java - 从 JTable 写入文本文件，以及从文本文件读取到 JTable

Question

我觉得我已经很接近了，但仍然遇到了我不明白的错误。我正在尝试使用保存按钮将数据从 JTable 写入文本文件。我还需要从文件加载回 JTable。任何关于为什么这不能正常工作的建议。

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.FileReader;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.JTextField;
import javax.swing.table.DefaultTableModel;


/*PayrollProgram.java
 * Created by: Steve Totten
 * Created on: 10-13-2013
 * This program allows the user to enter the contact's
 * name, Email address, age, and phone number.  Then the program
 * adds the information to a table for easy viewing
 * The program has a save button to save to a data file
 * it also has a load button that will read from a file to the table
 */

    public class ContactInfo {

    // Set up the size of the GUI window
    private static final int FRAME_WIDTH = 900;
    private static final int FRAME_HEIGHT = 400;
    static JTable tblContacts;
    static int ID = 0;

    public static void main(String[] args) {

        // Set up the user interface
        final JFrame frame = new JFrame();
        final JPanel buttonPanel = new JPanel();
        frame.add(buttonPanel);
        // I need this to be able to put the buttons where I want
        buttonPanel.setLayout(null);

        // Set up Add button and its location
        final JButton buttonAdd = new JButton(" Add ");
        buttonAdd.setBounds(50, 325, 100, 20);
        buttonPanel.add(buttonAdd);


        // Set up Exit button and its location
        final JButton buttonExit = new JButton("Exit");
        buttonExit.setBounds(200, 325, 100, 20);
        buttonPanel.add(buttonExit);

        // Method for exit button
        buttonExit.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {

                System.exit(0);
            }
        });

        // Set up Save button and its location
        final JButton buttonSave = new JButton("Save");
        buttonSave.setBounds(350, 325, 100, 20);
        buttonPanel.add(buttonSave);

        // Set up Save button method
        buttonSave.addActionListener(new ActionListener(){
            @Override
            public void actionPerformed(ActionEvent e) {
                try{

                    BufferedWriter bfw = new BufferedWriter(new FileWriter("C:\\Users\\Steve\\Workspace\\ContactInfo\\ContactInfo.txt"));

                      for (int i = 0 ; i < tblContacts.getRowCount(); i++)
                      {

                        for(int j = 0 ; j < tblContacts.getColumnCount();j++)
                        {
                            bfw.newLine();
                            bfw.write((String)(tblContacts.getValueAt(i,j)));
                            bfw.write("\t");;
                        }
                      }
                      bfw.close();
            }catch(Exception ex) {

                ex.printStackTrace();
            }
            }
        });
        // Set up Load button and its location
        final JButton buttonLoad =  new JButton("Load");
        buttonLoad.setBounds(500, 325, 100, 20);
        buttonPanel.add(buttonLoad);

        // Method for load button
        buttonLoad.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                String line;
                BufferedReader reader;
                    try{       
                        reader = new BufferedReader(new FileReader("C:\\Users\\Steve\\Workspace\\ContactInfo\\ContactInfo.txt"));
                        while((line = reader.readLine()) != null) 
                        {
                           tblContacts.add(null, line.split(", ")); 
                        }
                        reader.close();
                     }
                    catch(Exception ex){

                ex.printStackTrace();

                    }

            }
        });
        // Set up Labels for name, hours, and pay rate
        final JLabel lblFirst = new JLabel("Enter First Name: ");
        lblFirst.setBounds(20, 5, 150, 100);
        buttonPanel.add(lblFirst);

        final JLabel lblLast = new JLabel("Enter Last Name: ");
        lblLast.setBounds(20, 60, 150, 100);
        buttonPanel.add(lblLast);

        final JLabel lblAddress = new JLabel("Enter their Age: ");
        lblAddress.setBounds(20, 115, 150, 100);
        buttonPanel.add(lblAddress);

        final JLabel lblAge = new JLabel("Enter Email Address:");
        lblAge.setBounds(20, 170, 150, 100);
        buttonPanel.add(lblAge);

        final JLabel lblPhone = new JLabel("Enter phone number:");
        lblPhone.setBounds(20, 225, 150, 100);
        buttonPanel.add(lblPhone);


        // Set up textboxes for all expected inputs
        final JTextField txtFirst = new JTextField();
        txtFirst.setBounds(180, 40, 100, 25);
        buttonPanel.add(txtFirst);

        final JTextField txtLast = new JTextField();
        txtLast.setBounds(180, 95, 100, 25);
        buttonPanel.add(txtLast);

        final JTextField txtAge = new JTextField();
        txtAge.setBounds(180, 150, 100, 25);
        buttonPanel.add(txtAge);

        final JTextField txtPhone = new JTextField();
        txtPhone.setBounds(180, 260, 100, 25);
        buttonPanel.add(txtPhone);

        final JTextField txtEmail = new JTextField();
        txtEmail.setBounds(180, 210, 100, 25);
        buttonPanel.add(txtEmail);

        // Set up of columns in the table
        String[] columns = { "ID", "Last Name", "First Name", "Age", "Email Address",
                "Phone Number" };
        // Set up of the table with the appropriate column headers
        final DefaultTableModel model = new DefaultTableModel(null, columns);
        final JTable tblContacts = new JTable();
        tblContacts.setModel(model);
        JScrollPane scrollPane = new JScrollPane(tblContacts);
        scrollPane.setBounds(300, 20, 550, 300);
        buttonPanel.add(scrollPane);


        // Save button methods, including validation checking
        buttonAdd.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                if (txtFirst.getText().length() == 0) {
                    JOptionPane.showMessageDialog(null, "Error: no first name");
                    return;
                }

                if (txtLast.getText().length() == 0) {
                    JOptionPane.showMessageDialog(null, "Error: no last name");
                    return;
                }

                int age = 0;
                try {
                    age = Integer.parseInt(txtAge.getText());
                } catch (Exception ex) {
                    JOptionPane.showMessageDialog(null,
                            "Error: invalid age value");
                    return;
                }

                if (age < 0 || age > 120) {
                    JOptionPane.showMessageDialog(null,
                            "Error: invalid age range (0 - 120 allowed)");
                    return;
                }

                if (txtPhone.getText().length()==0){
                    JOptionPane.showMessageDialog(null, "Error: no phone number ");
                    return;
                }


                // Add an ID number to each entry and add the entry to the table
                ID++;
                model.addRow(new Object[] { String.valueOf(ID),
                        txtLast.getText(), txtFirst.getText(),
                        txtAge.getText(), txtEmail.getText(), txtPhone.getText() });


                // Once entry is added to the table, the text fields are cleared for the next entry
                txtLast.setText("");
                txtFirst.setText("");
                txtAge.setText("");
                txtPhone.setText("");
                txtEmail.setText("");



            }
        });     
        /*
         * This sets the size of the window along with the title and it sets up
         * the exit on close X button to close the window when the X is clicked.
         */

        frame.setSize(FRAME_WIDTH, FRAME_HEIGHT);
        frame.setTitle("Contact Information");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

}

Answer 1

调查结果和建议:

• 当您尝试使用 JTable 时，会遇到 NullPointException(NPE)。
• 这是因为您正在隐藏类字段 tblContacts。这意味着您在类中声明此字段，然后重新声明并在主方法中初始化它。那么，当您重新声明该变量时，它是一个新的且不同的变量，因此您将类字段保留为空。
• 解决方案是不要在其他地方重新声明变量。
• 此外，希望此代码只是演示代码，而不是您的真实程序。在您的真实程序中，您不会在静态主方法(或除非常简短的主方法之外的任何静态方法)中包含任何此类代码，并且大多数字段将是非静态实例字段。<

<小时/>

阴影示例:

public class Foo {
  private int bar;  // class field

  public Foo() {
     int bar = 3; // I've re-declared bar here and so the class field is still unassigned.
  }
}

纠正它:

public class Foo {
  private int bar;  // class field

  public Foo() {
     bar = 3; // bar not re-declared
  }
}

看出区别了吗？