Question

I have to do a synonyms dictionary using a TreeMap. The TreeMap is of <Word, ArrayList<Word>> type. That means that for each key represented by a Word there will be a list of synonyms. When I want to list the contents of the dictionary, by using the method below, I discover that the ArrayList returned is null. What can I do? I tried tracing the code but I don’t seem to find the error.
The method is :

public String listContent() {
    Set set = wordList.keySet();
    Iterator it = set.iterator();
    String result = new String();
    ArrayList<Word> words = new ArrayList<Word>();
    while (it.hasNext()) {
        Word temp = (Word) it.next();
        words = wordList.get(temp);
        if (words != null) {
            Iterator it2 = words.iterator();
            result += temp.getContent();
            result += " - ";
            int size = words.size();
            while (it2.hasNext()) {
                result += ((Word) it2.next()).getContent();
                if (size != 1)
                    result += ", ";
                size--;
            }
            result += "\n";
        }
    }
    return result;
}

The ArrayList returned by wordList.get(temp) is null for some of the inserted elements. I checked the watches but there, they’re not. What should I do ?

wordList is a TreeMap<Word, ArrayList<Word>>;

EDIT – the addWord method

public void addWord(String content1, String content2)
{
  Word w1 = new Word(content1);
  Word w2 = new Word(content2);
  Set set = wordList.entrySet();
  Iterator it = set.iterator();
  boolean ok=false;
  while(it.hasNext())
  {
    Map.Entry<Word,ArrayList<Word>> temp = (Map.Entry<Word,ArrayList<Word>>) it.next();
    if(temp.getKey().getContent().matches(content1))
    {
      ArrayList<Word> words = temp.getValue();
      Iterator it2 = words.iterator();
      if(words.isEmpty()) words.add(w2);
      else
      {
        boolean ok2=true;
        while(it2.hasNext())
        {
          Word tempy = (Word) it2.next();
          if(tempy.getContent().equals(content2))
          {
            ok2=false;
            break;
          }
        }
        if(ok2) words.add(w2);
      }
      ok=true;
    }
  }
  if(!ok) {
    ArrayList<Word> tempys = new ArrayList<Word>();
    tempys.add(w2);
    wordList.put(w1,tempys);
  }

}

EDIT 2 – Word Class

   public class Word implements Serializable,Comparable {

private String content;

public Word (String content)
{
    this.content = content;
}

public void setContent(String content)
{
    this.content=content;
}

public String getContent()
{
    return content;
}

@Override
public int compareTo(Object o) {
    if(((Word)o).getContent().equals(this.getContent())) return 0;
    return 1;
}

}

Answer 1

Your compareTo method is wrong. The contract is that if A > B, then you must have B < A. Your implementation always returns 1 if the contents are not equal.

You should implement it like this:

@Override
public int compareTo(Word w) {
    return this.content.compareTo(w.content);
}

(and the Word class should implement Comparable<Word>, not Comparable).

Since a TreeMap uses this method to tell if some word is bigger or smaller than another one, and since the method returns incoherent results, the Map also returns incoherent results.