Collection - The Deque Interface

Usually pronounced as deck, a deque is a double-ended-queue. A double-ended-queue is a linear collection of elements that supports the insertion and removal of elements at both end points. The Deque interface is a richer abstract data type than both Stack and Queue because it implements both stacks and queues at the same time. The Deque interface, defines methods to access the elements at both ends of the Deque instance. Methods are provided to insert, remove, and examine the elements. Predefined classes like ArrayDeque and LinkedListimplement the Deque interface.

Note that the Deque interface can be used both as last-in-first-out stacks and first-in-first-out queues. The methods given in the Deque interface are divided into three parts:

Insert

The addfirst and offerFirst methods insert elements at the beginning of the Deque instance. The methods addLast and offerLast insert elements at the end of the Deque instance. When the capacity of the Deque instance is restricted, the preferred methods areofferFirst and offerLast because addFirst might fail to throw an exception if it is full.

Remove

The removeFirst and pollFirst methods remove elements from the beginning of the Deque instance. The removeLast and pollLast methods remove elements from the end. The methods pollFirst and pollLast return null if the Deque is empty whereas the methodsremoveFirst and removeLast throw an exception if the Deque instance is empty.

Retrieve

The methods getFirst and peekFirst retrieve the first element of the Deque instance. These methods dont remove the value from the Deque instance. Similarly, the methods getLast and peekLast retrieve the last element. The methods getFirst and getLast throw an exception if the deque instance is empty whereas the methods peekFirst and peekLast return NULL.

The 12 methods for insertion, removal and retieval of Deque elements are summarized in the following table:

Deque Methods

Type of Operation	First Element (Beginning of the Deque instance)	Last Element (End of the Deque instance)
Insert	addFirst(e) offerFirst(e)	addLast(e) offerLast(e)
Remove	removeFirst() pollFirst()	removeLast() pollLast()
Examine	getFirst() peekFirst()	getLast() peekLast()

In addition to these basic methods to insert,remove and examine a Deque instance, the Deque interface also has some more predefined methods. One of these is removeFirstOccurence, this method removes the first occurence of the specified element if it exists in the Dequeinstance. If the element does not exist then the Deque instance remains unchanged. Another similar method is removeLastOccurence; this method removes the last occurence of the specified element in the Deque instance. The return type of these methods is boolean, and they return true if the element exists in the Deque instance.

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