Appending text to a JTextArea on button press?

Question

I've got a simple Swing GUI and I want to add a new line of text to a JTextArea after a button is pressed, simple right?

The Button and it's ActionListener function correctly (printing stuff to the console works fine), but when I use .append()or .setText() to add text to the textarea, I get a nullpointer exception.

As always, code it below. Any input would be greatly appreciated, thanks!!!

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import javax.swing.border.LineBorder;

public class GUI extends JFrame implements ActionListener {

private JFrame frame;
private JLabel paneHeader;
public JTextArea ipArea, portArea, outputLog, orderLog, cookLog;
private JButton newServer;

public String ipAddress, portNumber, cashierName, cookName;

public GUI() {
    initGUI();
}

public void initGUI() {

    frame  = new JFrame("Restaurant Overview");
    Container contentPane = frame.getContentPane(); 

    JLabel paneHeader = new JLabel("Restaurant Monitoring System");
    paneHeader.setBounds(200, 0, 200, 25);
    paneHeader.setFont(new Font("Calibri", Font.BOLD, 14));

    JLabel ipLabel = new JLabel("IP Address: ");
    ipLabel.setBounds(25, 30, 75, 20);
    ipLabel.setFont(new Font("Calibri", Font.PLAIN, 12));

    final JTextArea ipArea = new JTextArea();
    ipArea.setBorder(new LineBorder(Color.GRAY));
    ipArea.setBounds(105, 30, 100, 20);

    JLabel portLabel = new JLabel("Port Number: ");
    portLabel.setBounds(25, 55, 75, 20);
    portLabel.setFont(new Font("Calibri", Font.PLAIN, 12));

    final JTextArea portArea = new JTextArea();
    portArea.setBorder(new LineBorder(Color.GRAY));
    portArea.setBounds(105, 55, 100, 20);

    JButton newServer = new JButton("Create new Server");
    newServer.setBorder(new LineBorder(Color.GRAY));
    newServer.setBounds(250, 30, 150, 40);
    newServer.setActionCommand("createserver");
    newServer.addActionListener(this);

    JTextArea outputLog = new JTextArea(" ");
    outputLog.setBorder(new LineBorder(Color.GRAY));
    outputLog.setBounds(25, 90, 150, 150);
    //outputLog.setEditable(false);  

    JTextArea cashierLog = new JTextArea();
    cashierLog.setBorder(new LineBorder(Color.GRAY));
    cashierLog.setBounds(185, 90, 150, 150);
    //cashierLog.setEditable(false);  

    JTextArea cookLog = new JTextArea();
    cookLog.setBorder(new LineBorder(Color.GRAY));
    cookLog.setBounds(345, 90, 150, 150);
    //cookLog.setEditable(false);  

    contentPane.add(paneHeader);
    contentPane.add(ipLabel);
    contentPane.add(ipArea);
    contentPane.add(portLabel);
    contentPane.add(portArea);
    contentPane.add(outputLog);
    contentPane.add(cashierLog);
    contentPane.add(cookLog);

    contentPane.add(newServer);

    frame.setLayout(null);
    frame.pack();
    frame.setSize(600,400);
    frame.setVisible(true); 
}

public void test() {
    //ipAddress = ipArea.getText() + "\n";
    //portNumber = portArea.getText() + "\n";
    String text = "lemons";
    //System.out.println(text);
    outputLog.append(text);
    //outputLog.append(portNumber);
}


public void actionPerformed(ActionEvent e) {
    if ("createserver".equals(e.getActionCommand())) {
        //test();
        outputLog.append("lemons");
    } else {
        //Do Nothing
    }
}
}

Answer 1

You are likely shadowing a variable -- declaring it more than once, but initializing a local variable not the class field, and so the class field remains null.

Edit:
Yep, sure enough you do. In your constructor you have

JTextArea outputLog = new JTextArea(" ");

This re-declares the outputLog variable, and so you are initializing only the variable local to the constructor. The solution is not to redeclare the variable but instead initialize the class field. So change the above to

outputLog = new JTextArea(" ");

You will need to do this for every variable that needs to be accessed in the class scope. For those that are OK to declare locally, do so, but in the sake of safety, get rid of their corresponding class declaration so as not to risk causing the same error in the future.