Difference between static and non-static variables in Java

There are three types of variables in Java:

Local Variables
Instance Variables
Static Variables

The Local variables and Instance variables are together called Non-Static variables. Hence it can also be said that the Java variables can be divided into 2 categories:

Static Variables: When a variable is declared as static, then a single copy of the variable is created and shared among all objects at a class level. Static variables are, essentially, global variables. All instances of the class share the same static variable.

Important points for static variables :-

We can create static variables at class-level only. See here
static block and static variables are executed in order they are present in a program.

Below is the Java program to demonstrate that static block and static variables are executed in order they are present in a program.

// Java program to demonstrate execution 
// of static blocks and variables 
class Test { 
    // static variable 
    static int a = m1(); 
  
    // static block 
    static
    { 
        System.out.println("Inside static block"); 
    } 
  
    // static method 
    static int m1() 
    { 
        System.out.println("from m1"); 
        return 20; 
    } 
  
    // static method(main !!) 
    public static void main(String[] args) 
    { 
        System.out.println("Value of a : " + a); 
        System.out.println("from main"); 
    } 
} 

Output:

from m1
Inside static block
Value of a : 20
from main

Non-Static Variable

Local Variables: A variable defined within a block or method or constructor is called local variable.
- These variable are created when the block in entered or the function is called and destroyed after exiting from the block or when the call returns from the function.
- The scope of these variables exists only within the block in which the variable is declared. i.e. we can access these variable only within that block.
- Initialisation of Local Variable is Mandatory.
Instance Variables: Instance variables are non-static variables and are declared in a class outside any method, constructor or block.
- As instance variables are declared in a class, these variables are created when an object of the class is created and destroyed when the object is destroyed.
- Unlike local variables, we may use access specifiers for instance variables. If we do not specify any access specifier then the default access specifier will be used.
- Initialisation of Instance Variable is not Mandatory. Its default value is 0
- Instance Variable can be accessed only by creating objects.

Example :

// Java program to demonstrate 
// non-static variables 
  
class GfG { 
  
    // non-static variable 
    int rk = 10; 
  
    public static void main(String[] args) 
    { 
        // Instance created inorder to access 
        // a non static variable. 
        Gfg f = new Gfg(); 
  
        System.out.println("Non static variable"
                           + " accessed using instance"
                           + " of a class"); 
        System.out.println("Non Static variable "
                           + f.rk); 
    } 
} 

Output:

Non static variable accessed using instance of a class.
Non Static variable 10

The main differences between static and non static variables are:

Static variable	Non static variable
Static variables can be accessed using class name	Non static variables can be accessed using instance of a class
Static variables can be accessed by static and non static methods	Non static variables cannot be accessed inside a static method.
Static variables reduce the amount of memory used by a program.	Non static variables do not reduce the amount of memory used by a program
Static variables are shared among all instances of a class.	Non static variables are specific to that instance of a class.
Static variable is like a global variable and is available to all methods.	Non static variable is like a local variable and they can be accessed through only instance of a class.

My Personal Notes

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