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copied!<p>The <code>java.lang.IllegalStateException: Attempt to mutate in notification</code> is thrown because in these lines in <code>FileContentSubject</code>:</p> <pre><code>@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(); } </code></pre> <p>And this line at <code>FileContentSubject</code>'s constructor:</p> <pre><code>textArea.getDocument().addDocumentListener(this); </code></pre> <p><strong>show that you are trying to change the value of the <code>textArea</code> from inside the <code>DocumentListener</code> methods.</strong></p> <p>As pointed out <a href="https://stackoverflow.com/questions/6088069/java-cant-change-the-value-of-a-jtextfield-from-inside-of-documentlistener-met">here</a>, <strong>you should use a <code>DocumentFilter</code> for that purpose</strong>.</p> <p>Here's a simple working example of <code>DocumentFilter</code>'s use (it replaces all typed chars with their uppercase version):</p> <pre><code>//UpcaseFilter.java //A simple DocumentFilter that maps lowercase letters to uppercase. import javax.swing.*; import javax.swing.text.*; public class UpcaseFilter extends DocumentFilter { public void insertString(DocumentFilter.FilterBypass fb, int offset, String text, AttributeSet attr) throws BadLocationException { fb.insertString(offset, text.toUpperCase(), attr); } // no need to override remove(): inherited version allows all removals public void replace(DocumentFilter.FilterBypass fb, int offset, int length, String text, AttributeSet attr) throws BadLocationException { fb.replace(offset, length, text.toUpperCase(), attr); } public static void main(String[] args) { DocumentFilter dfilter = new UpcaseFilter(); JTextArea jta = new JTextArea(); JTextField jtf = new JTextField(); ((AbstractDocument) jta.getDocument()).setDocumentFilter(dfilter); ((AbstractDocument) jtf.getDocument()).setDocumentFilter(dfilter); JFrame frame = new JFrame("UpcaseFilter"); frame.getContentPane().add(jta, java.awt.BorderLayout.CENTER); frame.getContentPane().add(jtf, java.awt.BorderLayout.SOUTH); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setSize(200, 120); frame.setVisible(true); } } </code></pre> <p>(The above example was taken from the <a href="http://shop.oreilly.com/product/9780596004088.do" rel="nofollow noreferrer">Java Swing, 2nd Edition</a> book, <a href="http://examples.oreilly.com/jswing2/code/ch22/UpcaseFilter.java" rel="nofollow noreferrer">chapter 22</a>.)</p> <h2>That's the reason for the exception thrown, but you'll have to do more to correct you code:</h2> <p>In short, the problem is that this happens in your code:</p> <ul> <li>When an <code>Editor</code> instance's (let's call it <code>edt</code>) <code>textArea</code> changes...</li> <li>...the <code>FileContentSubject</code> (as it is a <code>DocumentListener</code>) notices the event and then notifies all its registered observers (<code>edt</code> included!) to call their <code>update()</code> method...</li> <li>...the <code>update()</code> method of <code>edt</code> changes its <code>textArea</code> and... voilá! You are trying to change the <code>textArea</code> who started the event (before the event ends)!</li> </ul> <p>All you have to do then is find a way of not notifying who started the event. The code below uses <code>Document.putProperty()</code> and <code>Document.getProperty()</code> for that: it detaches the event's source editor (<code>reference.detach(e);</code>), notifies everyone of the change, and reattaches it (<code>reference.attach(e);</code>).</p> <p>(Also, I replaced <code>FileContentSubject</code>'s <code>JTextArea</code> with a <code>String</code>, as just that <code>String</code> will suffice.)</p> <p>So, heres the changed <code>FileContentSubject</code> code:</p> <pre><code>import javax.swing.event.DocumentEvent; import javax.swing.event.DocumentListener; import javax.swing.text.BadLocationException; import javax.swing.text.Document; public class FileContentSubject implements Subject, DocumentListener { // private JTextArea textArea; // removed as a String field will suffice // private Document doc; // should not be a field! private String state; public String getState() { return this.state; } 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) { Document doc = (Document) arg0.getDocument(); try { // this.textArea.setText(doc.getText(0, doc.getLength()-1)); this.state = doc.getText(0, doc.getLength()); } catch (BadLocationException e) { // TODO Auto-generated catch block e.printStackTrace(); } Editor e = (Editor) doc.getProperty("ownerEditor"); reference.detach(e); // so it will not be notified of its own change notifyAllObservers(); // tell everybody else to catch up with the changes reference.attach(e); // reattaches the editor } @Override public void removeUpdate(DocumentEvent arg0) { Document doc = (Document) arg0.getDocument(); try { // this.textArea.setText(doc.getText(0, doc.getLength()-1)); this.state = doc.getText(0, doc.getLength()); } catch (BadLocationException e) { // TODO Auto-generated catch block e.printStackTrace(); } Editor e = (Editor) doc.getProperty("ownerEditor"); reference.detach(e); // so it will not be notified of its own change notifyAllObservers(); // tell everybody else to catch up with the changes reference.attach(e); // reattaches the editor } // public String getJTextArea() { // return textArea.getText(); // } } </code></pre> <p>The changed <code>Editor</code>'s constructor:</p> <pre><code>public Editor(FileContentSubject filecontentsubject) { super("Editor"); initComponents(); this.reference = filecontentsubject; textArea.getDocument().addDocumentListener(reference); textArea.getDocument().putProperty("ownerEditor", this); // <---- ADDED LINE } </code></pre> <p>and <code>update()</code>:</p> <pre><code>@Override public void update() { //textArea.setText(reference.getJTextArea()); //textArea.setText(reference.temp); textArea.setText(reference.getState()); // ADDED } </code></pre> <p>That's it!</p>

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