How can you make the efficient many-to-many -relation from fileID to Words and from word to fileIDs without database -tools like Postgres in Java?

I have the following classes. The relation from fileID to words is cheap, but not the reverse, since I need three for -loops for it.

My solution is not apparently efficient. Other options may be to create an extra class that have word as an ID with the ArrayList of fileIDs.

Reply to JacobM's answer

The relevant part of MyFile's constructors is:

            /**
             * Synopsis of data in wordToWordConutInFile.txt:
             * fileID|wordID|wordCount
             *
             * Synopsis of the data in the file wordToWordID.txt:
             * word|wordID
             **/        


    /**
     * Getting words by getting first wordIDs from wordToWordCountInFile.txt and then words in wordToWordID.txt.
     */
    InputStream in2 = new FileInputStream("/home/dev/wordToWordCountInFile.txt");
    BufferedReader fi2 = new BufferedReader(new InputStreamReader(in2));

    ArrayList<Integer> wordIDs = new ArrayList<Integer>();
    String line = null;
    while ((line = fi2.readLine()) != null) {
        if ((new Integer(line.split("|")[0]) == currentFileID)) {
            wordIDs.add(new Integer(line.split("|")[6]));
        }
    }
    in2.close();

    // Getting now the words by wordIDs.
    InputStream in3 = new FileInputStream("/home/dev/wordToWordID.txt");
    BufferedReader fi3 = new BufferedReader(new InputStreamReader(in3));

    line = null;
    while ((line = fi3.readLine()) != null) {
        for (Integer wordID : wordIDs) {
            if (wordID == (new Integer(line.split("|")[1]))) {
                this.words.add(new Word(new String(line.split("|")[0]), fileID));
                break;
            }
        }
    }
    in3.close();

    this.words.addAll(words);

The constructor of Word is at the paste.