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difference between an instance variable and static variable?

Aamir Abbass
Greenhorn

Joined: Jul 16, 2010
Posts: 1

Hye can anyone out there help me understanding that

1. what is the difference between an instance variable and a static variable?

2. when should we define any variable as static in a class?

Thanks in advance.
David Newton
Author
Rancher

Joined: Sep 29, 2008
Posts: 12617

Instance variables belong to an instance of the class. Static variables belong to the class itself, and may be used without instantiating the class.

You should use static members when you need functionality without creating an instance. An example where a lot of people use static members is when defining constants (like Math.PI) or utility methods (like StringUtils.isBlank()). Whether or not static methods are a good idea or not is debatable, at least in my opinion.
Campbell Ritchie
Sheriff

Joined: Oct 13, 2005
Posts: 39885
    
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And welcome to JavaRanch
lakmal padmakumara
Ranch Hand

Joined: Aug 08, 2009
Posts: 47
David is spot on.Another easy example would be think you have a class Call Egg .So you can create Egg objects using that class .Now you might be having several instance variables which describes a particular egg like eggColor .eggSize . So these are instance variables .Value of those variables are unique to that particular object .And You cannot have a eggColor unless you already got an Egg Object.So remember you can never use instance variables without creating a actual object.

Now you also want to keep track of how many eggs do you have in your System. So this howManyEggs variable is not unique to each and every egg object.It is a common attribute to all the eggs .

If you ask from egg1 :Hey What is your color ? He might say I'm white
If you ask from egg2: Hey what is your color ? He might say I'm yellow

But if you ask How many eggs are in the system by them both should give the same answer.So noOfEggs is a attribute represents the class Egg.So use a class member(attribute) to represent it.

Hope this will Help!


[Bsc in Computer Science and Engineering] [SCJP 80%][BCS certificate level]
Arun C. Giridharan
Ranch Hand

Joined: Jul 11, 2010
Posts: 96
lakmal .right on the top, egg represent object ( instance of a class ) ..if you want to know....how many objects(eggs) of that class type are created ... opt for static variable... since it is unique for a class (class level) ... and if you want to have a global variable kind mention use' public static 'keyword(behaves globally ...but not global).....
 
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