Variable and Constant in Java:-

Variable or Data Member:-  It is a special identifier whose value can be changed at compile-time or runtime.

Variable declaration Syntax:-
 access modifier + non access modifier +  datatype + identifier

 public                     static         int               x

int x =10 ;    //default instance int x=10 dynamic memory allocation using object
static int y=20 //  default static int y=20 one-time memory allocated at compile time
Types of variables:-
1 Instance or Dynamic:-
It will be declared without any static modifier and it will allocate memory at the time of execution by the object.
a variable that will be declared underclass or under instance method without using static modifier will be an instance variable

instance variable will never declare under the static method but it can be used.

this type of variable is also called dynamic data members in OOP'S Programming. means non-static, for instance, dynamic all are the same.

Example of nonstatic variable:-

class Dynamic

{

   int a,b,c;

   public static void main(String args[])

    {

      Dynamic obj= new Dynamic(); 

      obj.a=100;

      obj.b=200;

      obj.c=obj.a+obj.b;

      System.out.println(obj.c);

    }

}

class A
{
    int x=10; //global instance variable
    void fun()
    {
       int y=20; //local instance variable
    }
}
A obj = new A();  //object
Complete Explanation by Program:-
class Add
{
   int a=100,b=200,c;
   public static void main(String args[])
   {
      Add obj = new Add();
      obj.c=obj.a+obj.b;
      System.out.println(obj.c);
      Add obj1 = new Add();
      obj1.a=10;
      obj1.b=20;
      obj1.c=obj1.a+obj1.b;
      System.out.println(obj1.c);
   } 
}
2 Static:-
This type of variable will allocate memory after compile-time, static has constant memory but not the constant value, a value can be changed in the same memory multiple times.
static datatype identifier=value
static int x=100;
               x=10;
               x=12;
we can define static variable underclass using static keyword and static method without using the static keyword. if we define a static variable under the static method then no need to declare a static keyword.
class A
{
    static int a=100;
    void fun()
    {
        int b=20;
    }

   public static void main()
   {
      int c=30;
   }
}
Program of Static With Complete explanation?
class Add
{
   static int a=100,b=200,c;
   public static void main(String args[])
   {
       c=a+b;
      System.out.println(c);
      a=10;
      b=20;
      c=a+b;
      System.out.println(c);
   } 
}
......................................................................................................................................... 

Program explanation in Java:-

class Dynamic

{

    int a,b,c;

   

    public static void main(String args[])

    {

      Dynamic obj=new Dynamic();

      obj.a=100;

      obj.b=200;

      obj.a=20;

      Dynamic obj1=new Dynamic();

      obj1.a=2000;

      obj1.b=3000;

      obj=obj1;

      obj.c=obj.a+obj.b;

         System.out.println(obj.c);

    }

}

Constant:-
....................................................................................................................................................
If we can not change the value of the identifier at compile-time or run-time then it is called constant.
java provides the final keyword to a constant declaration.
final static datatype variablename=value;
constant memory always will be static by default.

Assignment:-

1)WAP to calculate electricity bill where unit price and total consumption will be assigned by the users?

2)  WAP to calculate salary where basic, ta, da, comm, pf,hra,noofleave will be assigned by the users?

3)  WAP to calculate the multiplication of complex numbers?

The solution to this program:-

class Complex

{

    public static void main(String args[])

    {

       int a,b,c,d,r,i;

       a=2;

       b=3;

       c=4;

       d=5;

       r = a*c-b*d;

       i = a*d+b*c;

       System.out.println(r + "+" + i + "i");

      }

}

Program to perform the addition of complex numbers?

class Complex

{

    public static void main(String args[])

    {

       int a,b,c,d,r,i;

       a=2;

       b=3;

       c=4;

       d=5;

       r = a+c;

       i = b+d;

       System.out.println(a + "+" + b + "i");

       System.out.println(c + "+" + d + "i");

       System.out.println(r + "+" + i + "i");

      }

}

4)  WAP to find the maximum number in any three-digit number?

int a= 457 ;  //7

5)  WAP to find middle numbers using three-digit numbers?

6)  WAP to calculate the area of a circle where pi will be constant and radius will be variable?

7)  WAP to calculate simple interest using instance type variable?

8)  WAP to convert temperature from Celsius to Fahrenheit using static data members globally?

c/5 = f-32/9