COMP 14 Introduction to Programming

COMP 14Introduction 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 Modifiersprivate 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!! • In order to copy object data, we need additional methods Player player1 = new Player(“Felton”); Player player2 = new Player(); player2 = player1; //player2 points to the same obj. public void Player.copy(Player otherPlayer)

Player.java • Write Player.copy()

Visibility Modifiersstatic • 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 Modifiersstatic • static methods • Operations that do not use object-dependent data • They can only use static data members!! (besides local variables) public static int factorial(int x) { … } static BufferedReader keyboard = … public static void main(…) { String str=keyboard.readLine(); … }

Visibility Modifiersstatic • To access static data members and methods from outside the class: • To access non-static data members and methods from outside the class: • In general, from inside the class, no need to write class name or reference variable. double res = Math.pow(2.0, 3.0); //ClassName.method(); player1.printInfo(); //referenceVariable.method();

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

COMP 14 Introduction to Programming