Monday, July 14, 2014

Understanding enum: Definition and need of enum : Chapter 1

4:34 PM

Chapter 1

Understanding enum: 

Definition and need of enum

Enum is a type of class which is final. It is created by using the keyword “enum”. It is used for defining set of named constants those represents a menu kind of item.

For Example:
Hotel Menu, Bar Menu, Course Menu etc…

Before java 5 these menu items are created by usin class. Class has a problem in accessing these menu items, that is it cannot return or print item name as it is declared instead it returns or prints its value.

For Example:

//Months.java
class Months
{
                static final int JAN =1;
                static final int FEB =2;

                public static void main(String[] args)
                {
                                System.out.println(Months.JAN);
                                System.out.println(Months.FEB);
                }
}

Prints
1
2

Enum is introduced in Java 5 to solve above problems, it means to print menu item name not value.

Syntax: to create enum data type in java 5 a new keyword “enum” is introduced.

enum <enumName> 
{
                //menu items names with "," seperator
                                                   ;
                //normal all 8 members which you define in a nnormal class
}

Note: For creating above name constants just use only names do not use any datatype

//Months.java
enum Months{
                JAN, FEB;
}

//year.java
class Year
{
                public static void main(String[]args)
                {
                                System.out.println(Months.JAN);
                                System.out.println(Months.FEB);
}

Compilation
>javac Year.java
>java Year

Prints:
JAN
FEB

What is the data type of named constants in enum ?
Its data type is current enum type. It is added by compiler automatically.

For example

In the enum Months, Its named constants are converted as shown below

public static final Months JAN;
public static final Months FEB;

Please Note:
  •  Enum is a final class hence cannot be inherited.
  •  It is sub class of Java.lang.Enum.
  •  It is a abstract class which is the default super class for every enum type classes. Also it is implementing from Comparable and Serializable interfaces.
  •  Since every enum type is Comparable so we can add its objects to collection TreeSet object, also it can be stored in file as it is Serializable type.

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