OOP

Question 1

What features does OOP support?

Answer 1

The bare minimum is it must support Classes, Objects, Encapsulation, Inheritance, Polymorphism.

Question 2

What is polymorphism?

Answer 2

When an object has many Is-A identities. Most common use is using a parent class reference to refer to child class object.

Question 3

Give example of polymorphism?

Answer 3

Dog is an Animal
Dog is a Dog
Dog is a Carnivore
Dog is an object

Dog d = new Dog();
Animal a = d;

Question 4

Give example of taking advantage of polymorphism?

Answer 4

Animal a = new Dog();
a.noise();

Compiler uses noise() in animal to validate. At run time though JVM uses overridden noise() in the Dog class.

Question 5

Advantage of using Polymorphism?

Answer 5

Useful for making lists - a list of animals could contain different sub animals.

Question 6

Difference between class and interface?

Answer 6

A class is a blueprint. Must be instantiated into object. Interface is more like a contract, can not be instantiated. Interfaces implemented, only one class can be extended. Interface variables are constants (static and final).

Question 7

Difference between abstract method and interface?

Answer 7

Abstract class cannot be instantiated. Interface neither. Interface all abstract methods and static final fields.

Question 8

When to choose interface over abstract class?

Answer 8

Choose abstract class if some default behavior shared amongst related classes. If providing common functionality to unrelated classes use interfaces.

Question 9

What are access modifiers?

Answer 9

Private: class only.
Package: class + package
Protected: class, package + subclass
Public: anybody