Dr. Zara Hamid

CLASSES: A DEEPER LOOK

c lass Over loadDemo {void tes t () {

System.out .pr in t ln("No parameters") ;/ / Over load tes t for one in teger parameter.vo id tes t ( in t a) {

System.out .pr in t ln("a: " + a) ;}/ / Over load tes t for two in teger parameters.void tes t ( in t a , in t b) {System.out .pr in t ln("a and b: " + a + " " + b) ;}/ / over load tes t for a double parameterdouble tes t (double a) {System.out .pr in t ln("double a: " + a) ;return a*a;} }

c lass Over load {publ ic s tat ic vo id main(St r ing args[ ] ) {Over loadDemo ob = new Over loadDemo() ;double resul t ;/ / ca l l a l l vers ions of tes t ( )ob. tes t () ;ob. tes t (10);ob. tes t (10, 20) ;resul t = ob. test (123.25);System.out .pr in t ln("Resul t o f ob. tes t (123.25) : “ + resul t ) ;}}

OVERLOADING METHODS

c lass Test {in t a , b ;Test( in t i , in t j ) {

a = i ;b = j ;}/ / re turn t rue i f o is equal to the invok ing objectboolean equals(Test o) {i f (o .a == a && o.b == b) return t rue;e lse return fa lse;}}

class PassOb {publ ic stat ic void main(Str ing args[]) {Test ob1 = new Test(100, 22);Test ob2 = new Test(100, 22);Test ob3 = new Test(-1, -1);System.out.print ln("ob1 == ob2: " + ob1.equals(ob2));

System.out.print ln("ob1 == ob3: " + ob1.equals(ob3)); } }

USING OBJECTS AS PARAMETERS

What happens when one object is put equal to other

class Test {int a;Test(int i) {

a = i;}

Test incrByTen() {Test temp = new Test(a+10);return temp;}}

class RetOb {public static void main(String args[]) {

Test ob1 = new Test(2);Test ob2;ob2 = ob1.incrByTen();

System.out.println("ob1.a: " + ob1.a);System.out.println("ob2.a: " + ob2.a);ob2 = ob2.incrByTen();

System.out.println("ob2.a after second increase: "+ ob2.a); }}

RETURNING OBJECTS

Declare a class date. It should have three int variables day, month, year. Declare a constructr that sets the default date to 14/08/1947. Declare another constructor that sets the date according to user requirements.

Declare a function addDate that adds two date objects.

CLASS ACTIVITY

DYNAMIC VARIABLES AND METHODS

All instance variables and methods we’ve created so far have been dynamic

Note: There is no “dynamic” keyword in JavaDynamic by default

In general, dynamic refers to things created at “run time” i.e. when the program is runningEvery object gets its own (dynamic) instance variables.Every object effectively gets its own copy of each dynamic methodThe absence of the keyword static before non-local variables and

methods means dynamic (one per object/instance)

STATIC VARIABLES

Static means “pertaining to the class in general”, not to an individual object

A variable may be declared (outside of a method) with the static keyword:

E.g. static int numTicketsSold;There is one variable numTickets for the class not one

per object!!! A static variable is shared by all instances (if any).

All instances may be able read/write it

A static variable that is public may be accessed by:ClassName.variableName

STATIC METHODS

A method may be declared with the static keyword

Static methods live at class level, not at object level

Static methods may access static variables and

methods, but not dynamic ones

public static int getNumSold(){return numTicketsSold;

}

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STATIC METHODS (CONTD..)

A static method that is public can be accessed ClassName.methodName(args) double result = Math.sqrt(25.0); int numSold = Ticket.getNumberSold();

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EXAMPLE: TICKETpublic class Ticket{ private static int numTicketsSold = 0; // shared private int ticketNum; // one per object public Ticket(){ numTicketsSold++; ticketNum = numTicketsSold; } public static int getNumberSold() { return numTicketsSold; } public int getTicketNumber() { return ticketNum; } public String getInfo(){ return "ticket # " + ticketNum + "; " + numTicketsSold + " ticket(s) sold."; }}

EXAMPLE: TICKET

Ticket.getNumberSold()

Ticket t1 = new Ticket(); t1.getTicketNum()

t1.getInfo()"ticket # 1; 1 ticket(s) sold." t1.getNumberSold()

Ticket t2 = new Ticket(); t2.getTicketNum()

t2.getInfo()"ticket # 2; 2 ticket(s) sold." t2.getInfo()"ticket # 2; 2 ticket(s) sold." t1.getInfo()"ticket # 1; 2 ticket(s) sold." Ticket.getNumberSold()

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STATIC CONTEXT

To have standalone Java Application we need a public static void main(String args[]) method

The main method belongs to the class in which it is writtenIt must be static because, before your program

starts, there aren’t any objects to send messages to

This is a static context (a class method) You can send messages to objects, if you have some

objects: d1.bark(); You cannot send a message to yourself, or use any

instance variables - this is a static context, not an object

Non-static variable cannot be referenced from a static context

EXAMPLE

public class JustAdd { int x; int y; int z;

public static void main(String args[]) { x = 5; y = 10; z = x + y; }}

all are wrong

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SOLUTION

public class JustAdd { int x; int y; int z;

public static void main(String args[]) { JustAdd myAdd = new JustAdd()

myAdd.doItAll()}

void doItAll() { x = 5; y = 10; z = x + y; }

}

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WHEN TO USE STATIC

A variable should be static if: It logically describes the class as a whole There should be only one copy of it

A method should be static if: It does not use or affect the object that receives the

message (it uses only its parameters)

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STATIC RULES

static variables and methods belong to the class in general, not to individual objects

The absence of the keyword static before non-local variables and methods means dynamic (one per object/instance)

A dynamic method can access all dynamic and static variables and methods in the same class

A static method can not access a dynamic variable (How could it choose or which one?)

A static method can not call a dynamic method (because it might access an instance variable)