Como chmar uma outra janela de dentro da principal

package Interface;

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;


public class Principal extends JFrame {  
   
    public Principal () {  
    super ("Principal");  
    initComponents();
    
    } 
    /*
    desktop = new JDesktopPane();
    createFrame(); //Create first window
    setContentPane(desktop);

    desktop.setDragMode(JDesktopPane.OUTLINE_DRAG_MODE);

    protected void createFrame() {
    MyInternalFrame frame = new MyInternalFrame();
    frame.setVisible(true);
    desktop.add(frame);
    try {
        frame.setSelected(true);
    } catch (java.beans.PropertyVetoException e) {}
}
  */
    private void initComponents() {
    	JMenuBar menuBar = new JMenuBar();
    	JMenu arquivo = new javax.swing.JMenu("Arquivo");
    	JMenuItem sair = new javax.swing.JMenuItem("Sair");
    	sair.addActionListener(new ActionListener(){  
    		public void actionPerformed(ActionEvent evt){  
    		System.exit(0);  
    		}  
    	});
    	
    	JMenu cadastro = new javax.swing.JMenu("Cadastro");
    	JMenuItem cliente = new javax.swing.JMenuItem("Cliente");
    	cliente.addActionListener(new ActionListener() {
    		public void actionPerformed(ActionEvent arg0) {
				
				
			}
		});
		
    	this.setJMenuBar(menuBar);
    	menuBar.add(arquivo);
    	arquivo.add(sair);
    	menuBar.add(cadastro);
    	cadastro.add(cliente);
    	  	            
    }
            
	public static void main (String [] args) {  
    	Principal janela = new Principal ();
        janela.setExtendedState(JFrame.MAXIMIZED_BOTH);
        janela.setVisible(true);
        
	
	}
	
}      

dessa dai eu queria chamar essa daqui

package Interface;

public class JanelaCadastroCliente extends javax.swing.JPanel {

    
    public JanelaCadastroCliente() {
        initComponents();
    }

    private void initComponents() {

        TelaCadastro = new javax.swing.JPanel();
        Codigo = new javax.swing.JLabel();
        Nome = new javax.swing.JLabel();
        Dtnasc = new javax.swing.JLabel();
        Cpf = new javax.swing.JLabel();
        Doc_id = new javax.swing.JLabel();
        Rua = new javax.swing.JLabel();
        Bairro = new javax.swing.JLabel();
        Cidade = new javax.swing.JLabel();
        Telfix = new javax.swing.JLabel();
        Telcel = new javax.swing.JLabel();
        jTextField5 = new javax.swing.JTextField();
        jTextField6 = new javax.swing.JTextField();
        jTextField7 = new javax.swing.JTextField();
        jTextField8 = new javax.swing.JTextField();
        jTextField9 = new javax.swing.JTextField();
        jTextField10 = new javax.swing.JTextField();
        jTextField11 = new javax.swing.JTextField();
        jTextField12 = new javax.swing.JTextField();
        jTextField13 = new javax.swing.JTextField();
        jFormattedTextField1 = new javax.swing.JFormattedTextField();

        Codigo.setText("Código:");
        Nome.setText("Nome:");
        Dtnasc.setText("Data de Nascimento:");
        Cpf.setText("CPF:");
        Doc_id.setText("ID:");
        Rua.setText("Rua:");
        Bairro.setText("Bairro");
        Cidade.setText("Cidade:");
        Telfix.setText("Telefone Fixo:");
        Telcel.setText("Telefone Celular");

        jTextField5.setFont(new java.awt.Font("Tahoma", 0, 10)); // NOI18N
        jTextField6.setFont(new java.awt.Font("Tahoma", 0, 10)); // NOI18N
        jTextField7.setFont(new java.awt.Font("Tahoma", 0, 10)); // NOI18N
        jTextField8.setFont(new java.awt.Font("Tahoma", 0, 10)); // NOI18N
        jTextField9.setFont(new java.awt.Font("Tahoma", 0, 10)); // NOI18N
        jTextField10.setFont(new java.awt.Font("Tahoma", 0, 10)); // NOI18N
        jTextField10.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jTextField10ActionPerformed(evt);
            }
        });

        jTextField11.setFont(new java.awt.Font("Tahoma", 0, 10)); // NOI18N

        jTextField12.setFont(new java.awt.Font("Tahoma", 0, 10)); // NOI18N

        jTextField13.setFont(new java.awt.Font("Tahoma", 0, 10)); // NOI18N

        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(TelaCadastro);
        TelaCadastro.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(jPanel1Layout.createSequentialGroup()
                        .addComponent(Telfix)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(jTextField9, javax.swing.GroupLayout.PREFERRED_SIZE, 87, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(Telcel)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(jTextField11, javax.swing.GroupLayout.PREFERRED_SIZE, 87, javax.swing.GroupLayout.PREFERRED_SIZE))
                    .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
                        .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING)
                            .addGroup(javax.swing.GroupLayout.Alignment.LEADING, jPanel1Layout.createSequentialGroup()
                                .addComponent(Rua)
                                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                                .addComponent(jTextField6, javax.swing.GroupLayout.DEFAULT_SIZE, 221, Short.MAX_VALUE))
                            .addGroup(javax.swing.GroupLayout.Alignment.LEADING, jPanel1Layout.createSequentialGroup()
                                .addComponent(Cidade)
                                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                                .addComponent(jTextField5, javax.swing.GroupLayout.PREFERRED_SIZE, 207, javax.swing.GroupLayout.PREFERRED_SIZE))
                            .addGroup(javax.swing.GroupLayout.Alignment.LEADING, jPanel1Layout.createSequentialGroup()
                                .addComponent(Nome)
                                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                                .addComponent(jTextField7, javax.swing.GroupLayout.DEFAULT_SIZE, 213, Short.MAX_VALUE)))
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                        .addComponent(Bairro)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(jTextField12, javax.swing.GroupLayout.DEFAULT_SIZE, 221, Short.MAX_VALUE))
                    .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING, false)
                        .addGroup(javax.swing.GroupLayout.Alignment.LEADING, jPanel1Layout.createSequentialGroup()
                            .addComponent(Codigo)
                            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                            .addComponent(jTextField10, javax.swing.GroupLayout.PREFERRED_SIZE, 22, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addGap(20, 20, 20)
                            .addComponent(Dtnasc)
                            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                            .addComponent(jFormattedTextField1))
                        .addGroup(javax.swing.GroupLayout.Alignment.LEADING, jPanel1Layout.createSequentialGroup()
                            .addComponent(Cpf)
                            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                            .addComponent(jTextField8, javax.swing.GroupLayout.PREFERRED_SIZE, 120, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                            .addComponent(Doc_id)
                            .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                            .addComponent(jTextField13, javax.swing.GroupLayout.PREFERRED_SIZE, 120, javax.swing.GroupLayout.PREFERRED_SIZE))))
                .addContainerGap())
        );
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addGap(21, 21, 21)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(Codigo)
                    .addComponent(Dtnasc)
                    .addComponent(jTextField10, javax.swing.GroupLayout.PREFERRED_SIZE, 15, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(jFormattedTextField1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(Nome)
                    .addComponent(jTextField7, javax.swing.GroupLayout.PREFERRED_SIZE, 15, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(Cpf)
                    .addComponent(jTextField8, javax.swing.GroupLayout.PREFERRED_SIZE, 15, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(Doc_id)
                    .addComponent(jTextField13, javax.swing.GroupLayout.PREFERRED_SIZE, 15, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addGap(6, 6, 6)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(Rua)
                    .addComponent(jTextField6, javax.swing.GroupLayout.PREFERRED_SIZE, 15, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(Bairro)
                    .addComponent(jTextField12, javax.swing.GroupLayout.PREFERRED_SIZE, 15, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(Cidade)
                    .addComponent(jTextField5, javax.swing.GroupLayout.PREFERRED_SIZE, 15, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(Telfix)
                    .addComponent(jTextField9, javax.swing.GroupLayout.PREFERRED_SIZE, 15, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(Telcel)
                    .addComponent(jTextField11, javax.swing.GroupLayout.PREFERRED_SIZE, 15, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addContainerGap(25, Short.MAX_VALUE))
        );

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(this);
        this.setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(TelaCadastro, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(270, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(TelaCadastro, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(118, Short.MAX_VALUE))
        );
    }

private void jTextField10ActionPerformed(java.awt.event.ActionEvent evt) {

}

    private javax.swing.JFormattedTextField jFormattedTextField1;
    private javax.swing.JLabel Codigo;
    private javax.swing.JLabel Nome;
    private javax.swing.JLabel Dtnasc;
    private javax.swing.JLabel Cpf;
    private javax.swing.JLabel Doc_id;
    private javax.swing.JLabel Rua;
    private javax.swing.JLabel Bairro;
    private javax.swing.JLabel Cidade;
    private javax.swing.JLabel Telfix;
    private javax.swing.JLabel Telcel;
    private javax.swing.JPanel TelaCadastro;
    private javax.swing.JTextField jTextField10;
    private javax.swing.JTextField jTextField11;
    private javax.swing.JTextField jTextField12;
    private javax.swing.JTextField jTextField13;
    private javax.swing.JTextField jTextField5;
    private javax.swing.JTextField jTextField6;
    private javax.swing.JTextField jTextField7;
    private javax.swing.JTextField jTextField8;
    private javax.swing.JTextField jTextField9;

}

Na linha 49 (evento de botão de "Cadastro->Cliente" adicione:

Vini, tudo bem contigo?
Essa questão do nosso amigo você já respondeu.
Eu sou iniciante em programação e estou tentando algo parecido.
Eu quero de um formulário “A”, ao clicar no envento de um botão, abrir um formulário “B”, porém, com os mesmos dados da tela em que estou no formulário “A”, visto que as informações do BD são as mesmas.
Não estou conseguindo, pois ao abrir o form “B” vai para o primeiro registro e aí há de se fazer nova pesquisa para se chegar aos dados que estavam abertos no form “A”.
Obrigado pela ajuda.
Pedro

Nesse caso, veja esse exemplo: http://www.guj.com.br/java/55710-jdialog-devolvendo-valor-pra-jinternalframe#292687

JMenu cadastro = new javax.swing.JMenu("Cadastro");
    	JMenuItem cliente = new javax.swing.JMenuItem("Cliente");
    	cliente.addActionListener(new ActionListener() {
    		public void actionPerformed(ActionEvent arg0) {
    		new JanelaCadastroCliente().setVisible(true);  		
				
			}
		});

não adianta quando eu clicko não chama a outra tela

Assim é para funcionar. Eu testei aqui e funciona.

public void actionPerformed(ActionEvent evt) {
new JanelaCadastroCliente().setVisible(true);

		}

mesmo desse ultimo jeito mudando para (evt) não deu certo aqui

Acredito ser o problema em seu código, porque eu peguei o seu exemplo de Tela Principal e coloquei um formulário de uma aplicação minha e abriu normalmente no menu cadastroClientes.

Então o problema está no JanelaCadastroCliente.