About final class

Question

When we declare all variables and methods in a class as final, we are restricted the concept of override.

Then, "what is the reason we want to restrict IS-A relationship in an OOP language( even we restrict IS-A relationship, this language has Has-A relationship) by using final class?"

My point is we can not restrict the accessing the members of our class any way, then give an easy access by allowing IS-A relationship. Otherwise it is a headache to create objects and all? (in my view 'access' not includes modifications)

If I miss any important theme of programming , please excuse me. And tell me, what is it?

Answer 1

Is there any necessity to declare a class as final class?

Necessity, no. Declare a class final if you want to forbid extending it. This can prevent programming errors. For example it's a recommended practice to make utility classes with only static methods to be final, as inheriting from such class wouldn't make sense, and be clearly a programming error.

Answer 2

Security would be one necessity. String class in Java is an Immutable/final class. No sub-classing is possible System class is another example.

Have more read here: Good reasons to prohibit inheritance in Java?

Answer 3

Is there any necessity to declare a class as final class?

Yes. If you are implementing a Singleton pattern with a class. If someone can sub-class your Singleton, then there may be more than one (which violates the Singleton pattern).