Why we use class in the program??what does class do?

Answer 1

classes are used for the OOP (Object oriented programming), using OOP you just write the code 1 time and call It when need It, and you're not repeating the code everytime you need It.
Example:

Myclass hi = new Myclass();
System.out.print(hi.sayhello);
//output Hello World.

Answer 2

give ur answer in scientific language and only in 2 lines????