SCJP Java OOP and Inheritance 4

Question 1

Which statement correctly describes the behavior of constructors in an inheritance hierarchy?

Accepted Answer

If no super() call exists, the compiler inserts super() only if the superclass has a no-arg constructor

Answer

If a subclass constructor does not explicitly call super(), the compiler inserts a call to the superclass no-arg constructor; if none exists, a compile error occurs.

Question 2

Given: class A { int x = 5; } class B extends A { int x = 10; } A obj = new B(); System.out.println(obj.x); — What is printed?

Accepted Answer

5

Answer

Field access is resolved at compile time based on the reference type, so obj.x refers to A's x, printing 5.

Question 3

Which of the following is true about abstract classes in Java?

Accepted Answer

An abstract class can have both abstract and concrete methods

Answer

An abstract class can contain any combination of abstract and concrete methods, and it can also have constructors.

Question 4

What is the result of calling a non-private, non-final instance method on a null reference?

Accepted Answer

NullPointerException at runtime

Answer

Invoking any instance method on a null reference causes a NullPointerException at runtime.

Question 5

class Animal { void speak() { System.out.println("Animal"); } } class Dog extends Animal { void speak() { System.out.println("Dog"); } } Animal a = new Dog(); a.speak(); — What is printed?

Accepted Answer

Dog

Answer

Method calls are resolved at runtime based on the actual object type (Dog), demonstrating runtime polymorphism.

SCJP - Sun Certified Java Programmer Practice Test

SCJP - Sun Certified Java Programmer Practice Test

SCJP Java OOP and Inheritance 4