In java a sub class of a/ can override a method of itssuper class and declare it . in that case the subclassmust be declared abstract.
abstract class, non abstract class
non abstract class, abstract
non abstract class , reference data type
overloaded class, private

non abstract class,abstract

Explanation:

In java a sub class of a concrete class or non abstract class can override a method of its upper class  and declare it abstract . This is because it does not happen very often, but it is useful when the implementation of  the method in the upper class is not valid in the  subclass .

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