/*

* An interface describing the operations to support the "double-ended

* queue" abstract data type, known as the deque and pronounced as

* "deck". Elements can be added to and removed from the head and the

* tail of the deque.

*

* In this interface, elements can be any Object type or null.

*/

public interface InstructuresDeque

{

/*

* Returns the number of elements in the deque.

*/

int size();

/*

* Returns `true` if there are no elements in the deque.

*/

boolean isEmpty();

/*

* Adds the given element to the top of the deque. (This operation

* is known as "push" for stacks, and "unshift" for sequences.)

*

* Throws IllegalStateException if this method is called when the

* deque is full.

*/

void addTop(Object element);

/*

* Adds the given element to the bottom of the deque. (This

* operation is known as "enqueue [as a verb]" for queues.)

*

* Throws IllegalStateException if this method is called when the

* deque is full.

*/

void addBottom(Object element);

/*

* Removes the deque's top-most element, returning its value. (This

* operation is known as "pop" for stacks, "shift" for sequences,

* and "dequeue [as a verb]" for queues.)

*

* Throws IllegalStateException if this method is called when the

* deque is empty.

*/

Object removeTop();

/*

* Removes the deque's bottom-most element, returning its value.

*

* Throws IllegalStateException if this method is called when the

* deque is empty.

*/

Object removeBottom();

/*

* If the deque is non-empty, returns the value of the deque's

* top-most element without removing it; returns null

* otherwise. (This operation is known as "peek" for stacks, and

* "first" for sequences.)

*

* Unlike `removeTop`, `top` does not throw an exception.

*/

Object top();

/*

* If the deque is non-empty, returns the value of the deque's

* bottom-most element without removing it; returns null

* otherwise. (This operation is known as "last" for sequences.)

*

* Unlike `removeBottom`, `bottom` does not throw an exception.

*/

Object bottom();

}