Material Sept 6. CS1706 Intro to Object Oriented Dev II - Spring 04

Transcription

1 CS1706 Intro to Object Oriented Dev II - Spring 04 Material Sept 6 Brief review last week Collections, Data Structures Interfaces, Implementations and Dependencies Bags, Sets

2 Assertions Document all pre-conditions in code and comments In code, add pre-conditions as assertions and as javadoc comments class Day {... /** month > 0 && month <= 12 */ Day(int year, int month, int day) { assert month > 0 && month <= 12 : Illegal month ; Use assertions in other places too, to assert conditions

3 Assertions or exceptions Should your program use assertions for preconditions or exceptions? What is the difference between them? Exceptions are part of the method signature, thus are part of the contract. A method terminating with an exception might still be behaving as expected, the condition just could not be avoided.

4 Is it good enough? To manage any process you must to be able to measure the quality of the process. The design process has too many variables... What to do? Use a table to do a Claims Analysis Design Guidelines Pro Con Encapsulation?? Cohesion??... more... Use list of cons to improve your code

5 Movie class... class Movie { private String title; private String[] actors; private Date release; public Movie(String t, String[] actors, Date release) { this.title = t; this.actors = actors; this.release = release; class design? How good is this String gettitle() { return title; String[] getactors() { return actors; Date getdate() { return release;

6 Claims Analysis example Design Guidelines Encapsulation Pro Con Date is a mutable object, aliasing problems Encapsulation proper use of accessors Encapsulation Completeness / Convenience Contract Actors list being returned, aliasing problems Actors[] is a list, no methods to manage it Precondition of the accessor methods? To improve this particular class design, you must refactor your code to remove the number of cons evaluations.

7 Movie class (2) class Movie { private String title; private String[] actors; private Date release; public Movie(String t, String[] actors, Date release) { this.title = t; this.actors = actors.clone(); this.release = new Date(release); String gettitle() { return title; Date getdate() { return release.clone(); String[] getactors() { return actors.clone(); // add some way of manipulating the actors list

8 What if?... String[] actors; Movie star_wars = new Movie( Phantom Menace, actors, new Date(2005, 6, 1));... for (int i = 0; i < actors.size; i++) System.out.println(actors[i]); What is wrong with this program? Does it reflect a problem in the class design?

9 Data Structures Programming constructs to implement collections Example: arrays What is characteristic of arrays? How are they created? How are they accessed? What about their size? Do we have other choices? Is a Vector a data structure? Yes? No?

10 Collections Linear collections - objects are stored in a straight line (e.g. arrays) Non-linear collections (e.g. tree) Interesting to consider order in which objects are added to collection relationship among objects in collection size (fixed or growable) Examples: line to buy pizza in Sbarro, group of students in classroom, plates in spring-loaded stack in cafeteria, names in phonebook

11 Data Structures in CS Classic collections in computer science Stack Queue Array List Hash Tree Binary tree Graphs first in, last out, unbounded first in, first out, unbounded limited size, direct access linear, sequential access, unbounded key-value pairs, key access, unbounded parent/child relation Tree with at most two children per node multiple connections (trees with loops)

12 Reuse What is a good definition for Code Reuse? Is copy/paste an example of reuse? Why should we care? Every time you modify code, you need to retest Testing/Debugging is expensive. Do it only once. Definition: reuse - existing code in the form of method, classes, or packages that can serve different purposes without recompilation

13 Java Interfaces What is an interface in java? public interface BagADT { public void add(object element); public Object removerandom(); public boolean contains(object element);... How can you use it? Which of this is legal? BagADT bag = new BagADT(); BagADT bag = getmeabag(); bag.add( hello ); bag.add(45); class MyBag implements BagADT {... BagADT bag = new MyBag(); if (bag.contains( hello )) {

14 Advantage of Interfaces What are the advantage of interfaces? Why can t I just use a class? Code dependencies How one piece of code depends on another? Coupling - how tight two programs are KEY: try to make your classes stand on their own How are interfaces related to coupling?

15 Code dependencies public void work() { Collection bag = new BagADT(); Iterator idx; getdata(bag); // get some data and put it in bag idx = bag.iterator(); while (idx.hasnext()) { System.out.println(idx.next()); What dependencies exist here?

16 Importance of Interfaces One more way to reduce code dependencies Approach in Lewis & Chase Interface to DS: BagADT Different implementations ArrayBag, LinkedBag Clients depend only (mostly) on BagADT New implementations possible and easily done Code reuse is high Client code depends only on interface (literally)

17 Managing Capacity: Array For the Bag defined in L&C Member variable private Object[] contents; In contructor contents = new Object[some size]; Two sizes to consider: # elements in collection (count), and size of array (contents.length) Extend array when count == contents.length Initial Array copy count available new size

18 Iteration Common operation over collections for (i = 0; i < size; i++) do something with element ith Problem: different collections access elements differently: arrays[i], link.next, etc. Iterator interface provides simple access to all collections public interface Iterator { boolean hasnext(); Object next(); Iteration becomes Iterator i = collection.iterator(); while (i.hastnext()) do something with i.next(); //gets current, advances index

19 Iterator in Bag The Bag creates/returns an iterator Iterator iterator() { return ArrayIterator(contents, size); How does it work? contents in use available count What does the ArrayIterator class needs to do: for (i = 0; i < contents.length; i++) do something [i] ArrayIterator needs access to contents array contents.length

20 What is wrong? BagADT b = new BagADT(); Iterator i = b.iterator(); What is wrong with this picture? hint: aliasing, encapsulation b i Also, what could happen if I add an object to the collection after I got the iterator?

21 Iterator Often work only if the collection has not been modified Modification of collection invalidates iterator Many Java s iterators throw exception if collection is modified and iterator is reaccessed Encapsulation BagADT and ArrayIterator are cooperating classes, developed together Share some private vars BUT, if no mod of contents, ref is to immutable object, and no other object can access contents[]

22 Iterator public class ArrayIterator implements Iterator { private int count; private int current; private Object[] items; public ArrayIterator(Object[] collection, int size) { items = collection; count = size; current = 0; public boolean hasnext() { return (current < count); public Object next() { if (! hasnext()) throw exception; current++; return items[current-1];

23 Bags and Sets What is the difference between a bag and a set? What need to be modified in the code to implement a set? Anything in the interface? What in the implementation?