Java 2

Question 1

Nested Classes:

public class FlightTicket {

public static class Business {

}

public static class Economy {

}

public static class First {

	} }

how do i create an object of any of the nested classes?

Answer 1

FlightTicket.Business business = new FlightTicket.Business();

etc.

Question 2

Inheritance principles: (7 in total)

Answer 2

1. inherits properties/variables and behavior/methods of another class
2. Great for code Reusability and Maintainability
   Example of this: When i have to set up API i can extend the properties such as the BaseURI of the SpartanBase Class
3. have to use extends keyword
4. IS-A relation between classes
   If Vehicle is parent class then
   Car IS-A Vehicle
   Bus IS-A Vehicle
5. Can only extend one class
6. Constructors are NOT Inherited

Question 3

Inheritance: Method Overloading

Answer 3

-2 methods with the same name and parameters
- Child class can have a different implementation of the same method signature as the parent.
- Method in child class must be at least as accessible or more accessible as the method in parent class.
- Therefore, cannot override a public method in parent class with a method that is private. etc.
- Method return must be the same of a covariant.
Example: A method that returns a person can be overridden with a method that returns student since Student IS-A Person

Question 4

Inheritance: Child Class
Can a child class use Parents Variables or methods?

Answer 4

• Can have its own instance variables and methods
• Can use parents variables and methods using:
• super.instanceVariable
• super.instanceMethod

Question 5

Method hiding:

Answer 5

• There is a difference between hiding a static method and overriding.
  The version of the overridden instance method that gets invoked is the one in the subclassthe version of the hidden static method that gets invoked depends on where it is invoked from the superclass or subclass.

if the child class defines the method as static with same method signature as parent class, then the method in the child class hides the one in parent class.

Example:
Parent Class:
			public static void testClassMethod(){
			Sysout(" A - Parent")
			}
			public void testInstanceMethod(){
			Sysout(" B - Child")
			}
Child Class:
			public static void testClassMethod(){
			Sysout(" A - Child ")
			}
			public void testInstanceMethod(){
			Sysout(" B - Child ")
			}

Parent parent = new Parent();
Child myChild = parent;

Child.testClassMethod(); // Sysout(“ A - Parent”)
myChild.testInstanceMethod; //Sysout(“ B - Child “)

Question 6

Multi-Level Inheritance
Hierarchical Inheritance
Multiple Inheritance

Answer 6

• Child class can extend itself to another class
• One class can serve as parent for many child classes
• Java only allows you to extend one class

Question 7

How does inheritance work given the same/different packages when it comes to access modifiers?

Answer 7

In same package:

• public
• protected
• default
• private is never inherited no matter what

Different Package:

• public
• protected
• default

Question 8

Inheritance: Super Keyword

Answer 8

Super keyword is used to call the constructor

• it can call any of the constructor of the parent as long as the same parameters are given.
• has to be the first line of the code
• benefits: reusability logic, no new code is needed

Question 9

Final class

Answer 9

cannot be extended, cannot be inherited

Question 10

Final method

Answer 10

cannot be overriden in sub class

Question 11

Final Variable

Answer 11

Cannot be reassigned to a new value

Question 12

Access modifiers, what are they and explain each one.

Answer 12

Private > default > protected > public

private: is only visible/accessible in same class. Private method and variables are not inherited

default- only visible/accessible in same package only. Not visible or accessible to any other class in different package. default variables/methods are only when sub class and parent class are in the same package

protected - visible/accessible in same package or sub classes in different packages

public visible/accessible to very class, in same package or another. all variables and methods are inherited

Question 13

Constructor and Inheritance

Answer 13

When we have a parent-child relation between two classes and we make an object of the child class. The parent’s constructor will run first and create an object of the Parent then of the child

Question 14

Abstraction - OOP

Answer 14

2 Ways to use abstraction:

• Abstract class
• Interface
• Abstract classes can be extended to child classes

Question 15

What keyword has to be at the top of the class for it to be considered abstract?

Answer 15

abstract keyword must be added to class Name
ex: abstract class Animal {

}

Question 16

Can abstract classes be instantiated to a object ?

Answer 16

Cannot create objects of abstract classes

Question 17

In the abstract what can/cannot do for method

Answer 17

Only declare method but cannot implement any code
implement will be done in child class
- Sub classes are called concrete class

Question 18

can an abstract class contain abstract methods and non-abstract methods

Answer 18

Yes they can

Question 19

Major Benefit of Abstract class

Answer 19

To store and reuse variables and methods and do a unique implementation in each child class

Question 20

Abstraction - Interface type

Answer 20

Must have the interface keyword on the top
- public interface Teach{

}

Question 21

Can you extend multiple interfaces ?

Can you create an object of Interface class?

Answer 21

Yes you can, its one of the benefits of interface

No you cannot create object

Question 22

For an interface class, can you have a constructor?

Answer 22

No you cannot have a constructor for interface class

Question 23

Rules of an Interface class

Answer 23

1. Default methods can only exist in an interface
2. default keyword is mandatory to declare a default method
3. default methods cannot be static, final or abstract
4. methods are automatically public and cannot be changed

Question 24

Static keyword in interface

Answer 24

- static method was added since java 8
public interface Teachable {
			public abstract void canLean();
			public static void practice(){
			 //implementation code here
			 }
			}

Question 25

What are the only fields accepted in interface class?

Answer 25

Yes, an Interface class can only have constant variables
public static final int CONSTANT = 10;

Question 26

How do you inherit and interface class?

Answer 26

Must use the implement keyword. Can implement multiple interfaces

Question 27

In an interface class are methods are…

Answer 27

public abstract