Iterating through sets

Question

I am writing a program that will receive a list of words. After that, it will store the repeated words and the non repeated into two different lists. My code is the following:

public class Runner 
{
    public static void run (Set<String> words)
    {

        Set<String> uniques= new HashSet<String>();

        Set<String> dupes= new HashSet<String>();

        Iterator<String> w = words.iterator();

        while (w.hasNext())
        {
            if (!(uniques.add(w.next())))
                    {
                        dupes.add(w.next());
                    }
            else
            {
                uniques.add(w.next());
            }


        }

        System.out.println ("Uniques: "+ uniques);
        System.out.println ("Dupes: "+ dupes);



    }

}

However, the output for the following:

right, left, up, left, down

is:

Uniques: [left, right, up, down]

Dupes: []

and my desired would be:

Uniques: [right, left, up, down]

Dupes: [ left ]

I want to achieve this using sets. I know it would be way easier to just an ArrayList but I am trying to understand sets.

Answer 1

The reason for your problem is that the argument words is a Set<String>. A set by definition will not contain duplicates. The argument words should be a List<String>. The code also makes the mistake of calling w.next() twice. A call to the next() will cause the iterator to advance.

public static void run(List<String> words) {
    Set<String> uniques= new HashSet<String>();
    Set<String> dupes= new HashSet<String>();
    Iterator<String> w = words.iterator();
    while(w.hasNext()) {
         String word = w.next();
         if(!uniques.add(word)) {
             dupes.add(word);
         }
    }
}

Answer 2

• You are doing uniques.add(w.next()) twice. Why?
• Also, don't keep calling w.next() - this makes the iteration happen. Call it once and keep a local reference.

Use:

String next = w.next();
if(uniques.contains(next)) {
    // it's a dupe
} else {
    // it's not a dupe
}