GridLayout的問題

Question

我想用Java創建一個表單，但是我得到了一個NullPointerException。這是我的代碼：

import java.awt.*; 
import java.awt.event.*; 
import javax.swing.*; 

public class Inscripcion extends JFrame implements ActionListener { 

    JLabel Nombre; 
    JLabel Sexo; 
    JTextField CampoTexto; 

    JRadioButton M; 
    JRadioButton F; 
    ButtonGroup Casillas; 

    JButton Aceptar; 

    JPanel A; 
    JPanel B; 
    JPanel C; 

    Inscripcion() { 

     super("Formulario de Inscripción."); 

     Container Contenedor = getContentPane(); 
     Contenedor.setLayout(new FlowLayout()); 

     Nombre = new JLabel("Nombre: "); 
     Contenedor.add(Nombre); 
     CampoTexto = new JTextField(20); 
     Contenedor.add(CampoTexto); 
     A = new JPanel(); 
     A.setLayout(new GridLayout(2, 1)); 
     Contenedor.add(A, BorderLayout.NORTH); 

     Sexo = new JLabel("Sexo: "); 
     Contenedor.add(Sexo); 
     M = new JRadioButton("M", false); 
     M.addActionListener(this); 
     Contenedor.add(M); 
     F = new JRadioButton("F", false); 
     F.addActionListener(this); 
     Contenedor.add(F); 
     B.setLayout(new GridLayout(3, 1)); 
     Contenedor.add(B, BorderLayout.CENTER); 

     Aceptar = new JButton("Aceptar"); 
     Aceptar.addActionListener(this); 
     B.setLayout(new GridLayout(3, 1)); 
     Contenedor.add(B, BorderLayout.SOUTH); 
     Contenedor.add(Aceptar, BorderLayout.CENTER); 

     setSize(300, 500); 
     setVisible(true); 

    } 

    public void actionPerformed(ActionEvent Evento) { 

     String Nom = CampoTexto.getText(); 
     String Sex = M.isSelected() ? "Masculino":"Femenino"; 
     System.out.println(Nom + Sex + " "); 

    } 

    public static void main(String[] Flogging) { 

     Inscripcion obj = new Inscripcion(); 
     obj.setDefaultCloseOperation(EXIT_ON_CLOSE); 

    } 

} 

唯一的例外出現在該行：提前

B.setLayout(new GridLayout(3, 1)); 

感謝。

Answer 1

B永遠不會初始化......請在發佈此類問題之前考慮查看您的代碼。