COMP 14 Introduction to Programming Miguel A Otaduy May 28 2004 Today Defining classes using objects public private static Assignment operator and objects jGRASP projects multiple files Java archives Multiple Classes So far we have added methods to the main class Classes extend the concept of data types data operations Define new classes and instantiate objects Multiple Classes Concept Basketball team players objects of class Player coach object of class Coach games Define each class in one file and instantiate objects in other classes or in the main method Basketball java Create main class Basketball Define Player class player name year operations Multiple files Good style 1 class per file Store all files in 1 directory Create jGRASP project gpj BasketballProject gpj Example of project creation Multiple Objects A class has 1 single definition We can instantiate multiple objects of 1 class Each object keeps its own copy of the data members Objects are accessed using reference variables To perform operations on objects referenceVariable method parameter1 parameter2 Basketball java Instantiate multiple Player objects Call methods of the Player objects from Basketball main Demonstrate flow through different methods with the debugger Visibility Modifiers private and public Public For members that should be accessible outside the class i e from other classes or the main method Usually employed with methods Private For members that should not be accessed outside the class Usually employed with data members Also for methods that are only used internally Typically data members are declared as private so we need methods to set and get data values Player java Make data members private Make methods public Demonstrate accessibility from Basketball main Assignment Operator Assignment expressions that involve reference variables do not copy object data Player player1 new Player Felton Player player2 new Player player2 player1 player2 points to the same obj In order to copy object data we need additional methods public void Player copy Player otherPlayer Player java Write Player copy Visibility Modifiers static static data members Data related to a class that does not need to be defined for each object public class Player static final int FRESHMAN 1 static final int SOPHOMORE 2 private int year Visibility Modifiers static static methods Operations that do not use objectdependent data public static int factorial int x They can only use static data members besides variables keyboard staticlocal BufferedReader public static void main String str keyboard readLine Visibility Modifiers static To access static data members and methods from outside the class double res Math pow 2 0 3 0 ClassName method To access non static data members and methods from outside the class player1 printInfo referenceVariable method In general from inside the class no need to write class name or reference variable Player java Add static data members and methods The toString Method Special method in Java classes Produces a String object based on the current object suitable for printing Mapped to the operator concatenation Also called when the object is a parameter in a print or println method There is a default toString method but it s better if we write our own Player java Write Player toString To do Homework 5 due Wednesday night Don t wait till Tuesday Homework 6 is going to be rather long Read ch 9
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