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Bai Giang Khoa CNTT
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Introduction to Class
Object-Oriented Programming
Module Introduction
1. Classes and Objects
2. Instance variables
3. Methods
FPT- APTECH 2/28FPT- APTECH 2/28
#1 – Classes and Objects
Class declaration
Constructors
Creating objects
FPT- APTECH 3/28
Creating Class
The template or blueprint from which objects are actually made.
Syntax:class ClassName{
//definition of class}
All code that you write in Java is inside a class.
Classes can be built by extending other classes
FPT- APTECH 4/28
Constructor - how to instance an object?
Same name as the class.
A class can have more than one constructor.
A constructor can take zero, one, or more parameters.
Has no return value.
Always called with the new operator.
Use for initializing variables and invoking any methods that may be for initialization.
If a class does not have an explicit constructor, a default constructor (no-argument) is provided.
FPT- APTECH 5/28
Name of class
1. Use Naming rules
2. Always captain first letter
3. Use Noun for naming
Data member
• What data does it need to know?
Constructor
• Define the way to instance object
1. Same name as the
class name
2. Like function in C but
have no return value
Methods
• What can it do?
1. Like function in C
2. Use Verb for naming
3. Always lower first letter
Dissecting the Rectangle class
FPT- APTECH 6/28
Creating objects (instance object)
An object must be created before using in a program.
1. Declare a variable to store the object reference.1. Objects can only be manipulated via references
2. Creating an object.1. Using the new operator in conjunction with a call to a constructor
FPT- APTECH 7/28
#2 – Data Member
Instance Variables
Class Variables
FPT- APTECH 8/28
Instance variables
The values of those variables define the state of object.
All instances of a class have the same instance variables May have different values
inside them.
Has an “access modifier” associated with it.
FPT- APTECH 9/28
Example of Instance variables
recA
length = 3.0width = 4.0
recB
length = 6.4width = 4.7
FPT- APTECH 10/28
Class Variables (Static Variables)
Variable that is accessed without using an object of a class.
Declare using the static keyword.
Only one copy of a static variable is shared by all the objects of the class. Change in the value of static
variable is reflected by all the objects of the class.
FPT- APTECH 11/28
Example of Static Variables
FPT- APTECH 12/28
#3 - Methods
Definition
Instance Method
Calling method and Passing Arguments by Value.
Calling method and Passing Arguments by Reference.
Static Methods
Variable Argument Methods
FPT- APTECH 13/28
A method is defined as the actual implementation of an operation on an object.
Syntax:
access_specifier modifier datatype method_name (parameter_list)
{//body of the method
}
The method name should begin with a lowercase verb and follow with a proper noun.
Method
FPT- APTECH 14/28
Instance Method
A function defined in a class
Invoked by an instance object and can access instance variables
Provide a mechanism for accessing the private data stored in an object
FPT- APTECH 15/28
Invoking Methods
Using the '.' operator
Syntax: <the object reference> ‘.’ <the method to be invoked>
FPT- APTECH 16/28
Calling method and Passing Arguments by Value.
Value from calling method is passed as an argument to the called method.
Any change made to that passed value in the called method will not modify the value in the calling method.
Variables of primitive types (int, float ..) are passed by value.
FPT- APTECH 17/28
Example of Calling method and Passing Arguments by Value.
FPT- APTECH 18/28
Calling method and Passing Arguments by Reference.
Called method to change the value of the parameter passed to if from the calling method.
When references are passed as parameters, the caller can change the values stored but not the reference variables.
FPT- APTECH 19/28
Example of Calling method and Passing Arguments by Reference
FPT- APTECH 20/28
Static Methods
Methods that do not operate on objects. Can be called without creating any objects of the class.
Call static method, supply the name of the class.
Declared using the static keyword.
Can directly refer only to static variables and other static methods of the class.
Cannot refer to non-static methods and variables
FPT- APTECH 21/28
Example of Static Methods
FPT- APTECH 22/28
static methods usage
When a method doesn't need to access the object state
When a method only needs to access static fields of the class.
FPT- APTECH 23/28
Variable Argument Methods
Allow calling a method with variable number of arguments.
FPT- APTECH 24/28
That’s about all for today!
Thank you all for your attention and patient !
Classes and Objects
Instance variables
Methods