Computer Science 112

1. Computer Science 112 Fundamentals of Programming II Interfaces and Implementations

2. What Is an Interface? • The public or external view of a resource • What you see when you run help on a resource • Example: Python’s math module – a set of function names and constant names

3. What Is an Implementation? • The private or internal view of a resource • What you see when you examine the full code of a resource Interface def fib(n): """Returns the nth Fibonacci number.""" if n == 1 or n == 2: return 1 else: return fib(n – 1) + fib(n – 2) Implementation

4. Types of Interfaces • Function – The function’s header and docstring • Class –The class and method headers and docstrings • Module –The class and function headers

5. Interface and Implementation • Users and implementers have different responsibilities • A interface provides an abstraction barrier between users and implementers User’s code Interface Implementer’s code

6. Why the Barrier? • The resource is easier to learn • Can plug and play resources • Can provide several implementations of the same resource (polymorphism) • Can maintain a resource without disturbing users’ code

7. Example Resource: A Bag Collection • A bag is a container where you can put things, see if they’re there, visit them all, or remove each one • There is one interface or set of methods for all bag implementations • Two common implementations use an array or a linked structure

8. UML Class Diagram for Bags BagInterface means implements an interface LinkedBag ArrayBag The Unified Modeling Language is a way of describing relationships among resources visually

9. The Interface for All Bags b.isEmpty() Returns True if empty, False otherwise len(b) Returns the number of items in the bag str(b) Returns a string representation of the bag item in b Returns True if item is present in the bag, or False otherwise foritem in b: Visit each item in the bag b.add(item) Adds item to the bag b.remove(item) Removes item from the bag b.clear() Removes all items from the bag b1 + b2 Combines items in a new bag and returns it b == anything True if equal, False otherwise

10. Actual Methods in Python b.isEmpty() isEmpty(self) len(b) __len__(self) str(b) __str__(self) item in b __contains__(self, item) foritem in b: __iter__(self) b.add(item) add(self, item) b.remove(item) remove(self, item) b.clear() clear(self) b1 + b2 __add__(self, other) b == anything __eq__(self, other)

11. Bag Implementations • Array-based • Linked structure • Always a single set of abstract operations bag1 = LinkedBag() bag2 = ArrayBag([1, 2, 3, 4])

12. Example Use of a Bag bag = LinkedBag() # or ArrayBag() bag.add("A string") bag.add("Another string") for item in bag: print(item) secondBag = LinkedBag(bag) thirdBag = bag + secondBag print(secondBag == thirdBag) for item in bag: secondBag.remove(item) print(len(secondBag)) We have a single set of abstract operations and two implementing classes, ArrayBag and LinkedBag

13. Docstrings and Preconditions A docstring states what the method does. A precondition states what must be true for a method to run correctly. The method raises an exception if a precondition is not true. defremove(self, item): """Removes item from self. Precondition: item is in self. Raises: KeyError if item is not in self."""

14. Implementation: Data • The first step is to decide what the attributes are and represent these using the appropriate data • Will name these data with class or instance variables • The __init__ method sets these up

15. Composition and Aggregation BagInterface * Node LinkedBag ArrayBag Array means “is composed of” * means “aggregates zero or more within”

16. Data for ArrayBag from arrays import Array classArrayBag(object): """An array-based bag implementation.""" # Class variable DEFAULT_CAPACITY = 10 # Constructor def__init__(self, sourceCollection = None): """Sets the initial state of self, which includes the contents of sourceCollection, if it's present.""" self._items = Array(ArrayBag.DEFAULT_CAPACITY) self._size = 0 ifsourceCollection: for item insourceCollection: self.add(item)

17. Some Accessor Methods # Accessor methods defisEmpty(self): """Returns True if len(self) == 0, or False otherwise.""" returnlen(self) == 0 def__len__(self): """Returns the number of items in self.""" returnself._size def__str__(self): """Returns the string representation of self.""" return "{" + ", ".join(map(str, self)) + "}" Try to run methods within an implementation, rather than accessing variables directly.

18. Some Mutator Methods # Mutator methods defclear(self): """Makes self become empty.""" self._size = 0 self._items = Array(ArrayBag.DEFAULT_CAPACITY) defadd(self, item): """Adds item to self.""" # Resize the array here if necessary self._items[len(self)] = item self._size += 1

19. Data for LinkedBag from node import Node classLinkedBag(object): """A link-based bag implementation.""" # Constructor def__init__(self, sourceCollection = None): """Sets the initial state of self, which includes the contents of sourceCollection, if it's present.""" self._items = None self._size = 0 ifsourceCollection: for item insourceCollection: self.add(item)

20. Some Accessor Methods # Accessor methods defisEmpty(self): """Returns True if len(self) == 0, or False otherwise.""" returnlen(self) == 0 def__len__(self): """Returns the number of items in self.""" returnself._size def__str__(self): """Returns the string representation of self.""" return "{" + ", ".join(map(str, self)) + "}" Hooray! No changes from the same methods in ArrayBag!

21. Some Mutator Methods # Mutator methods defclear(self): """Makes self become empty.""" self._size = 0 self._items = None defadd(self, item): """Adds item to self.""" self._items = Node(item, self._items) self._size += 1 The container is now a linked structure, so it must be manipulated differently.

22. For Friday Iterators