OCP Java Class Design and OOP 2

Question 1

Which keyword prevents a method from being overridden in a subclass?

Accepted Answer

final

Answer

The `final` keyword on a method prevents subclasses from overriding it.

Question 2

What happens when a class is declared `abstract` and contains no abstract methods?

Accepted Answer

The class cannot be instantiated but can be subclassed

Answer

An abstract class with no abstract methods is valid — it simply cannot be instantiated directly.

Question 3

Given: `class A { int x = 5; }` and `class B extends A { int x = 10; }`, what does `((A) new B()).x` evaluate to?

Accepted Answer

5

Answer

Field access is resolved at compile time based on the reference type, so `((A) b).x` gives A's field value 5.

Question 4

Which of the following correctly describes a covariant return type in Java?

Accepted Answer

An overriding method can return a subtype of the original return type

Answer

Java allows covariant return types, meaning an overriding method may return a subtype of the declared return type.

Question 5

What is the result of calling a `default` interface method when a class implements two interfaces that define the same default method?

Accepted Answer

Compilation error unless the class overrides the method

Answer

If two interfaces provide the same default method, the implementing class must override it or a compilation error occurs.

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