Learning the Set interface in Java

The Java Set interface is one of the two sub-interfaces of the Collection class, with the other being the List interface.

The Set interface describes the Set abstract data type on how it is implemented in Java.

A Set type is a data type that can store unique values without saving the order of when the values are added.

A Set cannot contain duplicate values. You need to use a List when you need to store duplicate values.

In Java, there are several classes that implement the Set interface:

• AbstractSet
• ConcurrentSkipListSet
• CopyOnWriteArraySet
• EnumSet
• HashSet
• JobStateReasons
• LinkedHashSet
• TreeSet

But among these implementing classes, only a few are used to create a Set in Java. They are:

• EnumSet
• HashSet
• LinkedHashSet
• TreeSet

The four classes enable you to create a Set instance with different specializations.

For example, the EnumSet is a specialized implementation of the Set interface to store Java enum values.

On the other hand, the HashSet class stores your values using a HashMap instance internally.

All classes that implement the Set interface have methods to help you manipulate the data contained in the type.

For example, there are add() and remove() methods to help you add and remove values from the type.

Here’s an example of creating a HashSet variable:

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

mySet.add("Nathan");
mySet.add("Jack");
System.out.println("mySet values: " + mySet);

mySet.remove("Jack");
System.out.println("mySet values after remove: " + mySet);

As you can see from the code above, you can create a new instance of HashSet for variables of Set type.

This is because Set is implemented on all Java classes used to create a Set data type.

The code above will generate the following output:

mySet values: [Nathan, Jack]
mySet values after remove: [Nathan]

The list of defined methods in the Set interface is as follows:

• add() - adds an element passed as its argument to the set
• addAll() - add all elements of a collection to the set. You need to pass a Collection as its argument
• iterator() - returns an iterator that can be used to access elements of the set
• remove() - removes the element you passed as the argument from the set
• removeAll() - remove all elements from the set that is present in the argument. Accepts a Collection as its argument
• retainAll() - retain all elements in the set that are also present in the argument. Accepts a Collection type
• clear() - remove all elements from the set
• size() - returns the number of elements present in the set
• toArray() - returns an array containing all elements stored in the set
• contains() - returns true if the set contains the specified element
• containsAll() - returns true if the set contains all elements of the specified argument. Accepts a Collection type
• hashCode() - returns a hash code value (address of the element in the set)

Now you’ve learned how the Set interface works in Java.

Please remember that you need to use one of the classes that implement the Set interface to create one.