A class is a definition for a specific organization of data and instructions (called methods) for manipulating the data.
An object is an instantiation of the class, that is, an allocation of computer memory to hold specific data.
Example --- class Book
A book is a sequentially-numbered set of pages bound together along one edge. A book has an author, a title, a publisher and an IsbnNumber.
A book can be printed, copied, and searched for specific information.
class Book
data: pageSet;
title;
author;
publisher;
isbn;
methods: getTitle;
getAuthor;
getIsbn;
getPublisher
searchKey;
Book Object
_________________________________________________
Once a book is defined and instantiated, it is accessed by sending
messages to and from it. A message is the name of a method
together with any data needed to make the method work.
book myBook;
myBook= new book();
myBook.searchKey("whale")
_________________________________________________________
A Java program consists of a set of classes, together with a message that starts it running.
Example --- Library Management
class Library
data: set of Book;
library_name;
methods:
find_book(title);
sort_by_title;
sort_by_author;
_________________________
class Myprogram
data: Library
methods: start;
options_menu; if ask_for_title selected call ask_for_title;
ask_for_title(title); ....
ask_for_author(author);
... if quit selected call quit;
quit;
___________________________________________________________
send message: Myprogram.start