LinkedHashSet and Queue

LinkedHashSet:

• A LinkedHashSet is a class in Java that:
• Implements the Set interface
• It is part of java.util package.

Key Features:

1. Unique Elements -> Like all sets, it does not allow duplicates.
2. Maintains Insertion Order -> Unlike HashSet (which is unordered), LinkedHashSet keeps elements in the same order you inserted them.
3. Allows null -> Can store one null element.
4. Performance -> Almost same as HashSet (O(1) average time for add, remove, contains).
5. Slightly slower than HashSet because it also maintains a linked list for order.

Methods in LinkedHashSet:

1. Since it extends HashSet, it has the same methods:
2. add(E e) – add element
3. remove(Object o) – remove element
4. contains(Object o) – check if present
5. size() – number of elements
6. clear() – remove all elements
7. isEmpty() – check if empty

Queue:

• Queue is a collection interface used to hold elements before processing, typically in FIFO (First-In-First-Out) order.
• It is part of the Java Collections Framework and defined in:
• java.util.Queue

Key Points:

1. Interface → Queue is an interface, not a class.
2. Order → Elements are usually processed in FIFO order, though some implementations (like PriorityQueue) order them differently.
3. Duplicates → Allowed.
4. Nulls → Some implementations allow one null (like LinkedList), but PriorityQueue does not.

Common Queue Methods:
(Specializes methods of Collection)

1. boolean add(E e) Inserts element, throws exception if full
2. boolean offer(E e) Inserts element, returns false if full
3. E remove() Removes head, throws exception if empty
4. E poll() Removes head, returns null if empty
5. E element() Retrieves head, throws exception if empty
6. E peek() Retrieves head, returns null if empty