Bai Giang Khoa CNTT

1. Introduction to Class
Object-Oriented Programming

2. Module Introduction
1.
Classes and Objects
2.
Instance variables
3.
Methods
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3. #1 – Classes and Objects

Class declaration

Constructors

Creating objects
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4. 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
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5. 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.
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6. Dissecting the Rectangle class
Name of class
1. Use Naming rules
Data member
2. Always captain first letter
• WhatConstructor
data does it need to
3. Use Noun for naming
know?
•
Define the way to
instance object
1. Same name as the
class name
Methods
2. Like function in C but
• Whathave no do? value
can it return
1. Like function in C
2. Use Verb for naming
3. Always lower first letter
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7. 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
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8. #2 – Data Member

Instance Variables

Class Variables
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9. 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.
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10. Example of Instance variables
recA
length = 3.0
width = 4.0
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recB
length = 6.4
width = 4.7
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11. 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.
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12. Example of Static Variables
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13. #3 - Methods

Definition

Instance Method

Calling method and Passing Arguments by Value.

Calling method and Passing Arguments by Reference.

Static Methods

Variable Argument Methods
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14. Method

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.
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15. 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
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16. Invoking Methods

Using the '.' operator

Syntax:
 <the object reference> ‘.’ <the method to be invoked>
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17. 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.
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18. Example of Calling method and Passing Arguments by
Value.
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19. 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.
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20. Example of Calling method and Passing Arguments by
Reference
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21. 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
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22. Example of Static Methods
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23. 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.
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24. Variable Argument Methods

Allow calling a method with variable number of arguments.
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25. That’s about all for today!

Classes and Objects

Instance variables

Methods
Thank you all for your attention and patient !