为什么我得到一个NullPointerException？

Question

我早些时候发布了这个代码，但被告知要清理它并重新发布。我得到的错误是：

Exception in thread "main" java.lang.NullPointerException 
    at java.awt.Container.addImpl(Unknown Source) 
    at java.awt.Container.add(Unknown Source) 
    at FVolume.createCustomers(FVolume.java:74) 
    at FVolume.<init>(FVolume.java:21) 
    at FVolume.main(FVolume.java:270) 

下面是代码：

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

public class FVolume extends JFrame implements ActionListener{ 
    private JTabbedPane jtabbedPane; 
    private JPanel Customers; 

    JTextArea NameTextCustomers, ExistTextCustomers, NameTextContractors, ExistTextContractors; 

    public FVolume(){ 
     setTitle("Volume Calculator"); 
     setSize (300, 200); 

     JPanel topPanel = new JPanel(); 
     topPanel.setLayout(new BorderLayout()); 
     getContentPane().add(topPanel); 

     createCustomers(); 

     jtabbedPane = new JTabbedPane(); 
     jtabbedPane.addTab("Customers", Customers); 
     topPanel.add(jtabbedPane, BorderLayout.CENTER); 

    } 
    /*  CREATE CUSTOMERS */ 

    public JPanel createCustomers(){ 
     Customers = new JPanel(); 
     Customers.setLayout(null); 

     NameTextCustomers = new JTextArea(); 
     NameTextCustomers.setBounds(10, 10, 350, 150); 
     NameTextCustomers.setLineWrap(true); 
     Customers.add(NameTextCustomers); 

     JButton Exit = new JButton("Exit"); 
     Exit.setBounds(30,170,80,20); 
     Exit.addActionListener(this); 
     Exit.setBackground(Color.white); 
     Customers.add(Exit); 

     JButton AddCustomers = new JButton("Add Customer"); 
     AddCustomers.setBounds(130,170,120,20); 
     AddCustomers.setBackground(Color.white); 
     Customers.add(AddCustomers); 

     JButton Refresh = new JButton("Refresh"); 
     Refresh.setBounds(260,170,80,20); 
     Refresh.setBackground(Color.white); 
     Customers.add(Refresh); 

     ExistTextCustomers = new JTextArea(); 
     ExistTextCustomers.setBounds(10, 200, 350, 60); 
     ExistTextCustomers.setLineWrap(true); 
     Customers.add(ExistTextCustomers); 

     final JTextArea custArea = new JTextArea(6, 30); 
     final JTextArea custMessage = null; 

     AddCustomers.addActionListener(new ActionListener() 
     { 
     public void actionPerformed(ActionEvent e) 
     { 
     new Customer("Customer"); 
    } 
     }); 
     Customers.add(custArea); 
     Customers.add(AddCustomers); 

     Customers.add(Refresh); 
     Customers.add(custMessage); 
     Refresh.setMnemonic('R'); 

     Refresh.addActionListener(new ActionListener() 
     { 
      public void actionPerformed(ActionEvent e) 
      { 
     custMessage.setText(""); 
     try 
     { 
      File custOpen = new File("customer.txt"); 
      FileReader custAreaIn = new FileReader(custOpen); 
      custArea.read(custAreaIn, custOpen.toString()); 
      custMessage.setText("The file exists and can be read from."); 
     } 
     catch (IOException e3){ 
     custMessage.setText("The file could not be read. " + e3.getMessage()); 
      } 
     } 
    } 
    ); 
     return Customers; 
} 
class Customer extends JFrame 
{ 
    private String[] states = {"AL", "AK", "AZ", "AR", "CA", "CO", "CT", "DE", 
      "FL", "GA", "HI", "ID", "IL", "IN", "IA", "KS", "KY", "LA", "ME", 
      "MD", "MA", "MI", "MN", "MS", "MO", "MT", "NE", "NV", "NH", "NJ", 
      "NM", "NY", "NC", "ND", "OH", "OK", "OR", "PA", "RI", "SC", "SD", 
      "TN", "TX", "UT", "VT", "VA", "WA", "WV", "WI", "WY"}; 
      private JComboBox StateList = new JComboBox(states); 
      private JTextField NameText = new JTextField(25); 
      private JTextField AddressText = new JTextField(25); 
      private JTextField CityText = new JTextField(25); 
      private JTextField ZipText = new JTextField(9); 
      private JTextField PhoneText = new JTextField(10); 
      private JTextField PopMessageText = new JTextField(30); 
      private static final long serialVersionUID = 1L; 

      private AddCustButtonHandler addCusHandler = new AddCustButtonHandler(); 

      public Customer(String who) 
      { 
       popUpWindow (who); 
      } 
      public void popUpWindow(final String who) { 
       final JFrame popWindow; 
       popWindow = new JFrame(who); 
       popWindow.setSize(425, 350); 
       popWindow.setLocation(100, 100); 
       popWindow.setVisible(true); 
       setDefaultCloseOperation(EXIT_ON_CLOSE); 

       Container c = new Container(); 

       popWindow.add(c); 

       c.setLayout(new FlowLayout()); 

       JPanel one = new JPanel(); 
       JPanel two = new JPanel(); 
       JPanel three = new JPanel(); 
       JPanel four = new JPanel(); 
       JPanel five = new JPanel(); 
       JPanel six = new JPanel(); 

       one.add(new JLabel(who + " Name ")); 
       one.add(NameText); 
       two.add(new JLabel("Address ")); 
       two.add(AddressText); 
       three.add(new JLabel("City ")); 
       three.add(CityText); 
       four.add(new JLabel("State ")); 
       StateList.setSelectedIndex(0); 
       four.add(StateList); 
       four.add(new JLabel("ZIP")); 
       four.add(ZipText); 
       four.add(new JLabel("Phone")); 
       four.add(PhoneText); 
       JButton addwho = new JButton("Add " + who); 
       addwho.setMnemonic('A'); 
       JButton close = new JButton("Close"); 
       close.setMnemonic('C'); 
       JButton deleteFile = new JButton("Delete File"); 
       deleteFile.setMnemonic('D'); 
       five.add(addwho); 
       five.add(close); 
       five.add(deleteFile); 
       PopMessageText.setEditable(false); 
       PopMessageText.setHorizontalAlignment(JTextField.CENTER); 

       six.add(PopMessageText); 
       c.add(one); 
       c.add(two); 
       c.add(three); 
       c.add(four); 
       c.add(five); 
       c.add(six); 

       deleteFile.setToolTipText("Delete File"); 
       addwho.setToolTipText("Add "+ who); 
       close.setToolTipText("Close"); 

       if (who == "Customer") 
        addwho.addActionListener(addCusHandler); 
       close.addActionListener(new ActionListener() { 
        public void actionPerformed(ActionEvent e) { 
         NameText.setText(""); 
         AddressText.setText(""); 
         CityText.setText(""); 
         ZipText.setText(""); 
         PhoneText.setText(""); 
         PopMessageText.setText(""); 
         popWindow.dispose(); 
        } 
       } 
       ); 
       deleteFile.addActionListener(new ActionListener() { 
        public void actionPerformed(ActionEvent e) { 
         PopMessageText.setText(""); 
         if (who == "Customer") { 
          File file = new File("Customer.txt"); 
          boolean cusFileDeleted = file.delete(); 
          if (cusFileDeleted) { 
           PopMessageText 
           .setText("Customer file has been deleted"); 
          } else { 
           PopMessageText 
           .setText("There was an error in deleting file"); 
          } 
          } 
         } 
        } 
       ); 
      } 
      class AddCustButtonHandler implements ActionListener { 
       public void actionPerformed(ActionEvent addCusHandler) { 
        int StateIndex; 
        try { 
         File file = new File("Customer.txt"); 
         boolean success = file.createNewFile(); 
         if (success) { 
          PopMessageText 
          .setText("Customer.txt file created file added"); 
         } else if (file.canWrite()) { 
          PopMessageText 
          .setText("Writing data to Customer.txt, file added"); 
         } else { 
          PopMessageText.setText("Cannot create file: Customer.txt"); 
         } 
         try { 
          FileWriter fileW = new FileWriter("Customer.txt", true); 
          fileW.write(NameText.getText()); 
          fileW.write(","); 
          fileW.write(AddressText.getText()); 
          fileW.write(","); 
          fileW.write(CityText.getText()); 
          fileW.write(","); 
          StateIndex = StateList.getSelectedIndex(); 
          fileW.write(states[StateIndex]); 
          fileW.write(","); 
          fileW.write(ZipText.getText()); 
          fileW.write(","); 
          fileW.write(PhoneText.getText()); 
          fileW.write("\r\n"); 
          fileW.close(); 
          PopMessageText.setText("A new Customer has been added!"); 
          FileReader fileR = new FileReader("Customer.txt"); 
          BufferedReader buffIn = new BufferedReader(fileR); 
          String textData = buffIn.readLine(); 
          buffIn.close();       
         } 
         catch (IOException e1) { 
          JOptionPane.showMessageDialog(null, e1.getMessage(), "ERROR", 2); 
         } 
         NameText.setText(""); 
         AddressText.setText(""); 
         CityText.setText(""); 
         ZipText.setText(""); 
         PhoneText.setText(""); 
         } 
        catch (IOException e1) { 
        } 
       } 
      } 
      public void actionPerformed(ActionEvent event){ 
      } 
       private void Exit_pressed(){ 
        System.exit(0); 
       } 
      } 
      public void actionPerformed(ActionEvent e) { 
      } 


       public static void main(String[] args){ 
        JFrame frame = new FVolume(); 
        frame.setSize(380, 350); 
        frame.setVisible(true); 
    } 

} 

Answer 1

这就是问题所在：

final JTextArea custMessage = null; 
... 
Customers.add(custMessage); 

那你希望会做什么？基本上，你不允许通过对add方法的空引用。

以供将来参考，线索来诊断这个自己是找到引用您的代码堆栈跟踪的第一位 - 在这种情况下是线：

Customers.add(custMessage); 

然后看看什么它使用（Customers和custMessage）并确定它们是否正常。在这种情况下，很明显custMessage的值为空，这是不允许的。