Static Instance Variable And Non-Static Instance Variable Difference

Introduction

Static is the keyword in Java which can be used with variables, methods and blocks. However, the meaning of the static keyword differs with the place it is being used.

In this article, I will explain the use of the static keyword with the variable, by providing you with the difference between the static instance variable and the non-static instance variable.

Static Instance Variable vs Non-static Instance Variable

First.java

public class First {
//static instance variable named "a"
static int a=0;
//non-static instance variable named "b"
int b=0;
void displayValue()
{
System.out.println("static variable a : "+a);
System.out.println("non-static variable b : "+b+"\n");
}
void incrementValue()
{
a++; //incrementing static variable "a" by 1
b++; //incrementing non-static variable "b" by 1
}
}

Second.java

public class Second extends First {
public static void main(String[] args) {
//Object creation of First Class
First f1 = new First();
First f2= new First();
//Object creation of Second Class
Second s1 = new Second();
Second s2 = new Second();
System.out.println("Object f1");
f1.incrementValue();
f1.displayValue();
System.out.println("Object f1-Again");
f1.incrementValue();
f1.displayValue();
System.out.println("Object f2");
f2.incrementValue();
f2.displayValue();
System.out.println("Object f2-Again");
f2.incrementValue();
f2.displayValue();
System.out.println("Object s1");
s1.incrementValue();
s1.displayValue();
System.out.println("Object s2");
s2.incrementValue();
s2.displayValue();
}
}

In this example, there are

Two classes, First and Second, where Second inherits First
One static variable a with value 0
One non-static variable b with value 0
incrementValue() method to increment a and b by 1
displayValue() method to display a and b value to the console
Two objects f1 and f2 for First Class
Two objects s1 and s2 for Second Class

Accessing two methods, incrementValue() and displayValue() by the First class object named "f1".

static variable a : 1

non-static variable b : 1
Accessing two methods, incrementValue() and displayValue() by the First class object named "f1 Again".

static variable a : 2

non-static variable b : 2
Accessing two methods, incrementValue() and displayValue() by the First class object named "f2".

static variable a : 3

non-static variable b : 1
Accessing two methods, incrementValue() and displayValue() by the First class object named "f2 Again".

static variable a : 4

non-static variable b : 2

From this, it is understood that the static variable is common to all the instances of the class and non-static variable is specific to the particular object.
Accessing two methods, incrementValue() and displayValue() by the First class object named "s1".

static variable a : 5

non-static variable b : 1
Accessing two methods, incrementValue() and displayValue() by the First class object named "s1".

static variable a : 6

non-static variable b : 1

From the above two steps, it is understood that the static variable is common to all the instances of the superclass and subclass.

Final output

Conclusion

In this article, we learned that the static variable is shared by every instance of the classes. So, it can be used for memory management purposes. But the non-static variable is specific to the object created and it occupies more space.