problem with threads

I am having problem with threads in this project, I have to create a thread that will sort the files and the output should be threadedsort.sort is sorted. I keep getting error message
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0
at SortTest.main(SortTest.java:24)
Java Result: 1

unsure where my problem is because I am very new to programming and unsure how to fix this. Any help would be greatly appreciated as to why the error message and also guidance on how to fix it.

public class Sort {

    /**
     * You are to implement this method. The method should invoke one or more
     * threads to read and sort the data from the collection of Files. The
     * method should return a sorted list of all of the String data contained in
     * the files.
     *
     * @param files
     * @return
     * @throws IOException
     */
    public static String[] threadedSort(File[] files) throws IOException {
        String[] sortedData = new String[0];

        ArrayList<SortedThread> ThreadArray = new ArrayList<SortedThread>();
        // loop that creates the thread
       
        for (File file : files) {
         SortedThread thread = new SortedThread(getData(file));   
            thread.start();
        }
        // waiting for loops to be finish
        // loop that merges into a single file by calling the join method
        for (SortedThread thread : ThreadArray) {
            if (thread.isAlive()) {
                try {
                    thread.join();
                     sortedData = MergeSort.merge(sortedData, thread.threadData());
                } catch (InterruptedException ex) {
                    Logger.getLogger(Sort.class.getName()).log(Level.SEVERE, null, ex);
                }
            }

        }
        
        //return single value
        return sortedData;
    }

   
    /**
     * This method will read in the string data from the specified file and
     * return the data as an array of String objects.
     *
     * @param file the file containing the String data
     * @return String array containing the String data
     * @throws IOException thrown if any errors occur reading the file
     */
    private static String[] getData(File file) throws IOException {

        ArrayList<String> data = new ArrayList<String>();
        BufferedReader in = new BufferedReader(new FileReader(file));

        // Read the data from the file until the end of file is reached
        while (true) {
            String line = in.readLine();
            if (line == null) {
                // the end of file was reached
                break;
            } else {
                data.add(line);
            }
        }

        //Close the input stream and return the data
        in.close();
        return data.toArray(new String[0]);

    }
}

public class SortedThread extends Thread {

    String[] threadeddata = new String[0];
    String[] data;

    SortedThread(String[] data) {
        this.data = data;

    }

    @Override
    public void run() {
     threadeddata = MergeSort.mergeSort(data);
    }

    public String[] threadData() {
        return threadeddata;



    }
}


public static void main(String[] args) throws IOException {
  
    File[] files = {new File("enable1.txt"), new File("enable2k.txt"), new File("lower.txt"), new File("mixed.txt")};

    // Run Sort.sort on the files and test to ensure the data is sorted
    String[] sortedData = Sort.sort(files);
    for (int i=0; i<sortedData.length-1; i++) {
      if (sortedData[i].compareTo(sortedData[i+1]) > 0) {
        System.out.println("The data returned by Sort.sort is not sorted");
        throw new java.lang.IllegalStateException("The data returned by Sort.sort is not sorted");
      }
    }
    System.out.println("The data returned by Sort.sort is sorted");
    
    // Run Sort.threadedSort on the files and test to ensure the data is sorted
    String[] threadSortedData = Sort.threadedSort(files);
    for (int i=0; i<sortedData.length-1; i++) {
      if (threadSortedData[i].compareTo(threadSortedData[i+1]) > 0) {
        System.out.println("The data return by Sort.threadedSort is not sorted");
        throw new java.lang.IllegalStateException("The data returned by Sort.threadedSort is not sorted");
      }
    }
    System.out.println("The data returned by Sort.threadedSort is sorted");
    
    
  }
  
}

public class MergeSort {
  
  // The mergeSort method returns a sorted copy of the
  // String objects contained in the String array data.
  /**
   * Sorts the String objects using the merge sort algorithm.
   * 
   * @param data the String objects to be sorted
   * @return the String objects sorted in ascending order
   */
  public static String[] mergeSort(String[] data) {

    if (data.length > 1) {
      String[] left = new String[data.length / 2];
      String[] right = new String[data.length - left.length];
      System.arraycopy(data, 0, left, 0, left.length);
      System.arraycopy(data, left.length, right, 0, right.length);
    
      left = mergeSort(left);
      right = mergeSort(right);
    
      return merge(left, right);
      
    }
    else {
      return data;
    }
    
  }
  
  /**
   * The merge method accepts two String arrays that are assumed
   * to be sorted in ascending order. The method will return a
   * sorted array of String objects containing all String objects
   * from the two input collections.
   * 
   * @param left a sorted collection of String objects
   * @param right a sorted collection of String objects
   * @return a sorted collection of String objects
   */
  public static String[] merge(String[] left, String[] right) {
    
    String[] data = new String[left.length + right.length];
    
    int lIndex = 0;
    int rIndex = 0;
    
    for (int i=0; i<data.length; i++) {
      if (lIndex == left.length) {
        data[i] = right[rIndex];
        rIndex++;
      }
      else if (rIndex == right.length) {
        data[i] = left[lIndex];
        lIndex++;
      }
      else if (left[lIndex].compareTo(right[rIndex]) < 0) {
        data[i] = left[lIndex];
        lIndex++;
      }
      else {
        data[i] = right[rIndex];
        rIndex++;
      }
    }
    
    return data;
    
  }
  
}