Classes Objects 03

Objects and Classes ISYS350 Instructor: Lei Jin

Defining Class Variables and Methods: static public class TestMath public class Math { public final static double PI=3.14159; { public static void main(String[] args) … {System.out.println(Math.PI); public static double sqrt(double a) System.out.println(Math.sqrt(32)); {…} System.out.println(Math.random()*100); public static double random() {…} } } }

•Class variables(static variables) belongs to the class, not an individual object of the class. •All objects of the same class have the same value for its class variable. if one object changes the value of a static variable, the value of all object are affected. •Static variables should be initialized when they are declared. •Static methods cannot use instance variables or call instance methods.

Access Class Variables and Invoke Class Methods • Access class variables Classname.variableName Math.PI Color.black

• Calling class methods ClassName.methodName(argumentList); or ClassName.methodName(); Math.random(); MyInput.readInt();

You don’t have to create an object of the class to access class variables or class methods !!!

Class Design Principles • • •

Always keep data private - enforce data encapsulation Always initialize data; Don’t use too many basic types in a class, instead, organize them into a new class if necessary private String street; private string city; private string state; private int zip;

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Break up classes with too many responsibilities CardDeck

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class Address

CardDeck + Card

Make the names of your classes and methods reflect their responsibilities class BillingAddress;

setSalary();