This is java

Dequeue. A double-ended queue or deque (pronounced "deck") is a generalization of a stack and a queue that supports adding and removing items from either the front or the back of the data structure. Create a generic data type Deque that implements the following API:

public class Deque implements Iterable { public Deque() // construct an empty deque public boolean isEmpty() // is the deque empty? public int size() // return the number of items on the deque public void addFirst(Item item) // add the item to the front public void addLast(Item item) // add the item to the end public Item removeFirst() // remove and return the item from the front public Item removeLast() // remove and return the item from the end public Iterator iterator() // return an iterator over items in order from front to end public static void main(String[] args) // unit testing (required) } 

Corner cases. Throw a java.lang.NullPointerException if the client attempts to add a null item; throw a java.util.NoSuchElementException if the client attempts to remove an item from an empty deque; throw a java.lang.UnsupportedOperationException if the client calls the remove() method in the iterator; throw a java.util.NoSuchElementException if the client calls the next() method in the iterator and there are no more items to return.

Performance requirements. Your deque implementation must support each deque operation (including construction) in constant worst-case time and use space linear in the number of items currently in the deque. Additionally, your iterator implementation must support each operation (including construction) in constant worst-case time. Hint: you'll want to use a doubly-linked list.