1 Stacks and Queues Starring: IndexOutOfBOundsException Co-Starring: NoSuchElementException

2 Purpose: In this lecture series we will discuss two more ADTs The Stack and Queue data types are used to process data in a linear fashion

3 Resources: Barrons Chapter 9 p.300 (EXCEPT Priority Queuesp ) Java Essentials Chapter 19 p.757 Java Essentials Study Guide Chapter 16 p.281 Java Methods Chapter 3 p.61

4 Handouts: 1. Stack and Queue Interfaces & Implementations 2.The Following Java Code: Stack.java Queue.java ArrayStack.java ListQueue.java

5 Intro: A STACK is a linearly ordered list A STACK is a data structure used for storing and retrieving data elements in such a way that the element stored last will be retrieved first. LIFO --- last in first out

6 Intro: A QUEUE is a data structure used for temporary storage from which the data elements are retrieved in the same order as they were stored. FIFO – first in first out Used for processing events that have to be processed in the order of their arrival (events are buffered in the queue)

7 Stacks: Think of the mountain of clothes piled in a corner of your room As you hurry to dress so as not to be late for school, you begin your search from the TOP of the clothes pile The last stuff to be thrown on to the pile is the first to be examined This is a “messy” stack

8 TO implement a Stack ADT, you must provide a way to store (push) and remove (pop) elements from a Stack Elements on the stack should be of the same type (or in the same class hierarchy) A Stack has no limits as to its size

9 TO add an element on to the stack you “Push” that item and it goes to the front of the stack TO remove an element from the stack, you can only “Pop” off the element currently at the top of the stack PUSH --- adds elements to the top of the stack POP --- removes the element from the top of the stack

10 These operations should be in Constant time O(1) Other behaviors of the Stack should return the top element of the Stack without removing it (peekTop) and to see if the Stack has any elements left in it (isEmpty)

11 For testing purposes, we will use the following as the standard Stack Interface (similar to ListNode when dealing with Linked Lists)

12 public interface Stack { // postcondition: returns true if stack is empty, false otherwise boolean isEmpty(); // precondition: stack is [e1, e2,..., en] with n >= 0 // postcondition: stack is [e1, e2,..., en, x] void push(Object x); // precondition: stack is [e1, e2,..., en] with n >= 1 // postcondition: stack is [e1, e2,..., e(n-1)]; returns en // throws an unchecked exception if the stack is empty Object pop(); // precondition: stack is [e1, e2,..., en] with n >= 1 // postcondition: returns en // throws an unchecked exception if the stack is empty Object peekTop(); }

13 RUNTIME STACK Remember the “runtime stack” ? It uses stacks to handle function calls The stack holds the function specific data PLUS the memory address of the NEXT instruction to execute Elements are “on the stack”

14 The stack is controlled by the two operations:PUSH & POP Possible Implementation of a Stack is a singly linked list enhanced by an additional reference to the tail of the list Elements are added at the tail of the list and removed at the head of the list. However, lets look at an ArrayList Implementation

15 public class ArrayStack implements Stack { private ArrayList array; public ArrayStack() { array = new ArrayList(); } public void push(Object x) { array.add(x); } public Object pop() { return array.remove(array.size() - 1); } public Object peekTop() { return array.get(array.size() - 1); } public boolean isEmpty() { return array.size() == 0; }

16 To use the Stack: // SPVM MyStack mS = new ArrayStack(); mS.push(new String (“Billy”); mS.push(new String (“Joe”); mS.push(new String (“Bob”); mS.push(new String (“Sally”); mS.push(new String (“Lucy”); The Stack Should now look like this: Lucy Sally Bob Joe Billy

17 Object o = mS.pop( ); String s = (String)mS.pop( ); String y = (String)mS.peekTop( ); Boolean empty = mS.isEmpty( ); The Stack Should now look like this: Bob Joe Billy The String y contains “Bob” The Boolean empty is False

18 Errors: The pop and peekTop methods can throw an Unchecked Exception IndexOutOfBOundsException or NoSuchElementException in cases where the stack is empty

19 If you use the ArrayList or the LinkedList Java class to implement your Stack, then these errors are handled, for you, from within these classes

20 Queues: Its lunch time and you leave class early to get into the food line “Dr.Nick” dashes as fast as he can and is first to grab a tray and get into line After Dr.Nick, “Naum” trips over a floor tile and skins his knee

21 As a result, “Alden” stops to help wipe away Naum’s tears and to give him a Scooby Doo bandaid So, Alden winds up behind Dr.Nick and Naum follows Since the line is implemented as a Queue, Dr.Nick gets the best lunch choices and Naum gets the crumbs

22 TO implement a Queue ADT, you must provide a way to store (enqueue) and remove (dequeue) elements from a Queue Elements on the Queue should be of the same type (or in the same class hierarchy) A Queue has no limits as to its size

23 TO add an element on to the Queue you “enqueue” that item and it goes to the end of the Queue TO remove an element from the Queue, you can only “dequeue” off the element currently at the top of the Queue

24 ENQUEUE --- adds elements to the end of the Queue DEQUEUE --- removes the element from the top of the Queue These operations should be in Constant time O(1)

25 Other behaviors of the Queue should return the top element of the Queue without removing it (peekFront) and to see if the Queue has any elements left in it (isEmpty) For testing purposes, we will use the following as the standard Queue Interface:

26 public interface Queue { // postcondition: returns true if queue is empty, false otherwise boolean isEmpty(); // precondition: queue is [e1, e2,..., en] with n >= 0 // postcondition: queue is [e1, e2,..., en, x] void enqueue(Object x); // precondition: queue is [e1, e2,..., en] with n >= 1 // postcondition: queue is [e2,..., en]; returns e1 // throws an unchecked exception if the queue is empty Object dequeue(); // precondition: queue is [e1, e2,..., en] with n >= 1 // postcondition: returns e1 // throws an unchecked exception if the queue is empty Object peekFront(); }

27 Ring Buffer An array used in a circular manner. Adjust the pointer that defines the “logical” first array element. The state of the queue is maintained with the help of 2 indices, FRONT and REAR.

28 Front points to the first element in the queue (returned by the next call to the dequeue) Rear points to the empty slot following the last stored element. Enqueue method stores the next element in the slot pointed to by the rear and increments the rear index.

29 PC’s have a keyboard queue implemented as a ring buffer. When a key is pressed its code does not go directly to the active program but is placed in the keyboard buffer until the program requests it.

30 Lets look at a LinkedList Implementation:

31 public class ListQueue implements Queue { private LinkedList list; public ListQueue() { list = new LinkedList(); } public void enqueue(Object x) { list.addLast(x); } public Object dequeue() { return list.removeFirst(); } public Object peekFront() { return list.getFirst(); } public boolean isEmpty() { return list.size() == 0; }

32 To use the Queue: // SPVM MyQueue mQ = new ListQueue(); mQ.enqueue(new String (“Billy”); mQ.enqueue(new String (“Joe”); mQ.enqueue(new String (“Bob”); mQ.enqueue(new String (“Sally”); mQ.enqueue(new String (“Lucy”); The Stack Should now look like this: Billy Joe Bob Sally Lucy

33 Object o = mQ.dequeue( ); String s = (String)mQ.dequeue( ); String y = (String)mQ.peekFront( ); Boolean empty = mQ.isEmpty( ); The Stack Should now look like this: Bob Sally Lucy The String y contains “Bob” The Boolean empty is False

34 Queues are best when simulating bank lines, or any other system where there is a “First Come First Served” basis

35 Lets look at Barrons Redial Telephone Feature on Page 305

36 TPS:Implement a Queue using an Array (or ArrayList) Implement the Possible Unchecked Exceptions Draw array implementation on board (RingBuffer)

37 AP AB Subset Requirements: Although there is no standard Java Interface we need to be able to work with the AP interfaces for Stacks and Queues (handouts) Students are also responsible for understanding that these ADT’s can be implemented as an array or a linked list, although you will NOT be tested on any SPECIFIC implementation

38 AP AB Subset Requirements: For our reference purposes we will look at an Array implementation of a stack called ArrayStack.java and we will look at a LinkedList implementation of a Queue called ListQueue.java

39 AP AB Subset Requirements: You will see Multiple Choice questions regarding analysis of Stack and Queue implementations and processes Multiple Choice questions may focus on your ability to evaluate the effectiveness of and to determine the best use of Stacks and Queues

40 Projects: Barrons M/C Questions Chapter 9 p.310 Do ALL Questions Except the following: # 14, 15 Decimal to Binary Expression McDonalds Palendrome

41 TEST FOLLOWS COMPLETION OF PROJECTS !!!