SCJP Java OOP and Inheritance 5

Question 1

Which of the following correctly describes method hiding in Java?

Accepted Answer

A subclass static method with the same signature as a superclass static method hides it

Answer

Static methods are not overridden; when a subclass defines a static method with the same signature as a superclass static method, it hides it, and selection is based on the reference type, not the object.

Question 2

class Base { Base() { display(); } void display() { System.out.println("Base"); } } class Child extends Base { int x = 10; void display() { System.out.println(x); } } new Child(); — What is printed?

Accepted Answer

0

Answer

When the Base constructor calls display(), the Child's overridden display() runs, but x has not yet been initialized (instance initializers run after super()), so x is 0.

Question 3

Which of the following is a valid use of the 'super' keyword?

Accepted Answer

super.method() to invoke the direct superclass's overridden method

Answer

super.method() can be used in a subclass instance method to explicitly call the direct superclass's version of an overridden method.

Question 4

An interface I declares a method m(). Class A implements I and provides m(). Class B extends A. Which statement is true about B?

Accepted Answer

B inherits A's implementation of m() and satisfies the interface contract

Answer

B inherits A's concrete implementation of m(), which satisfies the interface contract without B needing to redeclare it.

Question 5

What is the effect of casting a superclass reference to a subclass type when the actual object is of the superclass type?

Accepted Answer

ClassCastException at runtime

Answer

Downcasting a reference to a subclass type when the actual object is not of that subclass results in a ClassCastException at runtime.

SCJP - Sun Certified Java Programmer Practice Test

SCJP - Sun Certified Java Programmer Practice Test

SCJP Java OOP and Inheritance 5