java-collections-queue

queue notes

• https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Queue.html

• interface Queue<E> extends Collection<E>

• methods

  • inserting
    • returns true if the element was added to this queue, else false
    • boolean add(E e)
      • IllegalStateException if the element cannot be added at this time due to capacity restrictions
    • boolean offer(E e)
  • removing
    • returns the head of this queue
    • E remove()
      • NoSuchElementException if this queue is empty
    • E poll()
      • returns null if this queue is empty
  • examining
    • returns the head of this queue
    • E element()
      • NoSuchElementException if this queue is empty
    • E peek()
      • returns null if this queue is empty

• for every action we have two methods, because they deal with extreme cases differently

• what can be extreme cases? for example examining element when queue is empty or adding element when queue is full

• summary

  	throws exception	returns special value
  insert	add(e)	offer(e)
  remove	remove()	poll()
  examine	element()	peek()

• the latter form of the insert operation (offer) is designed specifically for use with capacity-restricted Queue implementations

• in most implementations, insert operations cannot fail

• offer method is designed for use when failure is a normal, rather than exceptional occurrence, for example, in fixed-capacity (or "bounded") queues

• queue implementations generally do not allow insertion of null elements (null in queue usually makes no sense)

• implementations generally do not define element-based versions of methods equals and hashCode (for the same elements we may have different ordering properties, so element-based equality is not always well defined)

• the line of customers in a shop is an example of a queue

• types

  • unbounded queue - unlimited length (LinkedList)
  • bounded queue - limited length (BlockingQueue): https://github.com/mtumilowicz/java11-concurrency-blockingqueue-producer-consumer-problem

• head - exit point of a queue

• tail - entry point

• head and tail may be the same

• LIFO (stack) - head and tail are the same