Few OOP design clarifications

Question

I was wondering about few of the below OOP concepts 1) abstract class cannot have final constructor. Why cant subclasses have same same constructor without rewriting them 2) abstract method cannot be private . We can still implement the abstract class in same file. So why not allow this

Answer 1

If a class has a final constructor, it isn't abstract.

If a class has a private method, no derived class could override it, thus the base class isn't abstract.

Perhaps you'd like

• a non-abstract final base class with an abstract interface
• a protected method in the base class which allows derived classes to override it

instead.

Answer 2

In Java, no constructor can be final. Constructors are not inherited at all.

Private methods are not inherited, so a private abstract method could not be implemented. Compare this to C++, in which private methods may oddly be overridden by derived classes.

From the Java Language Specification

8.8.3 Constructor Modifiers Unlike methods, a constructor cannot be abstract, static, final, native, strictfp, or synchronized. A constructor is not inherited, so there is no need to declare it final and an abstract constructor could never be implemented.

8.4.3.1 abstract Methods ... It is a compile-time error for a private method to be declared abstract. It would be impossible for a subclass to implement a private abstract method, because private methods are not inherited by subclasses; therefore such a method could never be used.