Global Variable vs Class Variable vs Instance Variable vs Local Variable in Java

In this post, we'll develop some basic understanding of different kind of variable in java and how they are differ from each other with the help of some examples.

Local variable
A local variable lives only within the method that declared the variable.

Variable's can be used only within the read() method. In other words, the variable is in scope only within its own method. No other code in the class can see.
When you call a same method second time, it recreates the local variables, and reinitialized them.

Before using local variable, it must be initialized.

Local variables, including primitives, always, always, always must be initialized before you attempt to use them.

• Just don't forget that while the local variable is on the stack, if the variable is an object reference, the object itself will still be created on the heap. There is no such thing as a stack object, only a stack variable.
• Local variable declarations can't use most of the modifiers that can be applied to instance variables, such as public (or the other access modifiers), transient, volatile, abstract, or static, but local variables can be marked final.

Class Variables
A class variable is a variable defined in a class (i.e. a member variable) of which a single copy exists, regardless of how many instances of the class exist.
Sometimes, you want to have variables that are common to all objects. This is accomplished with the static modifier. Fields that have the static modifier in their declaration are called static fields or class variables. They are associated with the class, rather than with any object. 

• Every instance of the class shares a class variable, which is in one fixed location in memory.
• Any object can change the value of a class variable.
• Class variables can also be manipulated without creating an instance of the class.

Sample Output
c1 size : 10 c2 size : 10 c3 size :10
c1 size : 50 c2 size : 50 c3 size :50
Size = 50

The problem is that main() is itself a static method, and thus isn't running against any particular instance of the class, rather just on the class itself.
A static method can't access a nonstatic (instance) variable, because there is no instance!

That's not to say there aren't instances of the class alive on the heap, but rather that even if there are, the static method doesn't know anything about them. The same applies to instance methods; a static method can't directly invoke a nonstatic  method. Think static = class, nonstatic = instance. 


Instance Variable

• There are those variable that is associated with object.
• Instance variables are defined inside the class, but outside of any method, and are only initialized when the class is instantiated.
• Their values are unique to each instance of a class.
• An instance variable lives as long as the object does. If object is still alive, so are its instance variables.

You need to know that instance variables

• Can use any of the four access levels (which means they can be marked with any of the three access modifiers)
• Can be marked final
• Can be marked transient
• Cannot be marked abstract
• Cannot be marked synchronized
• Cannot be marked strictfp
• Cannot be marked native
• Cannot be marked static, because then they'd become class variables.

Sample Output
c1 size :2 c2 size:0 c3 size:0

c1 price :0 c2 price:100 c3 price:0

Global Variable
There is no direct concept of global variable in java, but you implement the same in different number of ways.

With the help of static keyword and public access modifier.

public class GlobalVariable {  
      public static int MAX_SIZE = 1000;  
      public static int MIN_SIZE = 1;  
 }

Now you can access MAX_SIZE and MIN_SIZE from anywhere by calling like this

globalVariable.MAX_SIZE 

globalVariable.MIN_SIZE 

With the help of interface

public interface GlobalVariable1 {  
      /**  
       * Variable are implicitly public, static, and final  
       */  
       int MAX_SIZE = 1000;  
       int MIN_SIZE = 1;  
 } 

Any class that needs to use these, can implement the interface.

public class GlobalVariable1Demo implements GlobalVariable1{  
      public static void main(String[] args) {  
           System.out.println("Max Size :"+MAX_SIZE);  
           System.out.println("Max Size :"+MIN_SIZE);  
      }  
 } 

The static modifier, in combination with the final modifier, is also used to define constants. The final modifier indicates that the value of this field cannot change.



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