Java OCA OCP Practice Question 3081

Question

Given this class definition: 

abstract class Base {
   public abstract Number getValue();
}

Which of the following two options are correct concrete classes extending Base class? 


a)class Deri extends Base {
       protected Number getValue() {
           return new Integer(10);
       }// w  ww .  ja v a 2s. com
   }

b)class Deri extends Base {
       public Integer getValue() {
           return new Integer(10);
       }
   }

c)class Deri extends Base {
       public Float getValue(float flt) {
           return new Float(flt);
       }
   }

d)class Deri extends Base {
       public java.util.concurrent.atomic.AtomicInteger getValue() {
           return new java.util.concurrent.atomic.AtomicInteger(10);
       }
   }



b)
d)

Note

option b) makes use of a co-variant return type (note that Integer extends Number), and defines the overriding method correctly.

option d) makes use of co-variant return type (note that AtomicInteger extends Number), and defines the overriding method correctly.

option a) attempts to assign a weaker access privilege by declaring the method protected when the base method is public, and thus is incorrect (results in a compiler error).

In option c) the method Float getValue(float flt) does not override the getValue() method in Base since the signature does not match, so it is incorrect (results in a compiler error).




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