Chapter 2 - Object Orientation

Question 1

Encapsulation

Answer 1

helps hide implementation behind an interface or API.

Question 2

Encapsulation code has 2 features

Answer 2

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

Question 3

IS-A

Answer 3

Refers to inheritance or implementation

Question 4

IS-A

Answer 4

is expressed with the keyword extends

Question 5

HAS-A

Answer 5

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

Question 6

Polymorphism means

Answer 6

many forms

Question 7

A reference variable

Answer 7

is always of a single, unchangeable type, but it can refer to a subtype object.

Question 8

A single object can be referred to by reference variables of many different types

Answer 8

as long as they are the same type or supertype of the object.

Question 9

The reference variables type (not the object type)

Answer 9

determines which methods can be called

Question 10

Polymorphic method invocations

Answer 10

apply only to overridden instance methods

Question 11

Methods can be overridden or overloaded

Answer 11

constructors can be overloaded but not overridden

Question 12

Abstract methods

Answer 12

must be overridden by the first concrete (non-abstract) subclass

Question 13

An overriding method

Answer 13

1. must have same arg list
2. must have same return type, the return type can be a subclass - known as a covariant return
3. must not have a more restrictive access modifier
4. my have a less restrictive access modifier
5. Must not throw new or broader checked exceptions
6. May throw fewer or narrower exceptions or any unchecked exception

Question 14

final methods

Answer 14

cannot be overridden

Question 15

Only inherited methods

Answer 15

may be overridden… private methods are NOT inherited

Question 16

overloading means

Answer 16

reusing a method name, but with different arguments

Question 17

Overloaded methods

Answer 17

1. must have different argument lists
2. May have different return types, if arg lists are also different
3. May have different access modifiers
4. May throw different exceptions

Question 18

Methods from a superclass

Answer 18

can be overloaded in a subclass

Question 19

Polymorphism applies to overriding

Answer 19

not to overloading

Question 20

Object type (not the reference variables type)

Answer 20

determines which overriding method is used at runtime

Question 21

Reference type determines

Answer 21

which overloaded method will be used at compile time

Question 22

Two types of reference variable casting

Answer 22

downcasting and upcasting

Question 23

Downcasting

Answer 23

Reference variables of subtypes must be explicitly cast, resulting in access to the subtypes members with this new reference variable.

Question 24

Upcasting

Answer 24

can be done explicitly or implicitly.

Question 25

When you are implementing an interface

Answer 25

you are fulfilling its contract

Question 26

You implement an interface properly when...

Answer 26

you concretely override all of the methods defined by the interface

Question 27

A single class can implement

Answer 27

many interfaces

Question 28

Overloaded methods can change return types

Answer 28

overridden methods cannot, except in the case of covariant returns

Question 29

An array is a legal return type

Answer 29

both to declare and return as a value

Question 30

Methods with primitive return types

Answer 30

any value that can be implicitly converted to the return type can be returned

Question 31

Nothing can be return from a void

Answer 31

but you can return nothing by simply saying return

Question 32

Methods with an object reference return type

Answer 32

can return a subtype

Question 33

Methods with an interface return type

Answer 33

can return any implementer

Question 34

A constructor is always invoked

Answer 34

when a new object is created

Question 35

Each superclass in an objects inheritance tree

Answer 35

will have a constructor called

Question 36

Every class,even abstract class

Answer 36

has at least one constructor

Question 37

Constructors must have the same name

Answer 37

as the class

Question 38

Constructors don't have a return type

Answer 38

If you see a return type, its a method with the same name as the class, its NOT a constructor

Question 39

Constructors can use

Answer 39

any access modifier (even private!)

Question 40

The compiler will create a default constructor

Answer 40

if you don't create one in your class

Question 41

The default constructor

Answer 41

is a no-arg constructor, if you don't create any constructors in your class

Question 42

The first statement in a constructor MUST

Answer 42

call either this()... an overloaded constructor or super()

Question 43

Instance members are accessible only

Answer 43

after the superclass constructor runs

Question 44

Abstract classes have constructors

Answer 44

that are called when a concrete subclass is instantiated

Question 45

Interfaces do not have

Answer 45

constructors

Question 46

If your superclass does not have a no arg constructor

Answer 46

you MUST create a constructor and insert a call to super() with args matching those of the superclass

Question 47

Constructors are never inherited

Answer 47

thus they cannot be overloaded

Question 48

A constructor can be directly invoked only by another constructor

Answer 48

using a call to super() or this()

Question 49

this

Answer 49

1. may appear only as the first statement in a constructor 2. the arg list determines which overloaded constructor is called

Question 50

constructors can call constructors, can call constructors ...

Answer 50

sooner or later a call to super() or the stack will explode

Question 51

calls to this() and super()

Answer 51

cannot be in the same constructor

Question 52

use static methods

Answer 52

to implement behaviors that are not affected by the state of any instances

Question 53

use static variables to hold data

Answer 53

that is class specific, there is only one copy of static variables

Question 54

All static variables belong to the class

Answer 54

not to an instance

Question 55

A static method cant access

Answer 55

instance variables directly

Question 56

Use the dot operator

Answer 56

to access static members

Question 57

static methods

Answer 57

can't be overridden, but they can be redefined

Question 58

coupling

Answer 58

refers to the degree to which one class knows about or uses members of another class

Question 59

lose coupling

Answer 59

is the desirable state of having classes that are well encapsulated

Question 60

tight coupling

Answer 60

is the undesirable state

Question 61

cohesion

Answer 61

refers to the degree in which a class has a single, well defined role or responsibility

Question 62

Hi cohesion is desirable

Answer 62

members support a single, well-focused responsibility

Question 63

low cohesion is undesirable

Answer 63

whose members support multiple, unfocused roles or responsibilities