Writing text file from JTable, and Reading from text file to JTable

Question

I feel like I am close but still running into errors I do not understand. I am attempting to write data from my JTable to a text file using the save button. I also need to load from the file back into the JTable. Any suggestions why this isn't working correctly.

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

Findings and suggestions:

• You're getting a NullPointException (an NPE) when you try to use your JTable.
• This is because you're shadowing the class field, tblContacts. Meaning you declare this field in the class, and then re-declare and initialize it in the main method. Well when you re-declare the variable, it's a new and distinct variable, and so you leave the class field null.
• The solution is to not re-declare the variable any where else.
• Also, hopefully this code is just a demonstration code and not your real program. In your real program, you would not have any of this code within the static main method (or any static method for that matter other than a very brief main method), and most of your fields would be non-static instance fields.

---

Example of shadowing:

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.
  }
}

To correct it:

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

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

See the difference?