Thread: can't add a file

import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;
import java.util.concurrent.*;
import java.util.AbstractCollection;
 
public class Wiki {
 
	// private ConcurrentHashMap<String, SynchronizedStack<TXTFile>> repository;
	private ConcurrentHashMap<String, Stack<TXTFile>> repository;
 
	public Wiki() {
		// repository = new ConcurrentHashMap<String,
		// SynchronizedStack<TXTFile>>();
		repository = new ConcurrentHashMap<String, Stack<TXTFile>>();
	}
 
	// HashMap to store files
	private HashMap<String, TXTFile> files;
 
	/**
	 * Method used to wait for input and invoke methods according to that input.
	 */
	public void Start()
	{
		System.out.println("Wiki started");
		String input = "";
		Scanner in = new Scanner(System.in);
		// Create the txt_files folder if it does not already exist.
		File dir = new File("txt_files");
		if(! dir.exists())
		{
			dir.mkdir();
		}
 
		// Enter a loop to parse all input.
		do{
			System.out.println(">");
			input = in.nextLine();
 
			// Perform string matching
			/*if(input.startsWith("commit"))
			{
				commit();
				continue;
			}*/
 
			if(input.startsWith("add"))
			{
				/* "\\s+" this groups all whitespaces as a delimiter... so if i have the 
				   string "Hello[space][tab]World", this should yield the strings 
				   "Hello" and "World" and omit the empty space between the space and the tab*/
				Add(input.substring(4).split("\\s+"));
				continue;
			}
 
 
			if(input.equals("show"))
			{
				list();
				continue;
			}
 
			// Fall through case
			System.out.println("Illegal command: " + input);
 
 
		}while(!input.equals("quit"));
		// Cleanup.
		System.out.println("Wiki is terminating...");
		in.close();		
	}
 
	/**
	 * Method used for listing all wiki files.
	 */
	public void list() {
		File dir = new File("txt_files");
		File[] wfiles = dir.listFiles();
		// No files in directory.
		if (wfiles == null) {
			// return;
			System.out.println("There no files !");
		}
		// Initialise streams
		FileInputStream fis = null;
		BufferedInputStream bis = null;
		BufferedReader reader = null;
 
		// Loop over all files in the directory.
		for (File f : wfiles) {
			// System.out.println(f.getName().endsWith(".txt"));
			System.out.println(f.getName());
		}
	}
 
	/**
	 * Method used for outputting the content of wiki files.
	 */
	/*
	 * private void list() { File dir = new File("txt_files"); File[] wfiles =
	 * dir.listFiles(); // No files in directory. if(wfiles == null) { return; }
	 * // Initialise streams FileInputStream fis = null; BufferedInputStream bis
	 * = null; BufferedReader reader = null;
	 * 
	 * // Loop over all files in the directory. for(File f : wfiles) {
	 * if(f.getName().endsWith(".txt")) { try{ fis = new FileInputStream(f); bis
	 * = new BufferedInputStream(fis); reader = new BufferedReader(new
	 * InputStreamReader(bis)); String entry;
	 * 
	 * while ((entry = reader.readLine()) != null) {
	 * System.out.println(f.getName().substring(0, f.getName().indexOf(".")) +
	 * "\t" + entry); }
	 * 
	 * }catch(Exception e){ e.printStackTrace(); }finally{ try{ fis.close();
	 * bis.close(); reader.close(); }catch(IOException e) {
	 * System.out.println("Error closing streams!"); e.printStackTrace(); } } }
	 * } System.out.println(); }
	 */
 
	// ?? load factor = initial-capacity x .75 = number of items (round up) that
	// HashMap supports
	/*
	 * commit method is used before adding a file to the repository if accepted
	 * version < Wiki's newest version, an Exception is thrown HashMap contains
	 * all files which will be added to the repository
	 */
 
	/*
	 * public void commit(HashMap<String, TXTFile> files) throws Exception{
	 * Iterator<String> keySet = files.keySet().iterator(); while
	 * (keySet.hasNext()) { TXTFile file = files.get(keySet.next()); add(file);
	 * } }
	 */
 
	/**
	 * Adds a file to the repository (1).
	 */
	public void Add(String[] tokens) {
		if (tokens.length < 2) {
			System.out.println("Illegal arguments");
			System.out.println("add [file_name] [content]");
			System.out.println();
			return;
		}
		// processing user input, creating a TXTFile
		String fileName = tokens[0];
		String content = tokens[1];
		TXTFile file = new TXTFile(fileName, content);
 
		// managing file versions
		Stack<TXTFile> stack = repository.get(file.getFileName());
		if (stack == null) // if there are no versions yet
		{
			stack = new Stack<TXTFile>();
			file.setVersion(1);
			TXTFile copy = file.clone();
			stack.push(copy);
			repository.put(file.getFileName(), stack);
		}
 
		else {
			int subVersion = file.getVersion();
			int latestVersion = stack.peek().getVersion();
 
			/*
			 * if(subVersion == -1) throw new Exception(
			 * "Your file has no Version yet. You need to checkout or update before submitting files"
			 * );
			 * 
			 * if(subVersion < latestVersion || subVersion > latestVersion)
			 * throw new Exception("Your file \"" +file.getFileName()
			 * +"\" has an outdated or not allowed version");
			 */
 
			file.setVersion(++latestVersion);
			stack.push(file.clone());
		}
 
	}
 
 
 
 
 
 
	/**
	 * Adds a file to the repository (2).
	 */
	/*
	 * public void add(TXTFile file) throws Exception { if
	 * (file.getFileName().length() < 1) throw new Exception("No FileName !");
	 * 
	 * //SynchronizedStack<TXTFile> stack = repository.get(file.getFileName());
	 * Stack<TXTFile> stack = repository.get(file.getFileName());
	 * 
	 * if(stack == null) { //stack = new SynchronizedStack<TXTFile>(); stack =
	 * new Stack<TXTFile>(); file.setVersion(0); TXTFile copy =
	 * file.clone(file); stack.push(file.clone(file));
	 * repository.put(file.getFileName(), stack); }
	 * 
	 * else { int subVersion = file.getVersion(); int latestVersion =
	 * stack.peek().getVersion();
	 * 
	 * if(subVersion == -1) throw new Exception(
	 * "Your file has no Version yet. You need to checkout or update before submitting files"
	 * );
	 * 
	 * if(subVersion < latestVersion || subVersion > latestVersion) throw new
	 * Exception("Your file \"" +file.getFileName()
	 * +"\" has an outdated or not allowed version");
	 * 
	 * file.setVersion(++latestVersion); stack.push(file.clone(file)); } }
	 */
 
	// removes a file from the repository
	public void remove(String fileName) throws Exception {
		if (!repository.containsKey(fileName))
			throw new Exception("The file \"" + fileName
					+ "\" doesnt exist in the Repository");
 
		repository.remove(fileName);
	}
 
	// Update method to update a single file to the newest version available
	public TXTFile update(String filename) throws Exception {
		if (!repository.containsKey(filename))
			throw new Exception("The file \"" + filename
					+ "\" doesnt exist in the Repository");
 
		// SynchronizedStack<TXTFile> stack = repository.get(filename);
		Stack<TXTFile> stack = repository.get(filename);
 
		TXTFile file = stack.peek();
 
		//try {
			return file.clone();
		//} catch (CloneNotSupportedException ex) {
		//	return null;
		//}
	}
 
	// Reverts a single file to the version given as parameter
	public TXTFile revert(String filename, int version) throws Exception {
		if (!repository.containsKey(filename))
			throw new Exception("The file \"" + filename
					+ "\" doesnt exist in the Repository");
 
		// SynchronizedStack<JavaCode> stack = repository.get(filename);
		Stack<TXTFile> stack = repository.get(filename);
 
		if (version < 0 || version >= stack.size())
			throw new Exception("Please check your version. Your Version: \""
					+ version + "\"");
 
		TXTFile file = stack.get(version);
 
		Boolean found = false;
		if (file.getVersion() != version) {
			Iterator<TXTFile> iterator = stack.iterator();
 
			while (iterator.hasNext())
				if (iterator.next().getVersion() == version) {
					found = true;
					break;
				}
			if (!found)
				throw new Exception("The version \"" + version
						+ "\" of the file \"" + filename
						+ "\" doesn't exist in the Repository");
		}
 
		return file.clone();
	}
 
}

public class TXTFile {
 
	private String fileName;
	private int version;
	private String content;
 
	public TXTFile(String fileName, String content){
		this.fileName = fileName;
		this.content = content;
	}
 
	public int getVersion() {
		return this.version;
	}
 
	public void setVersion(int version) {
		this.version = version;
	}
 
	public TXTFile(String fn){
		this.fileName = fn;
	}
 
	public String getFileName() {
		return this.fileName;
	}
 
	public void setFileName(String fileName) {
		fileName = fileName;
	}
 
	@Override public TXTFile clone(){
		try{
			return (TXTFile) super.clone();
		} catch (CloneNotSupportedException e){
			throw new AssertionError();
		}
	}
 
	/*public TXTFile clone(TXTFile file) throws CloneNotSupportedException{
		return (TXTFile)file.clone();
	}*/
 
	public String getContent() {
		return this.content;
	}
 
	public void setContent(String cont) {
		this.content = cont;
	}
 
 
}

public class Main {
	public static void main(String[] args) {
 
		// Start Wiki
		Wiki wiki = new Wiki();
		wiki.Start();
 
	}
 
}

Console output:
Wiki started
>
show
handler copy 2.txt
handler copy.txt
handler.txt
>
add example.txt content
Exception in thread "main" java.lang.AssertionError
at TXTFile.clone(TXTFile.java:37)
at Wiki.Add(Wiki.java:163)
at Wiki.Start(Wiki.java:59)
at Main.main(Main.java:6)

what's wrong ?

Clone is not supported on File...

You might want to look at the java.io package, linked below, in more detail

java.io (Java Platform SE 7 )

Have you tried implementing the cloneable interface?
read the API doc for the cloneable interface.