COMP 14 Introduction to Programming Miguel A. Otaduy May 24, 2004

COMP 14 So Far... Problem Solving Mathematical Calculations Output User Input File I/O Selection (if, if-else, switch) Loops (while, do...while, for)

COMP 14 Next... Object-Oriented Design Writing Methods –pieces of code that we give a name Writing Classes –organize related pieces of information Arrays –access multiple pieces of information with a single identifier

Object-Oriented Design What is it? Designing a solution to a problem by first identifying components called objects, and determining how the objects interact with each other

Objects VCR Example Use it without knowing how it's made Internal parts are hidden -- only interact with the provided buttons Can't modify the functions of a VCR -- record button always records, play button always plays Same is true for objects (like Strings) that are provided by Java

Objects Consists of data and operations on the data Data - descriptive characteristics Operations - what it can do (or what can be done to it) Example A coin that can be flipped so that its face shows either "heads" or "tails" –data: its current face (heads or tails) –operations: it can be flipped Operations can change data.

Objects And Methods and Classes We represent operations with methods –group of statements that are given a name We can use objects and their methods without knowing exactly how the methods work An object is an instance of a class. A class is the blueprint of an object. –the class provides the methods that can operate on an object of that class

Classes A class contains data declarations and method declarations A class is a description of an object –just a model, not an actual object –you can think of the concept of a book without thinking of a particular book A class is no more an object than a blueprint is an actual house

Object-Oriented Design Simplified Methodology 1. Write down detailed description of problem 2. Identify all (relevant) nouns and verbs 3. From list of nouns, select objects 4. Identify data components of each object 5. From list of verbs, select operation

Object-Oriented Design Example 1 Problem Statement –Write a program to input the length and width of a rectangle and calculate and print the perimeter and area of the rectangle

Example 1 Building a Rectangle Identify nouns –length, width, rectangle, perimeter, area Identify each class –length of a rectangle –width of a rectangle –perimeter of a rectangle –area of a rectangle

Example 1 Building a Rectangle Identify data members for each class –nouns: length, width, area, perimeter –what are the essential nouns for describing the rectangle? –area and perimeter can be computed if we know the length and width

Example 1 Building a Rectangle Identify operations for each class –input, calculate, print –setLength –setWidth –computePerimeter –computeArea –print –getLength –getWidth directly from problem statement customary to include operations to get the value of the data members

class Rectangle Data Members and Operations Last Step: design and implement an algorithm for each operation class name data members operations (methods)

Anatomy of a Class A class contains data declarations and method declarations int width; int length; Data declarations Method declarations (operations)

Classes and Objects Rectangle A class (the concept) length = 15, width = 3 An object (the realization) length = 20, width = 6 length = 15, width = 15 Multiple objects from the same class

Object-Oriented Design Example 2 A place to buy candy is from a candy machine. A new candy machine is bought for the gym, but it is not working properly. The candy machine has four dispensers to hold and release items sold by the candy machine and a cash register. The machine sells four products —candies, chips, gum, and cookies—each stored in a separate dispenser. You have been asked to write a program for this candy machine so that it can be put into operation. multiple objects/classes (pgs )

Non-Concrete Objects Objects in programs don't always have real-world analogs Example object: error message data: text of the error message operation: print the text of the error message to the screen

Next in Comp14 Tomorrow: writing methods Wednesday: review for mid-term –Bring laptops –Write questions before coming to class