Java OCA OCP Practice Question 2320

Question

Given the following class declaration: 

class ClassA<U> implements Comparable<U> {
  public int compareTo(U a) { return 0; }
}

Which class declarations below will compile without errors?.

Select the three correct answers. 

(a) class ClassB<U,V> extends ClassA<R> {}
(b) class ClassC<U,V> extends ClassA<U> {}
(c) class ClassD<U,V> extends ClassA<V, U> {}
(d) class ClassE<U> extends ClassA<Comparable<Number>> {}
(e) class ClassF<U extends Comparable<U> & Serializable> extends ClassA<Number> {}
(f) class ClassG<U implements Comparable<U>> extends ClassA<Number> {}
(g) class ClassH<U extends Comparable<U>> extends ClassA<? extends Number> {}
(h) class ClassI<U extends String & Comparable<U>> extends ClassA<U> {}
(i) class ClassJ<U> extends ClassA<Integer> implements Comparable<Number>{}



(b), (d), and (e)

Note

(a) R cannot be resolved.

(b) Ok.

(c) The ClassA is parameterized with only one type parameter.

(d) Ok.

(e) Ok.

The declaration says that the parameterized type ClassA<Number> is the super type of ClassF<U> for any concrete type U that satisfies the constraint U extends Comparable<U> & Serializable.

(f) The keyword implements cannot be used to specify bounds.

(g) A wildcard cannot be used to specify the type parameter for the superclass.

(h) String is Comparable<String>, therefore, Comparable cannot be implemented more than once with different arguments.

(i) From the declaration, superclass ClassA<Integer> implements Comparable<Integer>.

Subclass ClassJ thus implements Comparable<Integer>.

However, Comparable cannot be implemented more than once with different arguments.




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