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.