
        initComponents();
        NewClass nc = new NewClass();
        nc.setVisible(true);
    }
    
    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
     */
    // <editor-fold defaultstate="collapsed" desc=" Generated Code ">//GEN-BEGIN:initComponents
    private void initComponents() {
        jMenuBar1 = new javax.swing.JMenuBar();
        jMenu1 = new javax.swing.JMenu();
        jMenu2 = new javax.swing.JMenu();
        jMenu3 = new javax.swing.JMenu();
        jMenu4 = new javax.swing.JMenu();
        jMenu5 = new javax.swing.JMenu();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setExtendedState(6);
        jMenu1.setText("Programa");
        jMenuBar1.add(jMenu1);

        jMenu2.setText("Consultas");
        jMenuBar1.add(jMenu2);

        jMenu3.setText("Cadastros");
        jMenuBar1.add(jMenu3);

        jMenu4.setText("Usu\u00e1rios");
        jMenuBar1.add(jMenu4);

        jMenu5.setText("Ajuda");
        jMenuBar1.add(jMenu5);

        setJMenuBar(jMenuBar1);

        org.jdesktop.layout.GroupLayout layout = new org.jdesktop.layout.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
            .add(0, 1016, Short.MAX_VALUE)
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
            .add(0, 683, Short.MAX_VALUE)
        );
        setBounds(0, 0, 1024, 738);
    }// </editor-fold>//GEN-END:initComponents
    
    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    javax.swing.UIManager.setLookAndFeel("com.jgoodies.looks.windows.WindowsLookAndFeel");
                } catch (Exception e) {
                    System.out.println("Não foi possível carregar o Skin padrão.");
                }
                new Gui_Principal().setVisible(true);
            }
        });
    }
    
    // Variables declaration - do not modify//GEN-BEGIN:variables
    private javax.swing.JMenu jMenu1;
    private javax.swing.JMenu jMenu2;
    private javax.swing.JMenu jMenu3;
    private javax.swing.JMenu jMenu4;
    private javax.swing.JMenu jMenu5;
    private javax.swing.JMenuBar jMenuBar1;
    // End of variables declaration//GEN-END:variables
    
}