Should you check for a duplicate before inserting into a set

Question

I am learning to use sets. My question is : Sets do not contain duplicates. When we try to insert duplicates, it does not throw any error and automatically removes duplicates. Is it a good practice to check each value before inserting into set whether it exists or not? Or is it OK to do something like the below code? I think Java would be internally doing the check using .contains(value) . What do you think?

What would be the Big O complexity in both the cases considering there are n elements going into the set?

import java.util.HashSet;
import java.util.Set;

public class DuplicateTest {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
         Set<Integer> mySet = new HashSet<Integer>();

         mySet.add(10);
         mySet.add(20);
         mySet.add(30);
         mySet.add(40);
         mySet.add(50);
         mySet.add(50);
         mySet.add(50);
         mySet.add(50);
         mySet.add(50);
         mySet.add(50);

         System.out.println("Contents of the Hash Set :"+mySet);
    }

}

Answer 1

As per the docs:

public boolean add(E e)

Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if this set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.

So the add() method already returns you a true or a false. So you don't need to do the additional check.

Answer 2

Compare with the API documentation of Set.add(E)

The add method checks if the element is already in the Set. If the element is already present, then the new element is not added, and the Set remains unchanged. In most situations, you don't need to check anything.

The complexity of the method depends of the concrete implementation of Set that you are using.

Answer 3

Its ok not to check. This is the main advantage over Sets of Lists, as they will automatically filter out duplicates.

HashSet has constant time performance (http://docs.oracle.com/javase/8/docs/api/java/util/HashSet.html)

This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets