How do I append text to a JTextArea from another class?

Question

I'm trying to put a console-type thing on my GUI and for that I'm pretty sure that I have to append text to the JTextArea. For the console to actually be worthwhile I'll have to append text from a different class. For that I built a method to append a string to my console but it threw a NullPointerException and failed.

What I'm wondering is how I can append text to my console (JTextArea) from other classes.

Here is my code:

package com.robot;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.ScrollPaneConstants;

public class GUI extends JFrame implements Runnable {

static JTextArea console;

//defines the line break
static String newline = System.getProperty("line.separator");

//start of the constructor method for GUI
public GUI() {

    //makes the program unable to be resized
    this.setResizable(false);

    //allows the user to close the program with the x button
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    //sets the title of the program
    this.setTitle("ROBOT Alpha Alfred Version 3.0");

    //creates panels to hold the elements of the GUI
    JPanel mainPanel = new JPanel();
    JPanel buttonPanel = new JPanel();
    JPanel consolePanel = new JPanel();

    //creates buttons
    JButton runDemo = new JButton("Run Demo");
    JButton runLive = new JButton("Run Live");
    JButton scan = new JButton("Scan Market");
    JButton findPatterns = new JButton("Find Patterns");
    JButton cleanFolder = new JButton("Clean Up Folder");
    JButton configureSettings = new JButton("Configure Settings");

    //creates the console
    JTextArea console = new JTextArea(6, 40);

    //sets the default text of the console
    console.setText("----------------------- ROBOT Console -----------------------" + newline);

    //makes the console unable to be edited
    console.setEditable(false);

    //sets the line wrapping of the console
    console.setLineWrap(true);
    console.setWrapStyleWord(true);

    //creates scroll bars
    JScrollPane scrollBar = new JScrollPane(console);
    scrollBar.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_AS_NEEDED);
    scrollBar.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED);

    //adds buttons to the buttonPanel
    buttonPanel.add(runDemo);
    buttonPanel.add(runLive);
    buttonPanel.add(scan);
    buttonPanel.add(findPatterns);
    buttonPanel.add(cleanFolder);
    buttonPanel.add(configureSettings);

    //adds the console to the console panel
    consolePanel.add(scrollBar);

    //adds panels to the main panel
    mainPanel.add(buttonPanel);
    mainPanel.add(consolePanel);

    //adds the main panel to the frame
    this.add(mainPanel);

    //packs the GUI
    this.pack();

    //sizes the GUI
    this.setSize(600, 400);

    //centers the GUI
    this.setLocationRelativeTo(null);

    //sets the GUI to be visible
    this.setVisible(true);

}

public void run() {

}

public static void add(String string) {
    console.append(string + newline);
}

}

Here is the method for appending the text to the console:

public static void add(String string) {
    console.append(string + newline);
}

Here is a part that you are really going to want to pay attention to (well, still pay attention to the append method):

static JTextArea console;

Here is how the add method is call and where it throws the NullPointerException:

//main method start
public static void main(String[] args) throws InterruptedException, IOException, AWTException {

    //opens up the GUI
    (new Thread(new GUI())).start();

    GUI.add("Text to add");

    //possible methods
    //ScanMarket.scanMarket(); //scans market for data
    //FindPattern("Images"); //finds pattern among images in image folder labeled Images

}//end of main method

By the way I tried changing

console.setEditable(false);

to

console.setEditable(true);

and a NullPointerException was still thrown. Thanks so much for your help!

Answer 1

1. Your code shadows the console variable since you re-declare it in the constructor. Don't do this.
2. That static add(String...) method should not be static. Make it an instance method.

ie, you have

class Foo {
  private Bar bar; // this guy is null

  public Foo() {
    Bar bar = new Bar(); // the class field is *still* null
                       // since this bar variable is local to the constructor only.
  }
}

By re-declaring the bar variable in the constructor, the instance field in the class remains null. You should not re-declare the variable in the constructor, like so:

class Foo {
  private Bar bar; // class field is null here

  public Foo() {
    bar = new Bar();  // now no longer null. *** note the difference
  }
}

---

In fact none of the methods or fields you are using above should be static other than a main method and one or two supporting methods, that's it.

Answer 2

Don't use a static reference, it has no meaning. As stated in your last question, a component can only have a single parent, using static components prevents you from having more the once instance of the class, but this is just a single side effect

You're declaring console twice. Once as a static field...

static JTextArea console;

Once as a local variable in the constructor...

JTextArea console = new JTextArea(6, 40);

Change static JTextArea console; to private JTextArea console; and console = new JTextArea(6, 40); , this way the instance field will actually be initialised and added to the UI as you expect it

Instead of using static , pass a reference of the GUI to whatever needs to append content.

Don't forget that Swing is a single threaded environment and all updates to the UI should be made from within the context of the Event Dispatching Theead