What is the difference between an instance and a class (static) variable in Java

Question

The title of this question is actually a previous examination question and I am looking for clarification / an answer to it.

Please note that I am learning Java and am becoming familiar with its syntax.

I understand that this question may have been asked before and if so can someone please show me where I may access the question if possible? Also please accept my apologies if this is the case. To show that I have been researching this area, my own understanding is that instance variables belong to the objects / instances of a certain class (template) and can be changed (mutated) within that instance / object as and when required.

A class variable is a variable that has only one copy and can be accessed but not be modified (mutated?), but is available to all classes as required?

Am I on the right track here?

Also, what exactly does the 'static' do? Is an instance of a class only static if it resides within the main instance of a class?

Many thanks.

You are almost right. Class variables *can* be mutated, though. — Code-Apprentice, Mar 18 '13 at 20:36
You are mostly correct. (A static variable *can* be mutated, if not final.) The specific term "static" is somewhat of a tradition, slightly abused in this context. — Hot Licks, Mar 18 '13 at 20:36
http://stackoverflow.com/questions/413898/what-does-the-static-keyword-do-in-a-class — Code-Apprentice, Mar 18 '13 at 20:37
(There is no "main instance" of a class -- all instances are equal (even if some are occasionally more equal than others).) — Hot Licks, Mar 18 '13 at 20:37
This is great feed back and answers. I am beginning, albeit slowly getting my head around it!! Thanks all. — PrimalScientist, Mar 18 '13 at 20:39
May I ask why this question has been downgraded please? The question seems straight forward and the answers were okay I think? — PrimalScientist, Mar 19 '13 at 13:03

score 20 · Accepted Answer · edited Nov 08 '15 at 22:52

20

A static variable is shared by all instances of the class, while an instance variable is unique to each instance of the class.

A static variable's memory is allocated at compile time, they are loaded at load time and initialized at class initialization time. In the case of an instance variable all of the above is done at run time.

Here's a helpful example:

An instance variable is one per object: every object has its own copy of its instance variable.

public class Test{

   int x = 5;

 }

Test t1 = new Test();   
Test t2 = new Test();

Both t1 and t2 will have their own copy of x.

A static variable is one per class: every object of that class shares the same static variable.

public class Test{

   public static int x = 5;

 }

Test t1 = new Test();   
Test t2 = new Test();

Both t1 and t2 will share the same x.

edited Nov 08 '15 at 22:52

Choylton B. Higginbottom

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answered Mar 18 '13 at 20:36

Ajay S

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Thanks for your feedback. This is helping me understand as it can be quite confusing!! Thanks!! – PrimalScientist Mar 18 '13 at 20:38
That is a great example. Helps me to understand the answer. MAny thanks. – PrimalScientist Mar 18 '13 at 20:47
Thank you all for your help here. It is really appreciated. – PrimalScientist Mar 18 '13 at 20:49
1

Static variables do not 'load at compile time'. They are loaded at load time and initialized at class initialization time. – user207421 Mar 19 '13 at 04:55

score 5 · Answer 2 · edited Jul 03 '15 at 07:46

5

You can make multiple instances of your class. When you declare an instance variable, each instance gets its own unique copy of that variable. When you declare a static variable, it is the same variable and value for all instances.

public class Foo
{
    public int instanceVariable;
    public static int staticVariable;
}

Foo instance1 = new Foo();
Foo instance2 = new Foo();
instance1.staticVariable = 1;
instance1.instanceVariable = 2;
instance2.instanceVariable = 3;

instance1.staticVariable == 1 // true
instance2.staticVariable == 1 // true

instance1.instanceVariable == 2 //true
instance2.instanceVariable == 3 //true

edited Jul 03 '15 at 07:46

i.am.jabi

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answered Mar 18 '13 at 20:35

Robert Harvey

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Thats great. Thanks for clarification. – PrimalScientist Mar 18 '13 at 20:37
This answer is more better then first one :) – Manjeet May 18 '16 at 18:58
Great explanation u have solved my 1 month long confusion. magical 2.25 lines for the explanation. – jax Feb 20 '17 at 11:05

score 1 · Answer 3 · answered Mar 18 '13 at 20:41

1

A static variable stores a value that is shared between all instances (or the non-instance) of the Class it is defined in. Where as instance variable by name create a new copy of it everytime.When the new object is created.

answered Mar 18 '13 at 20:41

Pragnani

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Many thanks for your feedback there. Starting to get it now. – PrimalScientist Mar 18 '13 at 20:42

What is the difference between an instance and a class (static) variable in Java

3 Answers3

Here's a helpful example:

Linked