Java- language Lecture 6

JAVA
FUNDAMENTALS
Eng : Hatem Abd El-Salam
Lecture 6

Agenda
 Overloading & Overriding
 This
 Super
 Getters and setters
© 2016 by Eng. Hatem Abd El-Salam

Overloading
 If a class have multiple methods by same name but different parameters, it is known
as Method Overloading.
 Advantage of method overloading?
Method overloading increases the readability of the program.
 Different ways to overload the method
There are two ways to overload the method in java
1. By changing number of arguments
2. By changing the data type

Overloading (Cont.)
1) Example of Method Overloading by changing the no. of arguments

Overloading (Cont.)
2) Example of Method Overloading by changing data type of argument

Overloading (Cont.)
Why Method Overloading is not possible by changing the return type of method?
 In java, method overloading is not possible by changing the return type of the
method because there may occur ambiguity. Let's see how ambiguity may occur:
 because there was problem:

Overloading (Cont.)
 Method Overloading and Type Promotion
 As displayed in the above diagram, byte can be promoted to( short, int, long, float or
double). The short data type can be promoted to (int,long,float or double). The char
data type can be promoted to (int, long, float or double )and so on.

Overloading (Cont.)

Overriding
 If subclass (child class) has the same method as declared in the parent class, it is
known as method overriding in java.
 In other words, If subclass provides the specific implementation of the method that
has been provided by one of its parent class, it is known as method overriding.
 Usage of Java Method Overriding
1. Method overriding is used to provide specific implementation of a method that is already provided
by its super class.
2. Method overriding is used for runtime polymorphism
 Rules for Java Method Overriding:
1. method must have same name as in the parent class
2. method must have same parameter as in the parent class.
3. must be IS-A relationship (inheritance).

Overriding (Cont.)
 Understanding the problem without method overriding

Overriding (Cont.)

Overriding (Cont.)
EX:
 Consider a scenario, Bank is a class that provides functionality to get rate of
interest. But, rate of interest varies according to banks. For example, SBI, ICICI and
AXIS banks could provide 8%, 7% and 9% rate of interest.

Overriding (Cont.)
 Can we override static method?
No, static method cannot be overridden. It can be proved by runtime polymorphism,
so we will learn it later.
 Why we cannot override static method?
because static method is bound with class whereas instance method is bound with
object. Static belongs to class area and instance belongs to heap area.
 Can we override java main method?
No, because main is a static method.

Overriding (Cont.)
 Difference between method Overloading and Method Overriding in java

This

This (Cont.)
1. This keyword can be used to refer current class instance variable.

This (Cont.)
2. this() can be used to invoked current class constructor
The this() constructor call can be used to invoke the current class constructor
(constructor chaining). This approach is better if you have many constructors in the
class and want to reuse that constructor.

This (Cont.)
 Where to use this() constructor call?
The this() constructor call should be used to reuse the constructor in the constructor.
It maintains the chain between the constructors
it is used for constructor chaining.

This (Cont.)

This (Cont.)
3) The this keyword can be used to invoke current class method (implicitly).
You may invoke the method of the current class by using the this keyword. If you
don't use the this keyword, compiler automatically adds this keyword while invoking
the method.

This (Cont.)
4) this keyword can be passed as an argument in the method.
The this keyword can also be passed as an argument in the method. It is mainly
used in the event handling.

This (Cont.)
5) The this keyword can be passed as argument in the constructor call.
We can pass the this keyword in the constructor also. It is useful if we have to use
one object in multiple classes.

This (Cont.)
6) The this keyword can be used to return current class instance.
We can return the this keyword as an statement from the method. In such case,
return type of the method must be the class type (non-primitive).

This (Cont.)
 Proving this keyword
Let's prove that this keyword refers to the current class instance variable. In this
program, we are printing the reference variable and this, output of both variables
are same.

Super

Super (Cont.)
 super is used to refer immediate parent class instance variable.

Super (Cont.)

Super (Cont.)
 2) super is used to invoke parent class constructor.

Super (Cont.)
 Note: super() is added in each class constructor automatically by compiler.
As we know well that default constructor is provided by compiler automatically but it
also adds super() for the first statement . If you are creating your own constructor
and you don't have either this() or super() as the first statement, compiler will
provide super() as the first statement of the constructor.

Super (Cont.)
 3) super can be used to invoke parent class method

Getters and setters
 When implementing a method of a class, use the class’s set and get methods to
access the class’s private data. This simplifies code maintenance and reduces the
likelihood of errors.
 When necessary, provide public methods to change and retrieve the values of
private instance variables. This architecture helps hide the implementation of a class
from its clients, which improves program modifiability.

Getters and setters (Cont.)
 Set methods
 Also known as mutator methods
 Assign values to instance variables
 Should validate new values for instance variables
• Can return a value to indicate invalid data
 Get methods
 Also known as accessor methods or query methods
 Obtain the values of instance variables
 Can control the format of the data it returns

Encapsulation
 Encapsulation in Java is a mechanism of wrapping the data (variables) and code
acting on the data (methods) together as a single unit. In encapsulation, the
variables of a class will be hidden from other classes, and can be accessed only
through the methods of their current class. Therefore, it is also known as data
hiding.
To achieve encapsulation in Java :
 Declare the variables of a class as private.
 Provide public setter and getter methods to modify and view the variables values.

Encapsulation(Cont.)
Benefits of Encapsulation
 The fields of a class can be made read-only or write-only.
 A class can have total control over what is stored in its fields.
 The users of a class do not know how the class stores its data. A class can change
the data type of a field and users of the class do not need to change any of their
code.
Example:
 I'll let you tell me what funny video we will watch on my computer. But I don't really
trust you enough to hand over my laptop and let you bring it up yourself.

Without setter and getter

With setter and getter

Name : Eng. Hatem Abd El-Salam
Email : Hatemabdelsalam@Hotmail.com
LinkedIn : linkedin.com/in/hatemabdelsalam
SlideShare : slideshare.net/HatemAbdElSalam
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