1. Topic: Classes and Objects
Course : JAVA PROGRAMMING Paper Code: ETCS-307 Faculty : Dr. Prabhjot Kaur Reader, Dept. of IT 1

2. Classes and Objects
A class consists of variables called fields together with functions called methods or member functions that act on those fields.

3. General Structure of a Class
Members of a class are:
fields or variables
a class or object's state or attributes
methods
a class or object's behaviour
constructors
to make objects
each has access control

4. General Structure of a Class
Types of Members
fields or variables
instance unique values for each object
class uses the static modifier same value for all objects
methods
instance work with both instance and static variables
class uses static modifier works with static variables

5. General Structure of a Class
Fields or Variables
field or variable holds a single value
is strongly typed: must be declared
type specifies the kind of value and the variable's behaviour
int i = 123; // integer primitive
Point pt = null; // object reference
pt = new Point (x, y); // coordinates pt has the value of the Point object's address, not the x, y coordinates

6. Classes and Objects
A class is a collection of fields (data) and methods (procedure or function) that operate on that data.
Circle
centre
radius
circumference()
area()

7. Classes and Objects
A class is a collection of fields (data) and methods (procedure or function) that operate on that data.
The basic syntax for a class definition:
Bare bone class – no fields, no methods
class ClassName [extends SuperClassName] {
[fields declaration]
[methods declaration] }
public class Circle {
// my circle class }

8. Adding Fields: Class Circle with fields
Classes and Objects
Adding Fields: Class Circle with fields
Add fields
The fields (data) are also called the instance variables.
public class Circle {
public double x, y; // centre coordinate
public double r; // radius of the circle }

9. Classes and Objects Adding Methods
A class with only data fields has no life. Objects created by such a class cannot respond to any messages.
Methods are declared inside the body of the class but immediately after the declaration of data fields.
The general form of a method declaration is:
type MethodName (parameter-list) {
Method-body; }

10. Adding Methods to Class Circle
Classes and Objects
Adding Methods to Class Circle
public class Circle {
public double x, y; // centre of the circle
public double r; // radius of circle
//Methods to return circumference and area
public double circumference() {
return 2*3.14*r; }
public double area() {
return 3.14 * r * r;
Method Body

11. Classes and Objects Data Abstraction
Declare the Circle class, have created a new data type – Data Abstraction
Can define variables (objects) of that type:
Circle aCircle;
Circle bCircle;

12. Classes and Objects Class of Circle cont.
aCircle, bCircle simply refers to a Circle object, not an object itself.
aCircle
bCircle
null
null
Points to nothing (Null Reference)
Points to nothing (Null Reference)

13. Creating objects of a class
Classes and Objects
Creating objects of a class
Objects are created dynamically using the new keyword.
aCircle and bCircle refer to Circle objects
aCircle = new Circle() ;
bCircle = new Circle() ;

14. Creating objects of a class
Classes and Objects
Creating objects of a class
aCircle = new Circle();
bCircle = new Circle() ;
bCircle = aCircle;

15. Creating objects of a class
Classes and Objects
Creating objects of a class
aCircle = new Circle();
bCircle = new Circle() ;
bCircle = aCircle;
Before Assignment
Before Assignment P
aCircle
bCircle P
aCircle
bCircle Q Q

16. Automatic garbage collection
Classes and Objects
Automatic garbage collection Q
The object does not have a reference and cannot be used in future.
The object becomes a candidate for automatic garbage collection.
Java automatically collects garbage periodically and releases the memory used to be used in the future.

17. Accessing Object/Circle Data
Classes and Objects
Accessing Object/Circle Data
Similar to C syntax for accessing data defined in a structure.
ObjectName.VariableName
ObjectName.MethodName(parameter-list)
Circle aCircle = new Circle();
aCircle.x = 2.0 // initialize center and radius
aCircle.y = 2.0
aCircle.r = 1.0

18. Executing Methods in Object/Circle
Classes and Objects
Executing Methods in Object/Circle
Using Object Methods:
sent ‘message’ to aCircle
Circle aCircle = new Circle();
double area;
aCircle.r = 1.0;
area = aCircle.area();

19. Classes and Objects Using Circle Class
// Circle.java: Contains both Circle class and its user class
//Add Circle class code here
class MyMain {
public static void main(String args[])
Circle aCircle; // creating reference
aCircle = new Circle(); // creating object
aCircle.x = 10; // assigning value to data field
aCircle.y = 20;
aCircle.r = 5;
double area = aCircle.area(); // invoking method
double circumf = aCircle.circumference();
System.out.println("Radius="+aCircle.r+" Area="+area);
System.out.println("Radius="+aCircle.r+" Circumference ="+circumf); }
java MyMain
Radius=5.0 Area=78.5
Radius=5.0 Circumference =

20. Constructors
a special kind of method to construct an instance of an object from a class template
default constructor is ClassName()
created if the class has no constructors
instance fields are initialized to default values automatically (see next slide)
overloaded constructors: same name with different parameter lists
use of “this” to call another constructor

21. Constructors
Java automatically initializes class (static) and object (instance) fields
Primitive Data Type Default Value
boolean false
int, etc
Object Reference Type null not yet referring to an object

22. Constructors Constructors this keyword Referencing the current object
To call another constructor

23. Constructor initializes an object's instance fields
Constructors
Constructor initializes an object's instance fields
class MyClass { int maximum; int counter; boolean isBelowMax = true; // override default MyClass(int maximum) { this.maximum = maximum; } MyClass() { this(100); } // calls this class's constructor MyClass(int maximum, int counter) { this.maximum = maximum; // assign local field value this.counter = counter; // to this object's instance isBelowMax = counter < maximum; }

24. Method Overloading
overloading is supported: same method name, different parameter lists
parameters passed by value, i.e. by copy
primitive data types and object reference types
copy of object reference is not a copy of the object
type of primitive/object returned or void

25. Four Levels of Access to a Class or object's members
Classes and Objects
Four Levels of Access to a Class or object's members
private: accessible from within this class only
should be standard practice on instance fields to support OO encapsulation
default (if you don't specify): accessible from any class in the package
protected: accessible from any subclass or any class in the package
public: accessible from any class anywhere

26. Classes and Objects Access Control Levels Access Location Public
Protected
Default
Private protected
private
Same class
Yes
Subclass in same pkg. No
Other classes in same pkg.
Subclass in other Pkg.
Non-subclasses in other Pkg.

27. Static keyword
When objects of its class are declared, no copy of a static variable is made
Instead, all instances of the class share the same static variable
When a member is declared static, it can be accessed before any objects of its class are created, and without reference to any object
The most common example of a static member is main().
Outside of the class in which static methods are defined, static methods and variables can be used independently of any object.
To do so, you need only specify the name of their class followed by the .(dot) operator
ClassName.methodName() 27

28. Static keyword Restrictions:
Static methods can only call other static methods.
They can only access static data.
They can not refer to ‘this’ or ‘super’ in any way. 28

29. Static Blocks
Static block is a block of code prefixed by ‘static’ keyword
Gets executed only once
Used to initialize static variables
class A {
static int x;
static {
x = 10; }
public static void main (String args[]) {
System.out.println (“ x value is : ”+ x);
static block
initializes
x to 10
Output : x value is : 10 29

30. Example Program : Static Block
class A {
static int x = 10;
static int y;
static void call( int p) {
System .out. println(“ x value is :”+x);
System .out .println(“ y value is :”+y);
System .out .println(“ p value is :”+p); }
static {
System.out.println(“ static block initialized”);
y=x*2;
public static void main (String args[]) {
call(30);
Output
x value is : 10
y value is : 20
p value is : 30

31. Final keyword A variable can be declared as final
Doing so prevents its contents from being modified
This means that you must initialize a final variable when it is declared
Permits us to create typed constants
In usage, final is similar to const in C / C++ 31

32. Example final variables
Syntax
final type constName = value;
Example
final int FILE_NEW = 1;
final float PI = ;

33. Final Variables
Subsequent parts of the program may use the above final variables FILE_NEW and PI
It is common coding convention to choose all uppercase identifiers for final variables
Variables declared as final do not occupy memory on a per-instance basis
Thus, a final variable is essentially a constant

34. Example program : final variables
Correct program
Wrong program
class A {
final int X = 10;
public static void main (String args[]) {
System.out.println(“ X is “+X); }
class B {
final int X = 10;
public static void main(String args[]) {
X = 20;
System .out. println(“ X is “+X); }
final variable
should not change

35. Other Uses Of final The keyword final can also be applied to methods,
Its meaning is substantially different than when it is applied to variables

36. Varargs Newly implemented feature in Java 5.0 Basic syntax
type … variableName
Argument passed to a method is converted into an array of the same-typed values
sum (10,20) sum(new int[] {10,20})

37. Varargs Example public static void main(String[] args) {
System.out.println(“The sum is ” + sum(10,20,30)); }
public int sum (double … numbers)
int total = 0;
for (int i = 0; i < numbers.length; i++)
total += numbers[i];
return total;
OUTPUT: The sum is 60

38. Varargs Varargs is done with the … operator.
Example. The following constructor
public Person (String name, String details… )
Can be called with many different invocations:
new Person (“Alexander ”);
new Person (“Alexander ”, “Bell ”);
new Person (“Alexander ”,”Graham”, “Bell ”);

39. Exercise:
Do one program to demonstrate Varargs concept.

40. Thank You