Here are the key points about PriorityQueue and LinkedList in Java:

Purpose & Underlying Structure:

• PriorityQueue: Primarily designed to keep elements in a priority-based sequence. It’s backed by an array-based binary heap.
• LinkedList: A general-purpose list implementation based on a doubly-linked list data structure.

Ordering:

• PriorityQueue: Elements are ordered based on their natural ordering (if they implement Comparable) or by a provided Comparator. They are not ordered by their insertion order.
• LinkedList: Maintains the order of insertion. The list’s iteration order is the same as the insertion order.

Performance:

• PriorityQueue: Provides O(log n) time for insertion and O(1) time for retrieval of the highest-priority element, but O(n) for removal of an arbitrary element.
• LinkedList: Offers constant time (O(1)) for adding or removing at the beginning or end but linear time (O(n)) for operations at arbitrary positions.

Null Elements:

• PriorityQueue: Doesn’t support null elements.
• LinkedList: Allows null elements.

Usage Scenarios:

• PriorityQueue: Best when you need to frequently access the highest (or lowest) priority element.
• LinkedList: Suited for situations where list functionalities and deque operations are needed, and there are frequent insertions and deletions at the beginning or end.

Interfaces:

• PriorityQueue: Implements the Queue interface.
• LinkedList: Implements both the List and Deque (double-ended queue) interfaces, making it more versatile.

Iterators:

• PriorityQueue: The iterator provided does not guarantee any specific order.
• LinkedList: The iterator follows the sequence in which elements were inserted (or added).

Concurrency:

• Both are not thread-safe by default. External synchronization is needed if accessed concurrently by multiple threads.

Memory Overhead:

• PriorityQueue: Due to its array-based nature, it typically has less overhead than a linked list.
• LinkedList: Has more memory overhead because each entry needs two additional references (next and previous) for navigation.

Difference Between PriorityQueue and LinkedList in Java

Criteria	PriorityQueue	LinkedList
Underlying Data Structure	Heap (binary heap)	Doubly-linked list
Ordering of Elements	Elements are ordered according to their natural ordering or by a provided comparator	Elements are stored in the order they were inserted
Main Use-case	Implementing a priority queue where elements are retrieved by priority	General-purpose list where elements are added or removed frequently from the front or back
Head Element	The least element with respect to the defined order	The first element that was inserted
Performance	Insertion: O(log n), Removal: O(log n), Peek: O(1)	Insertion at end/start: O(1), Insertion at middle: O(n), Removal: O(1) for start/end and O(n) for middle, Access: O(n)
Null Elements	Does not allow null elements	Allows null elements
Interfaces Implemented	Queue	List, Deque
Duplicates	Allows duplicates	Allows duplicates

Example: Difference Between PriorityQueue and LinkedList in Java

import java.util.LinkedList;
import java.util.PriorityQueue;
import java.util.Queue;

public class PriorityQueueVsLinkedListDemo {

    public static void main(String[] args) {

        // Create a LinkedList and add elements
        Queue<String> linkedList = new LinkedList<>();
        linkedList.add("Apple");
        linkedList.add("Banana");
        linkedList.add("Cherry");

        System.out.println("LinkedList:");
        while (!linkedList.isEmpty()) {
            System.out.println(linkedList.poll());
        }

        // Create a PriorityQueue and add elements
        Queue<String> priorityQueue = new PriorityQueue<>();
        priorityQueue.add("Apple");
        priorityQueue.add("Banana");
        priorityQueue.add("Cherry");

        System.out.println("\nPriorityQueue:");
        while (!priorityQueue.isEmpty()) {
            System.out.println(priorityQueue.poll());
        }
    }
}

Output:

LinkedList:
Apple
Banana
Cherry

PriorityQueue:
Apple
Banana
Cherry

Explanation:

1. LinkedList: A LinkedList is a doubly-linked list implementation of the List and Deque interfaces. It provides insertion, deletion, and access operations in constant time for both ends. Elements are added in the order they are inserted and are retrieved in the same order using the poll() method.

2. PriorityQueue: A PriorityQueue is a queue based on the natural ordering of its elements or based on a comparator provided at queue construction. It doesn't guarantee any specific order of elements but ensures that the highest priority element is served by the poll() method. If natural ordering is used, like in this example, then the smallest element (according to their natural order) is served first.

3. In the provided example, both the LinkedList and PriorityQueue have the same entries added. Since the natural order of strings is lexicographical order and the strings "Apple", "Banana", and "Cherry" are added in that order, the output for both the LinkedList and PriorityQueue appears the same.

4. In scenarios where you just need a list or queue structure without any order priority, LinkedList is a suitable choice. However, if you need to process elements based on some priority, then PriorityQueue should be used.