Object Orientation

Question 1

What is encapsulation?

Answer 1

Encapsulation helps hide implementation behind an interface. It has two features;

• Instance variables are kept protected (usually with the private modifier).
• Getter and setter methods provide access to instance variables.

Question 2

What is IS-A?

Answer 2

It refers to inheritance or implementation, expressed with the keyword extends or implements.”inherits from,” and “is a subtype of” are all equivalent expressions.

Question 3

What is HAS-A?

Answer 3

An instance of one class “has a” reference to an instance of another class or another instance of the same class.

Question 4

What are the fundamental concepts of OOP?

Answer 4

• Encapsulation
  
  • Inheritance
  • Polymorphism
  • Abstraction

Question 5

What is Inheritance?

Answer 5

Inheritance allows a class to be a subclass of a superclass and thereby inherit public and protected variables and methods of the superclass. ıt is a key concept that underlies IS-A, polymorphism, overriding,
overloading, and casting.

Question 6

Which classes are subclasses of Object?

Answer 6

All classes (except class Object) are subclasses of type Object, and therefore they inherit Object’s methods.

Question 7

What is Polymorphism?

Answer 7

Polymorphism means “many forms.” A reference variable is always of a single, unchangeable type, but it can refer to a subtype object. It applies to overriding, not to overloading.

Question 8

What determines which methods can be called?

Answer 8

The reference variable’s type - not the object’s type.

Question 9

Polymorphic method invocations apply only to..

Answer 9

overridden instance methods.

Question 10

Can constructors be overriden?

Answer 10

No. Can only be overloaded.

Question 11

What are the 6 rules for overriding?

Answer 11

❑ Must have the same argument list
❑ Must have the same return type, except that, as of Java 5, the return type can be a subclass, and this is known as a covariant return
❑ Must not have a more restrictive access modifier
❑ May have a less restrictive access modifier
❑ Must not throw new or broader checked exceptions
❑ May throw fewer or narrower checked exceptions, or any unchecked exception.

Question 12

Can final methods be overridden?

Answer 12

No.

Question 13

Can private methods be overridden?

Answer 13

No.

Question 14

How can a subclass call superclass version of an overriden method?

Answer 14

super.overriddenMethodName()

Question 15

What are the 4 rules for overloading?

Answer 15

❑ Must have different argument lists
❑ May have different return types, if argument lists are also different
❑ May have different access modifiers
❑ May throw different exceptions

Question 16

What determines which version of “overriden” method is called?

Answer 16

Object type (not the reference variable’s type).

Question 17

What determines which version of “overloaded” method is called?

Answer 17

Reference type.

Question 18

What is downcasting?

Answer 18

Downcasting or type refinement is the act of casting a reference of a base class to one of its derived classes. You must make an explicit cast to do this.
Animal animal = new Animal();
Dog d = (Dog) animal;

However, if the compiler knows with certainty that the cast could not possibly work, compilation will fail.
String s = (String) animal; // animal can’t EVER be a String

Question 19

What is upcasting?

Answer 19

Casting up the inheritance tree to a more general

type. It works implicitly (that is, you don’t have to type in the cast).

Question 20

How many interfaces can a class implement?

Answer 20

Many.

Question 21

Can overloaded change return type?

Answer 21

Yes.

Question 22

Can overriden change return type?

Answer 22

No, except in the case of covariant returns.

Question 23

Can a method with a primitive return type return a type other than the one in its signature?

Answer 23

For methods with primitive return types, any value that can be implicitly converted to the return type can be returned.

Question 24

Methods with an object reference return type can return..

Answer 24

a subtype.

Question 25

Methods with an interface return type can return..

Answer 25

any implementer.

Question 26

Does each class have a constructor?

Answer 26

Yes. Every class, even an abstract class, has at least one constructor.

Question 27

How are the constructors executed?

Answer 27

❑ The constructor calls its superclass constructor, which calls its superclass constructor, and so on all the way up to the Object constructor.
❑ The Object constructor executes and then returns to the calling constructor, which runs to completion and then returns to its calling constructor, and so on back down to the completion of the constructor of the actual instance being created.

Question 28

Can constructors use access modifiers?

Answer 28

Yes, constructors can use any access modifier (even private!).

Question 29

What happens if you don’t create any constructors for your class?

Answer 29

The compiler will create a default constructor if you don’t create any constructors in your class.

Question 30

What is a default consturctor?

Answer 30

The default constructor is the no-argument constructor with a no-arg call to super() automatically generated unless you define another constructor.

Question 31

What is the first statement of a consturctor?

Answer 31

The first statement of every constructor must be a call either to this() (an overloaded constructor) or to super(). The compiler will add a call to super() unless you have already put in a call to this() or super().

Question 32

When are the instance members accessible?

Answer 32

Only after the super constructor runs.

Question 33

When are the constructor for an abstract class is called?

Answer 33

When a concrete subclass is instantiated.

Question 34

Do interfaces have constructors?

Answer 34

No.

Question 35

What happens if super class does not have a no-arg constructor?

Answer 35

You must create a constructor and insert a call to super() with arguments matching those of the superclass constructor.

Question 36

Can constructors be inherited?

Answer 36

Constructors are never inherited; thus they cannot be overridden.

Question 37

Can a call to super and this be in the same constructor?

Answer 37

No, you can have one or the other, but never both. Constructors can call constructors, and so on, but sooner or later one of them better call super() or the stack will explode.

Question 38

When are the “static init” blocks are run?

Answer 38

Runs once, when the class is first loaded. Multiple blocks run from the top down.

Question 39

When are the “init” blocks are run?

Answer 39

Runs for every new instance, right after all the super constructors have run. Multiple blocks run from the top down.

Question 40

How many copies are there for a static variable?

Answer 40

Only one.

Question 41

Can we access a static variable using instance variable?

Answer 41

Yes but d.doStuff();
becomes
Dog.doStuff();

Question 42

Can static methods be overriden?

Answer 42

No. can be overloaded.