Static class members Static members l Recall that

Static class members

Static members l Recall that a static method is one l l That can be invoked through its class name For example, the methods of Math class are static result = Math. sqrt(25) l Variables can be static as well l We declare static methods and variables l Using the static modifier

The static modifier l So far, we have used instance variables l Where each instance of the class l l has its own version of the variable § Each object has distinct memory space for each variable Static variable called class variable l is shared among all instances of a class l There is only one copy of a static variable for all objects l Changing the value of a static variable in one object § Changes it for all of the others

Static Variables l Normally, each object has its own data space, l but if a variable is declared as static, l only one copy of the variable exists private static float price; l Memory space for a static variable l l All objects instantiated from the class l l is created when the class is first referenced share its static variables Changing the value of a static variable l in one object changes it for all others 4

Static Methods class Helper { public static int cube (int num) { return num * num; } } Because it is declared as static, the method can be invoked as value = Helper. cube(5); 5

Static class members l l Static methods cannot reference l Instance variables l Because they don’t exist until an object is created However, a static method l Can reference static variables

Static members: example l The following example l l Keeps track of how many Slogan objects l Have been created using a static variable l And makes this information available using a static method Refer to Slogan. Counter. java & Slogan. java

Problem l Problem statement l Prints the first 20 numbers in Fibonacci sequence l Each number is formed by adding previous 2 numbers l Starting with the numbers 0 and 1

Fibonacci series public class Fibonacci { public static void mainspring[] args) { int current, prev = 1, prev = 0; // variables for(int i = 0; i < 20; i++) { // times current = prev + prev; // of previous two System. out. print(current + " "); // prev = prev; // becomes 2 nd previous prev = current; // becomes previous } System. out. println(); // and flush output } } Initialize some Loop exactly 20 Next number is sum Print it out First previous And current number Terminate the line,

Outline Software Development Activities Identifying Classes and Objects Static Variables and Methods Class Relationships Interfaces Enumerated Types Revisited Method Design Testing GUI Design and Layout

Class Relationships l Classes in a software system can have various types of relationships to each other l Three of the most common relationships: l Dependency: A uses B l Aggregation: A has-a B l Inheritance: A is-a B l Let's discuss dependency and aggregation further l Inheritance is discussed in detail in Chapter 8

Dependency l A dependency exists when one class l l We don't want l l numerous or complex dependencies among classes Nor do we want l l relies on another in some way complex classes that don't depend on others A good design strikes the right balance

Dependency l Some dependencies occur l l between objects of the same class A method of the class may accept l an object of the same class as a parameter l For example, the concat method of the String class str 3 = str 1. concat(str 2); l This drives home the idea that l the service is being requested from a particular object

Dependency l The following example defines l l A rational number is a value l l l that can be represented as the ratio of two integers Some methods of the Rational class l l a class called Rational to represent a rational number accept another Rational object as a parameter See Rational. Tester. java (page 302) See Rational. Number. java (page 304)

Aggregation l An aggregate is an object l l Therefore aggregation is a has-a relationship l l A car has a chassis An aggregate object contains l l that is made up of other objects references to other objects as instance data The aggregate object is defined in part l by the objects that make it up

Aggregation l Student object is composed of Address objects l In fact each student has two addresses l School address and home address l See Student. Body. java (page 309) See Student. java (page 311) See Address. java (page 312) l An aggregation association is shown in a UML class l l l diagram using an open diamond at the aggregate end

Aggregation in UML Student. Body + main (args : String[]) : void Student - first. Name : String last. Name : String home. Address : Address school. Address : Address + to. String() : String Address - street. Address : String city : String state : String zip. Code : long + to. String() : String