Chap2 class,objects

Class fundamentals
 A class is a template that defines the form of an
object.
 It specifies both the data and code that will operate
on that data.
 Objects are instance of class
 Memories and variables that constitute a class are
called members
 Also called as instance variables.

General form of class
class classname{
type var1;
type var2;…
type varN;
type method1(parameters){
}
type method2(parameters){
}
….
}

 A main() method is required only if that class is
starting point for your program
 Java applications, such as applets don’t require a
main() function.
 Defining a class:
 Ex: class vehicle{
int passengers; ///no. of passengers
int fuel; //fuel capcity
int mileage; //fuel consumption per mile
}

 A class definition creates a new data type.
 Here data type is named as vehicle
 To create an object, you will use the statement such as
 Vehicle minivan=new Vehicle();
 Now, minivan is an instance of vehicle
 To access the data members or methods defined in the
class, we should use dot operator
 Object.member
 In order to assign the value for no. of passengers in
mininvan we should use
 minivan.passengers=15;

class vehicle{
int passengers; ///no. of passengers
int fuel; //fuel capcity
int mileage; //fuel consumption per mile
}
class VehicleDemo{
public static void main(String args[]){
Vehicle minivan=new Vehicle();
int range;
minivan.passengers=7;
minivan.fuel=16;
minivan.mileage=7;
range=minivan.fuel*minivan.mileage;
System.out,print(“Minivan can hold “+minivan.passengers+”
with the range”+range);
}
}

 Creating multiple objects of the same class wont affect
the value unless it refers to same object.
 Creating two vehicles named minivan and sportscar of
type vehicle class

class vehicle{
int passengers;
int fuel;
int mileage;
}
class VehicleDemo{
public static void main(String
args[]){
Vehicle sportscar=new Vehicle();
int range1,range2;
minivan.fuel=16;
minivan.mileage=7;
range1=minivan.fuel*minivan.mile
age;
System.out,print(“Minivan can
hold “+minivan.passengers+”
with the range”+range);
sportscar.passengers=2;
sportscar.fuel=14;
sportscar.mileage=12;
range2=sportscar.fuel*sportscar
.milege;
hold
“+sportscar.passengers+”
with the range”+range2);
}
}

How objects are created
 Vehicle minivan=new Vehicle();
 It declares a variable called minivan of the class type
Vehicle
 It’s a variable that can refer to an object
 New operator , creates a physical copy of object and
assigns to minivan a reference to that object.
 New operator dynamically allocates memory for an
object and returns a reference to it.
 Vehicle minivan;
 Minivan=new Vehicle();

Reference variables and
assignment
 Assigning one object to another, does not copy the
contents but it refers to the same object.
 Vehicle car1=new Vehicle();l
 Vehicle car2=car1;
 Here the contents of car1 is not copied to car2, instead
both car1 and car2 refer to the same object
 car1.mileage=26;
 System.out.printnln(car1.mileage);
 System.out.println(car2.mileage);

Methods
 Methods are subroutines that manipulate the data
defined by the class
 It contains one or more statements.
 Each method should perform only one tasks.
 Each method has a name and it is used to call in the
main() function
 Java keywords cannot be used as method name
 Method will have parenthesis after its name
ret_type name(parameter_list){
}

 ret_type specifies the type of data returned by the
method
 Method which does not return anything, should return
a type called void.
 parameter_list is a sequence of type and identifier
pairs seperated by commas.
 If the method has no parameters, the parameters list
will be empty.

Adding a method to the vehicle
classclass vehicle{
int passengers;
int fuel;
int mileage;
void range(){
System.out.println(“Range
is”+fuel*mileage);
}
}
class VehicleDemo{
minivan.fuel=16;
minivan.mileage=7;
hold “+minivan.passengers);
Minivan.range();
sportscar.fuel=14;
System.out,print(“Sportscar can
hold “+sportscar.passengers);
Sportscar.range();
}
}

Returning from a method
 There are two forms of return
 One for use in void methods
 One for returning methods
 In a void method, the immediate termination of the
method can be done by using this form of return
 A void method will terminate when it comes across the
closing curly braces.
void myMet(){
int i;
for(i=0;i<10;i++)
if(i==5) return;
System.out.println();
}

Returning a value
 Most of the methods will return a value.
 return value;
 Here value is the returned
 Non-void method must return a value by using this
form of return.

Return type for the methodclass vehicle{
int passengers;
int fuel;
int mileage;
int range(){
return fuel*mileage;
}
}
class VehicleDemo{
int range1,range2;
minivan.fuel=16;
minivan.mileage=7;
hold “+minivan.passengers);
range1=Minivan.range();
System.out.println(range1);
sportscar.fuel=14;
System.out,print(“Sportscar can
hold “+sportscar.passengers);
range2=Sportscar.range();
System.out.println(range2);
}
}

Using parameters
 Value passed to a method is called argument
 More than a value can be passed into a method.
 Inside a method, the variable that receives the
argument is called parameters.
 Simple example that uses a parameter
 Inside Chksum class, a method isEven() returns true if
the value passed to it is even otherwise it returns false.

class chkSum{
boolean isEven(int x)
if(x%2==0)
return true;
else
return false;
}
class demo{
chkSum e=new chkSum();
if(e.isEven(10))
System.out.println(“10 is even”);
else
System.out.println(“10 is not even”);
}
}

Adding parameterized method to
vehicle class
class Vehicle{
int passengers;
int fuel;
int milage;
int range(){
return milage*fuel;
}
double fuelneeded(int miles){
returns (double) miles/milage;
}
}
class Computefuel{
int dist=252;
minivan.fuel=16;
minivan.milage=23;
sportsacar.passengers=2;
sportscar.fuel=14;
sportscar.milage=12;
double gallons=minivan.fuelneeded(dist);
System.out.println(“Fuel required by
minivan and sportscar is
:”+minivan.fuelneeded(dist)+”t”+
sportscar.fuelneeded(dist));
}
}

Constructors
 Having multiple statements for assigning the data
members in the main class is error prone.
 A constructor initializes an object when it is created
 It has the same name as the class
 Syntax is similar to the method.
 Constructors do not have explicit return type.
 Java provides a default constructor that initializes all
the data members by the default values by it self.
 i.e. zero for numeric type, null for reference type and
false for boolean type.

class Myclass{
int x;
Myclass(){
x=10;
}
}
class CLassDemo{
Myclass m1=new Myclass();
Myclass m2=new Myclass();
System.out.println(m1.x+”t”+m2.x);
}
}

Parameterized constructors
 Constructors accept one or more parameters
 Parameters are added in the same way as the methods
 Declare them inside the parenthesis after the
constructors name
 Ex.

class Myclass{
int x;
Myclass(int i){
x=i;
}
}
class ParamCLassDemo{
Myclass m1=new Myclass(10);
Myclass m2=new Myclass(88);
System.out.println(m1.x+”t”+m2.x);
}
}

The new operator
 class_var =new class_name(arg_list);
 class_var is a variable of the class type being created.
 The class name is the name of the class that is being
instantiated.
 If a class does not define its own constructor , new will
use the default constructor supplied by java.
 The new operator returns a reference to a newly
created object, which is assigned to class_var.

Garbage collection and finalizers
 Objects are dynamically allocated from a pool of free
memory by using the new operator.
 Memory is not infinite, it can be exhausted.
 many languages like C++ memory reallocation is
handled manually, the programmer uses delete
operator to free the memory that was allocated by the
new operator
 Java uses a approach called garbage collection that
reclaims memory automatically
 When no references to an object exists, then object is
assumed to be no longer needed and memory
occupied by the object is released.

 The recycled memory can be used for further
allocation.
 The garbage collector will usually run when two
conditions are met: there are objects to recycle and
there is need to recycle them
 Java garbage collection requires times hence the Java
run-time will perform garbage collection when it is
appropriate.

The finalize() method
 A method that is called just before an objects final
destruction by the garbage collector.
 Can be used to ensure that an object terminates
cleanly.
 Inside the finalise method, we will specify those
actions that must be performed before an object is
destroyed.
protected void finalize()
{
//finalization code here
}

 The keyword protected is a specifier that prevents
access to finalize() by code defined outside the class.
 Finalize() is called just before garbage collection.
 This means we cannot predict when finalize function
will be called by the java runtime
 If program ends before the garbage collection occurs
then finalize() will not be executed.
 Hence used as backup procedure to ensure the proper
handling of the resources

The this keyword
 When a method is called, it is automatically passed an
implicit argument that is a reference to the invoking object,
this reference is called this reference.
class Pwr{
double b,val;
int e;
Pwr(double b,int e)
{
this.b=b;
this.e=e;
}
}

Controlling access to class
members
 There are two access modifiers: private and public
 A public member can be freely accessed by the code
defined outside the class.
 A private member can be accessed only by the other
methods defined in the class
 Restricting the access to class members is a
fundamental part of object oriented programming
because it helps to prevent the misuse of object.
 When correctly implemented, class creates a black box
that can be used, but the inner workings of which are
not open to tampering.

Java access modifiers
 Achieved through the use of three access modifiers:
 private
 public
 protected
 If no access modifier is specified, the default access setting
is assumed.
 When a member of class is public specifier, that member
can be accessed by any other code in the program
 When a member of class is private specifier , that member
can be accessed only by other members of its class.
 Default access modifier is same as public access modifier.

Chap2 class,objects

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Chap2 class,objects