Implementing inverted index in Java

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I am trying to implement inverted index in java from few days..but I am unable to implement it.the term and term frequencies are coming nicely but I am unable to retrieve the document Id's.I am not getting the idea how to use two treemap, or how to wrap one treemap inside another treemap.

I am attaching the code here.

import java.util.*;  
import java.io.*;  
 
public class invertindex{
 
public static void main (String[] args)
{
     TreeMap <String, Integer> t1 = new TreeMap<String, Integer>();
   // TreeMap <String, TreeSet> t2 = new TreeMap<String, TreeSet>();
     readFile(t1);  
     //print(t1);  
}
 
public static int getWord
        (String word, TreeMap <String, Integer> t1 )
{
 if (t1.containsKey(word))
 {
    return t1.get(word);
 }
 else {
     return 0;
 }
}
 
 
public static void readFile(TreeMap <String, Integer> t1 )
{
//    t1.clear();
    Scanner File;
    String word; 
    Integer count;
    String Docs [] = {"words.txt", "words2.txt","words3.txt", "words4.txt",};
   try  
      {      
for (int x=0; x<Docs.length; x++)  
{ 
   t1.clear();
 
          File f= new File(Docs[x]);
          BufferedReader br= new BufferedReader(new FileReader(f));
 
        // File = new Scanner(new FileReader(Docs[x]));  
 
   String str="";
      while ((str=br.readLine())!=null)  
      {   
//  word = File.next( );  
          StringTokenizer stk=new StringTokenizer(str, " ,.-");
          while(stk.hasMoreTokens())
          {
             word=stk.nextToken();
          word = word.toLowerCase(); 
 
  count = getWord(word, t1) + 1;  
  t1.put(word, count);  
          }
      }
 
   print(t1);
   }
       } 
 
      catch (Exception e)  
      {  
 System.err.println(e);  
 return;  
      }
   }
 
public static void print(TreeMap<String, Integer> t1)
{
    System.out.println("(Term, TermFrequency)");
    System.out.println("--------------------");
 
     for(String word : t1.keySet( ))  
      {  
         System.out.printf("(%s,%d);", word, t1.get(word));  
      }  
 
}
}