Question

Which data structure would be more preferable for making a dictionary in Java ? What would be better a tree or a hash-table ?

Answer 1

I would use something like

Map<String,Integer> dictionary = Collections.synchronizedMap(new TreeMap<String,Integer>());

Instead of Integer as the value of a String key, you can use a Class object which probably can hold a list that contains all the positions of that word inside the document
There are methods for easy retrieval of key values from the TreeMap. Following is the way you get an iterator out of TreeMap.

Set<Entry<String,Integer>> set = dictionary.entrySet();

Iterator<Entry<String,Integer>> entryItr = set.iterator();

Entry<String,Integer> entry = null;

while(entryItr.hasnext()){
entry = entryItr.next();
// Do whatever you want.
}