Another core interface provided by the collection framework is the Set interface. A Set is a collection of unique elements and all of its methods ensure this stays true. The collection framework provides different implementations of a Set (we’ll focus on a subset of them) that each have different use cases.

The HashSet implementation has the best performance when retrieving or inserting elements but cannot guarantee any ordering among them.

The TreeSet implementation does not perform as well on insertion and deletion of elements but does keep the elements stored in order based on their values (this can be customized).

The LinkedHashSet implementation has a slightly slower performance on insertion and deletion of elements than a HashSet but keeps elements in insertion order.

Let’s look at how we can create Set with a HashSet implementation:

Set<Integer> intSet = new HashSet<>();  // Empty set
intSet.add(6);  // true - 6  
intSet.add(0);  //  true - 0, 6 (no guaranteed ordering)
intSet.add(6);  //  false - 0, 6 (no change, no guaranteed ordering)

boolean isNineInSet = intSet.contains(9);  // false
boolean isZeroInSet = intSet.contains(0);  // true

In the example above we:

• Created a Set reference named intSet with a HashSet implementation.
• Called add(), which adds elements to the Set and returns true if an element was successfully added or false if not.
• Called add(), noting that the program will not throw an error if we try to insert a non-unique element into the set.
• Called contains(9), which returns false because the 9 does not exist in intSet.
• Called contains(0), which returns true because the 0 does exist in intSet.

We can iterate through a Set using the enhanced for-loop and notice that we can’t guarantee the ordering of elements looped. For example:

// Assuming `intSet` has elements -> 1, 5, 9, 0, 23
for (Integer number: intSet) {
  System.out.println(number);
}
// OUTPUT TERMINAL: 5 0 23 9 1

Let’s practice creating a Set and iterating through it.