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POJava - Observer Pattern in regards to some code. Mimic syncronized text update
 

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  1. POJava - Observer Pattern in regards to some code. Mimic syncronized text update
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    <p>The purpose of my code is to mimic the functionality of Google Docs - when a user types on one machine, the letters they type appear on another machine. For simplistic purposes, each machine types text in a gui, and a main class handles all changes.</p> <p><img src="https://i.stack.imgur.com/LYiCY.png" alt=""> Every machine has an "Editor" and is linked up to a total of 1 "File Content Subject". The "File Content Subject" is supposed to make the changes a user makes and send the updated code to all "Editors"</p> <p><img src="https://i.stack.imgur.com/atgx5.png" alt="enter image description here"></p> <p>This begins with a simple <strong>driver</strong> where I make a File Content Subject, create two Editors and connect these together</p> <pre><code>public class Driver { public static void main(String[] args) { FileContentSubject filecontentsubject = new FileContentSubject(); Editor e1 = new Editor(filecontentsubject); Editor e2 = new Editor(filecontentsubject); filecontentsubject.attach(e1); filecontentsubject.attach(e2); } } </code></pre> <p>The <strong>Editor</strong> looks like this (the two pop up windows, not the rest): <img src="https://i.stack.imgur.com/tq0PM.png" alt="enter image description here"></p> <p>The code that makes the Editor is here:</p> <pre><code>import javax.swing.*; import java.util.*; import javax.swing.event.DocumentEvent; import javax.swing.event.DocumentListener; import java.awt.BorderLayout; import java.awt.event.ActionEvent; import javax.swing.text.AbstractDocument; import javax.swing.text.BadLocationException; import javax.swing.text.Document; public class Editor extends JFrame implements DocumentListener, Observer { private FileContentSubject reference; private Document doc; private JScrollPane textAreaScrollPane; private JTextArea textArea; public Editor(FileContentSubject filecontentsubject) { super("Editor"); initComponents(); this.reference = filecontentsubject; textArea.getDocument().addDocumentListener(reference); } private void initComponents(){ textArea = new JTextArea(); textArea.setColumns(5); textArea.setLineWrap(true); textArea.setRows(50); textArea.setWrapStyleWord(true); textAreaScrollPane = new JScrollPane(textArea); setLocation(600,100); setSize(500,400); setVisible(true); setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); getContentPane().setLayout(new BorderLayout()); getContentPane().add(textArea, BorderLayout.CENTER); } @Override public void changedUpdate(DocumentEvent arg0) { } @Override public void insertUpdate(DocumentEvent arg0) { reference.insertUpdate(arg0); } @Override public void removeUpdate(DocumentEvent arg0) { reference.removeUpdate(arg0); } @Override public void update() { //textArea.setText(reference.getJTextArea()); //textArea.setText(reference.temp); } } </code></pre> <p>The only <strong>Problem</strong> right now with my code is within the <strong>File Content Subject</strong>, when I try to change the code that I get passed by through the Editor. I am getting many "cannot mutate within notification" and somethings a "null pointer exception" error. The code that generates this is below and is the part I have <strong>commented out.</strong></p> <pre><code>import java.util.ArrayList; import java.util.List; import javax.swing.JTextArea; import javax.swing.event.DocumentEvent; import javax.swing.event.DocumentListener; import javax.swing.text.BadLocationException; import javax.swing.text.Document; import javax.swing.text.Position; public class FileContentSubject implements Subject, DocumentListener { private JTextArea textArea; private Document doc; private SubjectImpl reference; @Override public void attach(Observer o) { reference.attach(o); } @Override public void detach(Observer o) { reference.detach(o); } @Override public void notifyAllObservers() { reference.notifyAllObservers(); } public FileContentSubject(){ reference = new SubjectImpl(); textArea = new JTextArea(); textArea.setTabSize(5); textArea.setLineWrap(true); textArea.setWrapStyleWord(true); textArea.getDocument().addDocumentListener(this); } @Override public void changedUpdate(DocumentEvent arg0) {} @Override public void insertUpdate(DocumentEvent arg0) { doc = (Document)arg0.getDocument(); // try { // //this.textArea.setText(doc.getText(0, doc.getLength()-1)); // } catch (BadLocationException e) { // // TODO Auto-generated catch block // e.printStackTrace(); // } notifyAllObservers(); } @Override public void removeUpdate(DocumentEvent arg0) { doc = (Document)arg0.getDocument(); // try { // this.textArea.setText(doc.getText(0, doc.getLength()-1)); // } catch (BadLocationException e) { // // TODO Auto-generated catch block // e.printStackTrace(); // } notifyAllObservers(); } public String getJTextArea(){ return textArea.getText(); } } </code></pre> <p>So my question is, how can I pass text in (by passing in a DocumentEvent through an Editor) into the File Content Subject, have it change the File Content Subject, and notify all Editors?</p> <hr> <p>My other classes that make this all happen (not important but shown for clarity):</p> <p><strong>Subject Interface</strong></p> <pre><code>/** * Interface */ public interface Subject { public void attach(Observer o); public void detach(Observer o); public void notifyAllObservers(); } </code></pre> <p><strong>Observer Interface</strong></p> <pre><code>public interface Observer { public void update(); } </code></pre> <p><strong>SubjectImpl class</strong></p> <pre><code>import java.util.ArrayList; import java.util.List; public class SubjectImpl implements Subject { private List &lt;Observer&gt; observers; public SubjectImpl(){ observers = new ArrayList&lt;Observer&gt;(); } @Override public void attach(Observer o) { observers.add(o); } @Override public void detach(Observer o) { observers.remove(o); } @Override public void notifyAllObservers() { for(Observer o: observers){ o.update(); } } } </code></pre> <hr> <hr> <hr> <h1><strong>ANSWER</strong></h1> <p>Needed to prevent the Editor that was editing itself from being updated. This was done with document properties and maintaining everything in a string rather than JTextArea. BIG thanks to acdcjunior for helping out and code answer are on the very last three code blocks in selected answer.</p>
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    1. COYes I turn system settings to low so my computer can run/render faster
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    2. COIt's not a good idea to use a class name the same as a JDK class, in your case `Observer` is also in `java.util` package
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    3. COThanks Bohemian, as its a simple problem I will not change it but I will remember this for the future
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