1、Queue（ One way queue ）

1.1 Definition

A common queue is FIFO（ fifo ） queue , You can delete the front end （peek/pop from front）, Back end add （push to back） The function of .

1.2 Realization （ You can use arrays - Sequential queue , You can also use a linked list - Chain queues ）

A subinterface ：BlockingQueue

1. Java The single queue in is realized by linked list ;
2. hinder Deque（ deque ） Inherited Queue, It's the point , It is also a good thing .
3. Queue Itself is an interface , Inherited Collection aggregate ;
4. There are basic queue abstraction methods , Three types of , Insert elements at the end of the team , Team leader takes out elements , And delete the team head element ;

Method ： Throw an exception / Return default (false / null)
Additive elements ：add / offer
Delete ：remove / poll
lookup ：element / peek

2、Deque（ deque ）

1.1 Definition

deque , seeing the name of a thing one thinks of its function , It's not just FIFO（ fifo ）, You can also use LIFO （LIFO）, There are also priority queues ...

1.2 Realization

deque （Deque） Inherited one-way queue （Queue）, It also inherits all the abstract methods , So it also includes the method of adding, deleting and checking the basic queue .

Implementation class ：LinkedList、ArrayDeque、ConcurrentLinkedDeque、LinkedBlockingDeque




Be careful ：ArrayDeque（ The bottom layer is a circular array ） and LinkedList（ The bottom layer is a two-way circular list ）

1. Take up memory ： Only data needs to be stored in the array , The bidirectional linked list also needs to store the addresses of the previous node and the next node , Therefore, the memory consumption is large .
2. efficiency ： For inquiry , Arrays naturally have advantages over linked lists , For addition and deletion , Only the header of the queue is deleted , Tail insertion ,ArrayDeque Just move the head or tail variable forward or backward by one bit , and LinkedList You also need to connect pointers , therefore ArrayDeque Is more efficient than LinkedList high .
3. Null value ：ArrayDeque Inserting... Is not allowed null,LinkedList Allow to insert null（ But it's best not to ）.
4. Security ： Both are thread unsafe .
   

1.3 Method

Deque The additional implementation methods are as follows ：（ Both the beginning and the end can be added, deleted and checked ）

Deque Can be viewed as Queue Use , First in first out queue .

Queue Can be viewed as Stack Use .

Deque There is also an additional method in to delete the first or last matched element .

boolean removeFirstOccurrence(Object o);
boolean removeLastOccurrence(Object o);

reference ：
1、Queue、Deque、LinkedList、ArrayDeque Relationship and method
2、 In depth understanding of Java Gather them together —Deque
3、 Why? java It is not recommended to use vector
4、 Analyze in simple terms ArrayDeque