/* * 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(); }