1. Sets

The Set Hierarchy

• Figure above shows the part of the Collections Framework that relates to sets. It includes interfaces Set, Sortedset, and Navigableset; abstract class AbstractSet; and actual classes HashSet, TreeSet, and ConcurrentSkipListSet. The HashSet is a set that is implemented using a hash table (discussed later). The TreeSet is implemented using a special kind of binary search tree, called the Red–Black tree (discussed in future chapters). The ConcurrentSkipListSet is implemented using a skip list (discussed in future chapters).

Interface java.util.Set

Modifier and Type	Method	Description
boolean	add(E e)	Adds the specified element to this set if it is not already present (optional operation).
boolean	addAll(Collection<? extends E> c)	Adds all of the elements in the specified collection to this set if they're not already present (optional operation).
void	clear()	Removes all of the elements from this set (optional operation).
boolean	contains(Object o)	Returns true if this set contains the specified element.
boolean	containsAll(Collection<?> c)	Returns true if this set contains all of the elements of the specified collection.
boolean	equals(Object o)	Compares the specified object with this set for equality.
int	hashCode()	Returns the hash code value for this set.
boolean	isEmpty()	Returns true if this set contains no elements.
Iterator<E>	iterator()	Returns an iterator over the elements in this set.
boolean	remove(Object o)	Removes the specified element from this set if it is present (optional operation).
boolean	removeAll(Collection<?> c)	Removes from this set all of its elements that are contained in the specified collection (optional operation).
boolean	retainAll(Collection<?> c)	Retains only the elements in this set that are contained in the specified collection (optional operation).
int	size()	Returns the number of elements in this set (its cardinality).
Object[]	toArray()	Returns an array containing all of the elements in this set.

Comparison of List and Set

• Collections implementing the Set interface must contain unique elements. Unlike the List.add method, the Set.add method will return false if you attempt to insert a duplicate item.
• Unlike a List, a Set does not have a get method. Therefore, elements cannot be accessed by index. So if setA is a Set object, the method call setA.get(0) would cause the syntax error method get(int) not found.
• Although you can’t reference a specific element of a Set, you can iterate through all its elements using an Iterator object. The loop below accesses each element of Set object setA. However, the elements will be accessed in arbitrary order. This means that they will not necessarily be accessed in the order in which they were inserted.

HashSet vs TreeSet vs LinkedHashSet

Features	HashSet	TreeSet	LinkedHashSet
Internal Working	HashSet internally uses HashMap for storing objects	TreeSet uses TreeMap internally to store objects	LinkedHashSet uses LinkedHashMap internally to store objects
When To Use	If you don’t want to maintain insertion order but want to store unique objects	If you want to sort the elements according to some Comparator then use TreeSet	If you want to maintain the insertion order of elements then you can use LinkedHashSet
Order	HashSet does not maintain insertion order	While TreeSet orders the elements according to supplied Comparator. By default, objects will be placed according to their natural ascending order.	LinkedHashSet maintains the insertion order of objects
Complexity of Operations	HashSet gives O(1) complexity for insertion, removing, and retrieving objects	While TreeSet gives the performance of order O(log(n)) for insertion, removing, and retrieving operations.	LinkedHashSet gives insertion, removing, and retrieving operations performance in order O(1).
Performance	The performance of HashSet is better when compared to LinkedHashSet and TreeSet.	TreeSet performance is better than LinkedHashSet except for insertion and removal operations because it has to sort the elements after each insertion and removal operation.	The performance of LinkedHashSet is slower than TreeSet. It is almost similar to HashSet but slower because LinkedHashSet internally maintains LinkedList to maintain the insertion order of elements
Compare	HashSet uses equals() and hashCode() methods to compare the objects	TreeSet uses compare() and compareTo() methods to compare the objects	LinkedHashSet uses equals() and hashCode() methods to compare it’s objects
Null Elements	HashSet allows only one null value.	TreeSet does not permit null value. If you insert null value into TreeSet, it will throw NullPointerException.	LinkedHashSet allows only one null value.
Syntax	HashSet obj = new HashSet();	TreeSet obj = new TreeSet();	LinkedHashSet obj = new LinkedHashSet();