Chapter 2 - Advanced Principles

Question 1

Difference of Object vs. value

Answer 1

value: abstract value, no lifecycle, no side effects, immutable (unveränderbar) state (integer, float, character), Object: describes a phenomenon in a problem domain, lifecycle (created, mutation, deletion), can e shared

Question 2

define “type”

Answer 2

abstract description of properties of a set of objects

Question 3

define “class”

Answer 3

implementation of a type

Question 4

What are the pre- and postconditions of the Liskov substitution principle?

Answer 4

preconditions must not be widened by a subtype, postconditions must not be narrowed (verschmälert) by a subtype

Question 5

explain subclass

Answer 5

T’ is a subclass of T if T’ is derived form T (Implementation inheritance)

Question 6

explain subtype

Answer 6

T’ is a subtype of T if T-objects can be replaced by T’-objects (subtype is more strict than subclass)

Question 7

what is covariance and contravariance?

Answer 7

Covariance: type of a method parameter is narrowed (eingeschränkt) in a subclass, Contravariance: type of a method parameter is widened (ausgeweitet) in a subclass. most oo-languages only support contravariance

Question 8

What types are there for Inheritance?

Answer 8

Specialization: derived concrete subclass from concrete base class / Specification: concrete subclass implements abstract base class / Restriction: subclass restricts the base class interface / Combination: multiple inheritance / Construction: Implementation inheritance

Question 9

How can you restrict the Inheritance Interface?

Answer 9

in order to prevent method overriding, you can use the keyword final

Question 10

Explain Inheritance vs. Composition

Answer 10

inheritance: is-a relationship, composition is has-a relationship

Question 11

What is the problem with Inheritance?

Answer 11

it violates encapsulation

Question 12

how does a Java interface work?

Answer 12

it declares only abstract methods

Question 13

how does a Java abstract class work?

Answer 13

it defines common and structural functional properties

Question 14

Explain Factorization

Answer 14

Sachverhalt nur an einer einzigen Stelle festhalten, eliminates duplicate code

Question 15

what is information hiding?

Answer 15

minimal coupling between classes, open-closed principle: class should be open for extensions but closed for modifications

Question 16

What are Generic classes?

Answer 16

formal type parameters like “public class DoubleObject”

Question 17

What is PECS?

Answer 17

producer extends, consumer super

Question 18

What is Meta-Information?

Answer 18

information on properties of objects, classes and the inheritance hierarchy –> Java Reflection provides meta-information at runtime

Question 19

Explain Aspect-oriented Programming (AOP)

Answer 19

dividing a problem into aspects rather than objects/classes

Question 20

Explain the joint-point model (in AOP)

Answer 20

Joint point: point in a running program where behaviour can be added, advice: defines the behavior, point cut: quantify join points for executing advice

Question 21

What is Polymorphism and which types are there

Answer 21

multiple implementations of a method, types: parametric: generic classes or methods, inheritance: serveral implementations along the inheritance hierarchy, overloading: namespace extension, coercion: implicit type conversion