CODEWITHSUNDEEP
javacollections
krsna
krsna

Reputation: 4333

Finding unique, duplicate words using Collections Java

I'm going through Java Collections in Oracle Docs Java. I'm not able to figure out few lines of code from the below.

This program splits the duplicate words, unique words. 

public class FindDups {
    public static void main(String[] args) {
        Set<String> uniques = new HashSet<String>();
        Set<String> dups    = new HashSet<String>();

        for (String a : args)
            if (!uniques.add(a))
                dups.add(a);

        // Destructive set-difference
        uniques.removeAll(dups);

        System.out.println("Unique words:    " + uniques);
        System.out.println("Duplicate words: " + dups);
    }
}

I'm not able to get these lines. Can anyone explain, what is actually happening here?

for (String a : args)
   if (!uniques.add(a))
       dups.add(a);

Upvotes: 2

Views: 328

Answers (3)

luk2302
luk2302

Reputation: 57124

From the docs regarding add:

Returns: true if this set did not already contain the specified element

That means the code "tries" to insert the object into uniques and if the add returns false the object was already there, meaning it is actually a duplicate - therefore it gets inserted into dups if uniques.add is false.

Afterwards all entries in dups get removed from uniques:

uniques.removeAll(dups);

After that uniques only contains the Strings present only once while dups contains all Strings that occurred multiple times.

Upvotes: 2

thatguy
thatguy

Reputation: 22089

From the Oracle docs for the add method of Set:

true if this set did not already contain the specified element

So what the code does is:

  1. for (String a : args): Iterate over all the input strings stored in the args array.
  2. if (!uniques.add(a)): If the add method returns true, the string is unique and successfully added to the Set uniques, as it is not contained, yet (see docs). Otherwise, the add method returns false (the current string a is already contained in the Set uniques) so...
  3. dups.add(a);: ...add it to the Set of duplicate elements.

As a note for a beginner, a Set can only contain an individual element once. So, if a is already in uniques, the call uniques.add(a) will just return true, but not actually add anything. If not, the element is added and it returns false.

Upvotes: 2

K Weston
K Weston

Reputation: 61

   if (!uniques.add(a))

uniques.add(a) will return True if the word that was added was NOT in the set already. False if it is inside it (this method has a return of type Boolean).

       dups.add(a);

So if it sees that the word is already in the set, it will add it to the dupes set.

Hope that helps!

Upvotes: 1

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