The Java Tutorials have been written for JDK 8.Java教程是为JDK 8编写的。Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.本页中描述的示例和实践没有利用后续版本中引入的改进，并且可能使用不再可用的技术。See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.有关Java SE 9及其后续版本中更新的语言特性的摘要，请参阅Java语言更改。
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.有关所有JDK版本的新功能、增强功能以及已删除或不推荐的选项的信息，请参阅JDK发行说明。
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 LinkedList implement 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:
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 are offerFirst and offerLast because addFirst might fail to throw an exception if it is full.
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 methods removeFirst and removeLast throw an exception if the Deque instance is empty.
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 retrieval of Deque elements are summarized in the following table:
| 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 occurrence of the specified element if it exists in the Deque instance. If the element does not exist then the Deque instance remains unchanged. Another similar method is removeLastOccurence; this method removes the last occurrence 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.