Java Class Design and OOP Flashcards

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

Answer: final

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

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

Answer: The class cannot be instantiated but can be subclassed

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

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

Answer: 5

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

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

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

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

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

Answer: Compilation error unless the class overrides the method

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

Which access modifier makes a member visible only within the same package and to subclasses in other packages?

Answer: protected

`protected` grants access within the same package and to subclasses regardless of package.

What is the effect of marking a constructor `private` in a class?

Answer: The class cannot have any subclasses and cannot be instantiated outside the class

A private constructor prevents both external instantiation and subclassing, since subclass constructors must call a superclass constructor.