mode 4

Question 1

What is a String?

Answer 1

• -An object that contains an array of characters
• -A String is immutable which means it can’t be changed
• A String is sometimes called a “psuedo primitive”

Question 2

Equivalency of String in Java: are we checking the memory address or the content?

Answer 2

We’re checking the memory address:
System.out.println(“comparison one: “ + (myStringTwo == “Donuts from Krispy Kreme”)); //prints false

Here we’re checking the CONTENTS of the objects
System.out.println(“comparison two: “ + myStringTwo.equals(“Donuts from Krispy Kreme”)); //prints true

Question 3

What is Stringbuilder and StringBuffer and what are their differences?

Answer 3

STRING BUILDER
StringBuilder is essentially a mutable counterpart to the String class

STRING BUFFER
StringBuffer is essentially a mutable counterpart to the String class

The difference is that StringBuffer is thread safe

Question 4

What makes a string immutable?

Answer 4

A String is immutable because of the combination of THREE facts:

	 * -All String modifying methods INTENTIONALLY NEVER alter the existing String, they instead create
	 * a new String and return a reference to that new String...leaving the original String unmodified
	 * -The class is final, so that NO ONE can EVER..create a child class and override the String's
	 * intended functionality
	 * -The char[]/byte[] inside String is final so you can NEVER point to a different array, to get around
	 * the immutability

Question 5

Why is a string immutable?

Answer 5

The answer is for efficiency

Question 6

What is Abstraction?

Answer 6

Abstraction - revealing WHAT something does, not HOW it does that something

Question 7

Similarities between Abstract Classes (AC) and Concrete Classes

Answer 7

Abstract classes and Concrete class have more similarities than they have differences.
Some of the differences include:
-Abstract classes cannot be instantiated
-AC can have abstract methods
-an abstract method MUST be overridden eventually

 Beyond these differences, the inheritance works exactly the same as a concrete class

BOTH OF THESE MAY HAVE STATIC METHODS

Question 8

Differences between Abstract Classes (AC) and Interfaces

Answer 8

Differences between Abstract Classes (AC) and Interfaces:

            interfaces                                              AC
       -------------------------                        --------------------------------
abstract method (until default)              concrete & abstract methods

implements keyword extends keyword

methods implicitly:
abstract & public no implicit modifiers

variables implicitly:
public, static, final

NO constructor YES constructor

Question 9

Why would we use Abstract Classes over Interface ?

Answer 9

PRO ac: can provide concrete implementations (including methods and instance variables)
(java 8 added default….so the interface can now have concrete methods)
PRO ac: constructor
PRO i: can implements multiple interfaces without taking up the ONE spot you have to extend a class

Question 10

Using Abstract classes w/ methods:

Answer 10

Abstract classes w/ methods:

• -You CAN have abstract methods inside of an abstract class
• -You CAN have concrete methods inside of an abstract class
• -You CAN have an abstract class with ZERO abstract methods
• -You can NOT have abstract methods inside of a concrete class

Question 11

Using Abstract classes w/ static methods and variables

Answer 11

Abstract classes w/ static methods:

• -You CAN have static methods in an abstract class
• -You can NOT have abstract static methods

Abstract classes w/ variables:

• -You CAN have variables in an abstract class
• -You can NOT have abstract variables

Question 12

Using Abstract classes w/ constructors and initializer blocks

Answer 12

Abstract classes w/ constructors:

• -You CAN have a constructor in an abstract class
• -You can NOT have an abstract constructor
• Abstract classes w/ initializer blocks:
• -You CAN have an initializer block in an abstract class
• -You can NOT have an abstract initializer block

Question 13

What is an interface?

Answer 13

An interface is a CONTRACT between itself and any implementing classes

Just like a real world contract, the implementing class needs to fulfill the contract’s requirements

Question 14

How can we use Interface on a class?

Answer 14

To utilize an interface on a class…you must use the keyword “implements”

Note, interfaces EXTENDS other interfaces (not implements)

Question 15

Can Interface implements multiple interfaces?

Answer 15

Java does NOT support multiple inheritance so you can’t extends multiple classes,
However, interfaces are NOT parent classes, they are contracts. So yes, you can implements multiple interfaces

public interface Edible extends Freezable , Heatable {

Question 16

What are the implicit values of variables and methods in interface

Answer 16

Variables in an interface are IMPLICITLY public & static & final   
/* public static final */ String myString= "hello interface world";

Methods in an interface are IMPLICITLY public & abstract
/* public abstract */ void breakDownIntoNutrients();

Question 17

Can Interfaces have Constructor, INITIALIZER BLOCKS, or Static INITIALIZER BLOCKS

Answer 17

NO!

Question 18

Can we have default and concrete static methods in an interface?

Answer 18

default void myDefaultMethodInAnInterface() {
System.out.println(“yes, we CAN have an implementation of a method in an interface”);
System.out.println(“using a default method”);
}

public static void myStaticMethodInAnInterface() {
	System.out.println("yes, we CAN have an implementation of a method in an interface");
	System.out.println("using a static method");

Question 19

Can we use the abstract method from an abstract class? If yes, how?

Answer 19

Yes. In order to access abstract classes, the child class MUST override any abstract methods before the object can be concrete.

If the child class is itself an abstract class then your child DOESN’T need to override this method….instead your concrete GRANDCHILD must override the method

(While your abstract child isn’t FORCED to override your abstract methods, it CAN to save your grandchild the headache of overriding)

Question 20

Can I have a private abstract method?

Answer 20

No, private means ONLY THIS CLASS can access
//	private abstract void myPrivateAbstractMethod();

Question 21

Can I have a final abstract method?

Answer 21

No, final means it can't be overridden, abstract means it needs to be overridden
//	public final abstract void finalAbstractMethod();

Question 22

Can I have a final abstract class or final interface?

Answer 22

NO!

Question 23

Can I extends multiple classes?

Answer 23

Java does NOT support multiple inheritance so you can’t extends multiple classes,

Question 24

What is a Marker Interface?

Answer 24

An interface with no abstract methods. E.g: serializable

Question 25

What is a Functional Interface?

Answer 25

An interface with one abstract method. E.g comparator