The Java.util package provides the Queue interface, which extends the Collection interface to hold elements based on the FIFO (first-in, first-out) principle. It maintains an ordered list where new elements are added at the end, and old elements are removed from the beginning. The LinkedList and PriorityQueue classes are popular concrete classes for implementing Queue. However, these classes are not thread-safe. For a thread-safe implementation, the PriorityBlockingQueue class can be used.
The Queue interface supports all the methods of the Collection interface, such as insertion, deletion, and more. LinkedList, ArrayBlockingQueue, and PriorityQueue are popular implementations of the Queue interface. However, any null operation on the blocking queues results in throwing a NullPointerException. The util package includes unbounded queues, while the util.concurrent package includes bounded queues.
The Queue interface allows inserting an element with the add() method, which returns true on success, or the offer() method, which inserts the element without throwing any exception. The remove() method retrieves and removes the queue's head, while the poll() method obtains and removes the queue's head, or returns null if the queue is empty. The element() method retrieves, but does not remove, the queue's head. Similarly, the peek() method obtains the head of the queue without removing it or returns null if the queue is empty.
On the other hand, the PriorityQueue class prioritizes objects according to their processing, as opposed to the FIFO pattern used by the Queue interface. PriorityQueue is an unbounded queue that does not permit null values. A priority queue cannot be created for non-comparable objects. It inherits from classes such as Collection, AbstractCollection, AbstractQueue, and Object. The head/front of the queue contains the least element according to the natural ordering.
The PriorityQueue class provides several methods such as add(), clear(), contains(), iterator(), offer(), peek(), poll(), size(), toArray(), and toArray(T[] a). However, the implementation of the priority queue is not thread-safe. Therefore, the PriorityBlockingQueue should be used for synchronized access.
In summary, the Queue interface and PriorityQueue class are two essential classes in Java's collection framework, each with its significance based on its implementation. While Queue maintains an ordered list of elements according to FIFO, PriorityQueue prioritizes the elements according to their importance, allowing for the quick retrieval of the most important element. Therefore, it is crucial to choose the appropriate implementation based on the specific requirements.