Classes and Objects CSC 171 FALL 2004 LECTURE 2

LABS Labs start this week. Go to your 2 nd (Wed or Thurs) lab Expect something like the programming exercises (P1.1 – P1.4) from chapter #1 You may use “notepad” and the “command prompt”

History The abacus

Questions What is an “object”? What is a “class”? What is the difference between an object and a class? How do I “write a class”? How do I “make an object”?

What are objects? Object: entity that you can manipulate in your programs (by invoking methods) Each object belongs to a class Class: Set of objects with the same behavior Class determines legal methods "Hello".println() // Error "Hello".length() // OK

Classes and Objects Object oriented programs – Define classes of objects – Make specific object out of class definitions – Run by having the objects interact (communicate – pass messages) A class is a type of thing – Instructor An object is a specific thing – Ted An object is an instance of a class

ANALOGY RUBBER STAMPING – The STAMP is the “Class definition” – The IMPRESSION is the “object”

DESIGN EXERCISE Think about the “class of things” we call “students” What is the “data”? What are the “behaviors”?

EXAMPLE A rectangle has a location, width, and height (5,10) 20 30

Making an object: CONSTRUCTION new Rectangle(5,10,20,30)

Reference to an object Rectangle cerealBox = new Rectangle(5,10,20,30);

Using references System.out.println(“The location is : (” + cerealBox.x + “,” + cerealBox.y);

Multiple references Rectangle cerealBox = new Rectangle(5,10,20,30); Rectangle r = cerealBox;

Programming Example The “Instructor” class – Instructors have names

Review A Computer Program //Really, A Class definition! public class myFirstProgram { public static void main(String args[]){ System.out.println(“Hello, CSC171”); }

Questions What is an “object”? – An instance of a class What is a “class”? – A definition of a set of objects What is the difference between an object and a class? How do I “write a class”? – With the “class” pattern How do I “make an object”? – By invoking a constructor