CS-2851Dr. Mark L. Hornick

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Stacks and QueuesBehavior vs. Structure

CS-2851Dr. Mark L. Hornick

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The Stack data structureThe semantics (behavior) of a Stack is

defined as follows:

1. Elements can only be inserted and removed from the front of a Stack

This is known as Last-In, First-Out (LIFO) Less commonly: First-Out, Last-In (FOLI)

2. Random-access is not defined – you cannot access the “middle” of the Stack

No get(index) behavior No add(index, element) behavior

CS-2851Dr. Mark L. Hornick

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Stack – behavioral methodsPrincipal methods that define behavior:1. push() – place an element on (top of) the

stack2. pop() – return and remove an element from

the (top of the) stack3. peek() – return the top element of the stack

Without removing (popping) it

The naming for the structure and the methods is an analogy for how the data elements within the structure are accessed

CS-2851Dr. Mark L. Hornick

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What is the best way to implement a Stack?

1. Derive from ArrayList? Advantages? Disadvantages?

2. Derive from LinkedList? Advantages? Disadvantages?

Does the internal implementation really matter as long as the resulting collection behaves like a Stack?

CS-2851Dr. Mark L. Hornick

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JCF implementation of Stack extends Vector

…with the 3 methods necessary for Stack behavior (push, pop, peek)

The extended Stack interface conforms to the idea of the stack abstraction, but… it is derived from Vector (like an ArrayList, but

less efficient!) All Vector methods are inherited by Stack

Thus there are methods to access the interior of the stack which violates the concept of a stack!

So a JCF Stack is not a “pure” stack

CS-2851Dr. Mark L. Hornick

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Queue data structureOnce again: an ordered collection of elements

Classical semantics (behavior):1. Elements can only be inserted at the

back and removed from the front of the collection

Alternately: inserted at the front and removed from the back

2. This is known as First-In, First-Out (FIFO)

CS-2851Dr. Mark L. Hornick

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Queue – behavioral methods offer() – place an element at the back of the queue

poll() or remove() – return and remove an element from the front of the queue poll() returns null if the queue is empty remove() throws an exception if the queue is empty

peek() or element() – return (without removing) the element at the front of the queue peek() returns null if the queue is empty element() throws an exception if the queue is empty

CS-2851Dr. Mark L. Hornick

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JCF Queue is an Interface Unlike Stack, there is no implementation of Queue by

itself. The interface just declares the methods that a class

implementing Queue would have

Queue extends the Collection interface:public interface Queue<E> extends Collection<E> {boolean offer(E o); // add to queueE element(); // get w/o remove E peek(); // get w/o remove E poll(); // get w/ remove E remove(); // get w/ remove What does this imply? What behavior does a Collection have?

CS-2851Dr. Mark L. Hornick

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JCF definition of Queue interface is weird

The Queue interface conforms to the idea of the queue abstraction, but it is extended from Collection, and so All Collection methods must be implemented by

anything that implements Queue There will be methods to access the interior of the

queue which violates the queue abstraction ! So you wouldn’t have a true queue

CS-2851Dr. Mark L. Hornick

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What is the best way to implement a Queue?

1. Derive from ArrayList? Advantages? Disadvantages?

2. Derive from LinkedList? Advantages? Disadvantages?

CS-2851Dr. Mark L. Hornick

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More weirdness: LinkedList implements the Queue interface!

So LinkedList is a Collection which is also a List which is also a Queue (and is also a Deque…) What does this imply?

LinkedList behaves like them all!