Urgent: invalid stream header: 5B47656E

Code :

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            if (fileDialog == null) {
		fileDialog = new FileDialog(frame);
	    }
	    fileDialog.setMode(FileDialog.LOAD);
	    fileDialog.show();
 
	    String file = fileDialog.getFile();
	    if (file == null) {
		return;
	    }
	    String directory = fileDialog.getDirectory();
	    File f = new File(directory, file);
	    if (f.exists()) {
		try {
		    FileInputStream fin = new FileInputStream(f);
		   [B] ObjectInputStream istrm = new ObjectInputStream(fin);[/B]
		    Document doc = (Document) istrm.readObject();
		    if(getEditor().getDocument() != null)
			getEditor().getDocument().removeUndoableEditListener
		            (undoHandler);
		    getEditor().setDocument(doc);
		    doc.addUndoableEditListener(undoHandler);
		    resetUndoManager();
		    frame.setTitle(file);
		    validate();
		} catch (IOException io) {
		    // should put in status panel
		    System.err.println("IOException: " + io.getMessage());
		} catch (ClassNotFoundException cnf) {
		    // should put in status panel
		    System.err.println("Class not found: " + cnf.getMessage());
		}
	    } else {
		// should put in status panel
		System.err.println("No such file: " + f);
	    }

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The code written above is throwing invalid stream header exception.Please help me out to get rid of this i had tried a lot , but on the line highlighted above its throwing exception.I already search a lot on internet but not get any solution.Its urgent for my project please help me out.