The last data structure we’ll cover in this chapter is HashSet
, a concrete implementation of the Set
interface.
The Set
interface in Java is a Collection
that is modelled after the mathematical concept of a set. They are collections of elements that don’t allow duplicate entries.
In this section we’ll cover some of the basic operations offered by the Set
interface using HashSet
as the concrete implementation for the examples.
Another common implementation of Set<E> is TreeSet<E> which we’ll not cover in this beginners course.

In the example code run the JavaBasicHashSetApp

Creating a HashSet
Let’s create a HashSet
that will hold a set of numbers to show some of the properties of this data structure:
Set<Integer> numbers = new HashSet<>();
System.out.println("Initial size of the set: " + numbers.size());
Output:
Initial size of the set: 0
Adding elements
To add elements to a Set<E>
we use the add(E element)
method in the same way we did in the ArrayList
section. For example, to add the number 10 we’d do:
numbers.add(10);
System.out.println("New size of the set: " + numbers.size());
Output:
New size of the set: 1
The add
method returns a boolean to indicate whether the element was added or not to the set.
For example, if we try to add the number 10 again to our set, we should get false
as a result as the number already exists in our set (remember Set
doesn’t allow duplicates):
boolean secondTenAdded = numbers.add(10);
System.out.println("Was the second 10 added?: " + secondTenAdded);
System.out.println("Size of the set after 2nd addition: " + numbers.size());
Output:
Was the second 10 added?: false
Size of the set after 2nd addition: 1
We can see that, as expected, the size of the set is still 1 after attempting to add the same element more than once.
For the purpose of this example, we’ll add a couple more numbers to our set:
numbers.add(20);
numbers.add(30);
Our set now contains the numbers 10, 20 and 30.
Iterating over elements
To iterate over the elements of a set we can use an enhancedfor
loop, for example, if we want to print out all the numbers in our example set, we could do the following:
for (Integer number : numbers) {
System.out.println("Number in set: " + number);
}
Output:
Number in set: 20
Number in set: 10
Number in set: 30
Note that the ordering of the output isn’t the same as the order in which the elements were inserted. This is because HashSet doesn’t provide a sense of ordering of elements, in contrast to an ArrayList where each element can be identified by an index.

As discussed before, since Java 8 there are more ways of doing this. These will be covered in a later chapter after we introduce the concept of lambdas. 
Checking if an element exists
In terms of performance, this is where a HashSet
(and HashMap
) excel as the lookup performance is of O(1)
(beware of the notes mentioned in the hash tables section).
To check if an element already exists in the Set
we make use of the contains(E element)
method, for example, to check if numbers 10 and 15 already exist:
System.out.println("Number 10 exists in the set?: " + numbers.contains(10));
System.out.println("Number 15 exists in the set?: " + numbers.contains(15));
Output:
Number 10 exists in the set?: true
Number 15 exists in the set?: false
Removing an element
To remove an element from a Set
(or a Collection
in general), we make use of the remove(Object element)
method. For example, to remove number 20 from our set we’d do the following:
numbers.remove(20);
Now, if we check if the set contains that number, we should get false as expected:
System.out.println("Number 20 exists in the set?: " + numbers.contains(20));
Output:
Number 20 exists in the set?: false
Other notes on HashSet

The
HashSet
implementation uses the same theory explained in the hash tables section. Bear in mind thehashCode
andequals
methods when using hashbased set implementations. 
Behind the scenes,
HashSet
uses aHashMap
to store its data.