Problem of the Day

1. Problem of the Day • At what times do the minute and hour hands on an analog clock line up?

2. Problem of the Day • At what times do the minute and hour hands on an analog clock line up?

3. CSC 212 – Data Structures Lecture 21:Stack ADT

4. Rest of the Year Abstract–List what is done, not how it is done Data– Access & use Collections of data Type– Will use in fields, parameters, & locals

5. Rest of the Year Abstract–List what is done, not how it is done Data–Access & use Collectionsof data Type– Will use in fields, parameters, & locals

6. ADTs Mean Interfaces • Each ADT is defined by single Interface • Guarantees methods exist & what they should do • But classes are free to implement however they want • Programmer knows from the interface: • Each of the method signatures • Value returned by the method • The effects of the method’s actions • Why Exceptions thrown by method

7. View of an ADT IOU

8. View of an ADT Other Coder You IOU

9. View of an ADT Other Coder You IOU ADT

10. Implementing ADT • ADTs must remain abstract • Any implementation okay, if it completes methods • Could implement an ADT with:

11. Implementing ADT • ADTs must remain abstract • Any implementation okay, if it completes methods • Could implement an ADT with: • Array

12. Implementing ADT • ADTs must remain abstract • Any implementation okay, if it completes methods • Could implement an ADT with: • Array • Linked list

13. Implementing ADT • ADTs must remain abstract • Any implementation okay, if it completes methods • Could implement an ADT with: • Array • Linked list • Trained monkeys

14. Implementing ADT • ADTs must remain abstract • Any implementation okay, if it completes methods • Could implement an ADT with: • Array • Linked list • Trained monkeys • College students

15. Is linked list an ADT? NO!

16. Why is linked list not ADT? • Linked lists have very specific implementation • Singly-, & doubly-linked versions exist… • … but implementation impossible using an array • No trained monkeys could do same work • Linked lists also do not specify functionality • No standard way to access or use data • In fact, there is no interface serving as guarantee!

17. Implementation vs. ADT Implementation ADT

18. Collection Classes • Superinterfacefor all our ADTs • Define methods common to all data structures • Access & usages patterns differ with each ADT

19. Collection Classes • Superinterfacefor all our ADTs • Define methods common to all data structures • Access & usages patterns differ with each ADT public interface Collection {public int size();public booleanisEmpty(); }

20. Collection Classes • Superinterfacefor all our ADTs • Define methods common to all data structures • Access & usages patterns differ with each ADT public interface Collection {public int size();public booleanisEmpty(); }

21. Collection Classes • Superinterfacefor all our ADTs • Define methods common to all data structures • Access & usages patterns differ with each ADT public interface Collection {public int size();public booleanisEmpty(); }

22. Stacks • Awwww… our first collection class • Works like PEZ dispenser: • Add by pushing data onto top • Pop top item off to remove • Accessing other values impossible • Top item only is available • Cheap plastic/private fields get in way

23. Applications of Stacks • Stacks are used everywhere • Back & Forward buttons in web browser • Powerpoint’sUndo & Redo commands • Methods’ stackframesused during execution • Java uses stacks to execute operations in program

24. Stack ADT • Defines two vital methods… • push(obj) add obj onto top of stack • pop() remove & return item on top of stack • … an accessormethod… • top() return top item (but do not remove it) • … and Collection’s methods… • size() returns number of items in stack • isEmpty() states if stack contains items

25. More Stack ADT • ADT also defines own exception public class EmptyStackException extends RuntimeException{public EmptyStackException(String err) {super(err);} } • EmptyStackExceptionis unchecked • Need not be listed in throws, but could be • Unchecked since there is little you can do to fix it • try-catchnot required, but can crash program

26. Stack Interface public interface Stack<E> extends Collection {public Etop()throws EmptyStackException;public Epop() throws EmptyStackException;public void push(E element); } • Any type of data stored within a Stack • Generics enable us to avoid rewriting this code • Minimum set of exceptions defined by interface • Classes could throw more unchecked exceptions

27. Using Stack • Last-In, First-Out principle used to access data • Also called LIFO ordering • Top of stack is where data added & removed

28. Using Stack • Last-In, First-Out principle used to access data • Also called LIFO ordering • Top of stack is where data added & removed • Pulling out tablecloth trick does not work

29. Your Turn • Get into your groups and complete activity

30. For Next Lecture • Read GT5.1.2 – 5.1.3 for Friday’s class • How can we use an array to implement a Stack? • Linked-list based approaches possible, too? • Why would we prefer one over the other? • Week #8 weekly assignment due on Tuesday • Programming assignment #1 also on Angel • Pulls everything together and shows off your stuff • Better get moving on it, since due in 10 days!