Lecture 2. Object Orientation

Transcription

1 Lecture 2 Object Orientation 1

2 Homework 0 Grades Homework 0 grades were returned earlier this week Any questions? 2

3 Homework 1 Homework 1 is due tonight at 11:59pm You will be graded on: Correctness: 15 points (passing all RSpec tests) Style: 5 points (having no Rubocop style offenses) Best Practices: 5 points (manually graded by TA) For this assignment, you will receive feedback but not actually lose any points Feedback by next Tuesday 3

4 Office Hours Weekday Time Location TA Monday 4-5pm Harrison Mezz Sanjana Sarkar 6-7pm Moore 100 Desmond Howard Tuesday 6-7pm Moore 100 Desmond Howard Wednesday 11am-12pm Moore 100 Sanjana Sarkar 4:30-5:30pm Moore 100 Jackie Askins Thursday 6-7pm Moore 100 Jackie Askins **These are pretty much final, but the locations may changes 4

5 Classes & Objects Classes define clusters of behavior or functionality *Almost* everything in Ruby is an object Every object is an instance of exactly one class You can call methods on almost anything! Because of this, there are no functions in Ruby There is always an object from which to call methods Even arithmetic operations are methods, Ruby adds syntactic sugar to an expression like 1.+(2) to allow us to call it like

6 Creating a Class Use the class and keywords and place class s code between them It is convention to use PascalCase when naming classes Instantiate a class with the new keyword class Person person = Person.new p person.class #=> Person 6

7 Instance Methods Methods that are inted to be called on a specific instance of the class Methods defined in a class are instance methods by default class Person def say_hello p 'Hello' person = Person.new person.say_hello #=> Hello 7

8 Instance Variables Instance variables allow you to remember state in individual objects Names always start with a They are only visible to the instance to which they belong Every instance has its own set of instance variables If you define an instance variable in one instance method in a class, you can access it in any other instance method in that class 8

9 Instance Variables Note that it is bad style in Ruby to prefix methods with set or get, we will fix this soon class Person def = name def person = Person.new person.set_name('jackie') p person.get_name #=> "Jackie" 9

10 Constructors If a method is named initialize, it will be executed when a class is being initialized class Person def initialize p 'Hi, I am being initialized!' Person.new #=> "Hi, I am being initialized!" 10

11 Constructors with Arguments Arguments passed into the new method are also passed into the constructor class Person def = name p "You've initialized a person with name: #{name}" Person.new('Jackie') #=> "You've initialized a person with name: Jackie" 11

12 Reader Methods What we call getters in other languages are referred to as reader methods in Ruby Used to return the value of a variable in a class Share name with variable being returned 12

13 Reader Methods class Person def = name def person = Person.new('Jackie') person.name #=> "Jackie" 13

14 Writer Methods What we call setters in other languages are referred to as writer methods in Ruby Used to set the value of a variable in a class Share name with variable being set followed by an equal sign Ruby uses syntactic sugar to allow us to call the method like it s an assignment 14

15 Writer Methods class Person def = name def def = name person = Person.new('Jackie') person.name = 'Sanjana' p person.name #=> "Sanjana" 15

16 Writer Methods Note that writer methods can do more than just assignment class Person def def name=(name) name *= = name.length > 10? 'Too long' : name person = Person.new person.name = 'Jackie' p person.name #=> "Too long" 16

17 Attr Readers It turns out that the use of reader methods is pretty common We can use the attr_reader method to generate a reader method class Person attr_reader :name def = name person = Person.new('Jackie') p person.name #=> "Jackie" 17

18 Attr Writers We can do the same with writers Using attr_writer will generate a writer method class Person attr_reader :name attr_writer :name person = Person.new person.name = 'Jackie' p person.name #=> "Jackie" 18

19 Attr Accessors It turns out that we can generate both a reader and writer method all in one line by using an attr_accessor class Person attr_accessor :name person = Person.new person.name = 'Jackie' p person.name #=> "Jackie" 19

20 Generating Multiple Methods Multiple symbols can be passed to all three of the attr methods This will generate a reader/writer (or both) for all of the symbols that you provided 20

21 Generating Multiple Methods class Person attr_reader :name, :age, :favorite_color def initialize(name, age, = = = favorite_color person = Person.new('Jackie', 21, 'blue') p person.name #=> "Jackie" p person.age #=> 21 p person.favorite_color #=> "blue" 21

22 Class Methods A method inted to be called on the class itself (as opposed to an instance) is called a class method Defined like an instance method, but starts with self. self indicates the class It cannot call any instance methods Similar to a static method in Java 22

23 Class Methods class Person def self.average_life_expectancy '71.5 years' p Person.average_life_expectancy #=> "71.5 years" 23

24 self This is similar to the this keyword in Java There s always exactly one current object or self The current object is determined by the scope 24

25 self At the top level: self refers to the default object, main Inside of a class definition: self refers to the class object itself Inside of a class method: self refers to the class object Inside of an instance method: self refers to the instance that the method is being called on 25

26 Implicit Receiver All methods must have a receiver When calling student.name, student is the receiver for name What does that mean for instance methods called in another instance method? It has an implicit receiver, and in a class, it is self This is important because: An instance method can be called with an implicit receiver It means there are no functions, only methods, in Ruby 26

27 Method Notation A quick note on method notation ClassName#instance_method Refers to the instance method instance_method in the class ClassName ClassName::class_method Refers to the class method class_method in the class ClassName 27

28 Private Methods By default, all methods are public But you don't want to be able to access every method defined in a class Methods can be made private with the private keyword In the body of a class, all methods physically under the private method are private 28

29 Private Methods class Person def say_hello "Hi, I'm #{name}" private def name 'Jackie' person = Person.new p person.say_hello #=> "Hi, I'm Jackie" p person.name #=> NoMethodError: private method `name' called for #<Person: 0x007fc525011ef8> 29

30 Class Variables Denoted with at the beginning Similar to static variables in Java They are preserved across all instances of the class All instances can access and modify them 30

31 Class Variables class Person = 0 def += 1 def p Person.count #=> 0 Person.new p Person.count #=> 1 They seem harmless, but they re actually pretty bad practice 31

32 Inheritance Classes can inherit from only one other class Classes can be thought of as hierarchical Inheritance is done with the < operator A class will gain all of its parent class s methods, both public and private class Person class Programmer < Person p Programmer.superclass #=> Person 32

33 A Problem with Class Variables It turns out that class variables are more accurately described as class hierarchy variables Not only does the class have access to the variables, its descants will too This is obviously bad As a general rule, don t use class variables unless you mean for this to happen 33

34 A Problem with Class Variables class Bird = 'EVERYWHERE' def self.location p Bird.location #=> "EVERYWHERE" class Penguin < Bird = 'Antarctica' def self.location p Penguin.location #=> "Antarctica" p Bird.location #=> "Antarctica 34

35 How do we work around this? Like I said earlier, almost everything in Ruby is an object All classes are instances of the Class class This means that classes are themselves objects class Person p Person.class #=> Class 35

36 Use Class Instance Variables Since classes are themselves objects, they can have their own instance variables We will refer to a class s instance variables as class instance variables Define class instance variables within the class definition body or within class methods with a Always use class instance variables, NEVER use class variables 36

37 Use Class Instance Variables class = 'EVERYWHERE' def class Penguin < = 'Antarctica' p Bird.location #=> "EVERYWHERE" p Penguin.location #=> "Antarctica 37

38 Classes Never Close Existing classes, including core classes, can be modified at any time just be declaring them my_string = 'hi' my_string.repeat(3) #=> NoMethodError class String def repeat(x) self * x p my_string.repeat(3) #=> "hihihi" This is dangerous and you should never do it unless you really know what you re doing 38

39 Modules I mentioned earlier that classes can inherit from only one class This seems really limiting at first, but that s where modules come in Modules are bundles of methods Declared with the module keyword Unlike classes, modules cannot be instantiated 39

40 Modules module MyModule def my_module_method 'Hello World' MyModule.new #=> NoMethodError 40

41 Mixing in Modules Modules get mixed into classes using include or ext Referred to as mix-ins When you mix a module into a class, that class gets access to all of the methods defined in that module 41

42 include vs. ext If you mix-in a module with: include: The module s methods will be available as instance methods ext: The module s methods will be available as class methods 42

43 Mixing in Modules module ClassMethodsModule def class_method 'Class Method' module InstanceMethodsModule def instance_method 'Instance Method' class MyClass ext ClassMethodsModule include InstanceMethodsModule p MyClass.class_method #=> "Class Method" p MyClass.new.instance_method #=> "Instance Method" 43

44 Separate Module Methods It should be clear if a method is meant to be an instance method or a class method It is extremely unlikely that a method is both Instance and class methods should be separated in the module with more, nested modules module MyModule module InstanceMethods def instance_method puts 'Instance Method' module ClassMethods def class_method puts 'Class Method' 44

45 Using Nested Modules The :: operator can be used to access a module in another module Thus, if a class wanted to access the module in the previous slide class MyClass include MyModule::InstanceMethods ext MyModule::ClassMethods MyClass.new.instance_method #=> "Instance Method" MyClass.class_method #=> "Class Method" 45

46 Requiring Files It's good practice for every class and module to have its own file To "import" other files, pass the path to the file without the extension as a string to the require method If the file foo.rb is in the same directory, put require './foo at the top of the file If it's in the parent directory, require '../foo Recall that requiring a file without the path imports a gem Note: "requiring" a file loads and runs that file 46

47 Homework 2 Homework 2 is released, it is designed to help you become more comfortable with Object Orientation You will be developing a workflow that you will see when we get to Rails 47

48 Homework 2 Passing a hash as an argument to a method def = = = params[:baz] 'Assigned foo, bar, and baz from params hash' p method_name(foo: 'foo', bar: 'bar', baz: 'baz') #=> "Assigned foo, bar, and baz from params hash" 48