← notes

scala

Objects in Scala

• The main way to create an object in Sclaa is to instantiate a class, like in java.
• There's also syntax to create objects right then and there
• Objects are by default lazy—Never used => never created

Object example in Scala:

(creating unique account nums starting from 2100000 at various places in the relative app) 
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object Acconuts:
    private var accountNumber = 2100000
    def newUniqueAccNumber()=
    accountNumber += 1 
    accountNumber

Using objects in Scala:

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Accounts.newUniqueAccNumber() // 2100001
Accounts.newUniqueAccNumber() // 2100002
Accounts.newUniqueAccNumber() // 2100003

Static Fields and Methods

• In java, static variables and method sare shared by all instances of the class of which they belong
• Scala does this same function using companion objcts

Static example in java:

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public class Person{
    public static int numberOfPeople;
    
    public static int getNumOfPeople(){
        return Person.numberOfPeople;
    }
    
    public person(){
        numberOfPeople++;
    }
}

You see, this is pretty shit. Scala does this more elegantly

Static example in Scala using "companion objects"

• companion objects are objects that have the same name as another class, and are defined in the same file

Companion object example:

Person class: 
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class person(val name: String):
    person.incrementNumPeople // constructor just calls incrememnt method on companion object

companion object for person class (is a replacement for static in java) 
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object Person:
    var numofPeople = 0
    private def incrmementNumPeople = numOfPeople =+ 1
    def conut = numOfPeople

usage: 
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var p1 = Person("Jake")
var p2 = Person("Yosra")
println(s"count: #{Person.count}") //outputs 2

Apply Method in Scala

• vv important

Basic Example:

We can create objects like this: 
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val foo = new Foo

Treating foo like a function... 
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foo()

- This should cause an error in Java, but not in Scala!!
- Scala will look for an apply method to activate
- if there's no apply, you'll get a syntax error

foo obj: 
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def foo:
    def apply() println(s"foo being foo")

Another example:

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object Greet:
    def apply(val name: String) = 
        println(s"gm {name}")

proxying the companion object

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class City(val name: String) //class

object City:                 //companion object
   def apply(name: String)
        new City(name)

New "city" object will appear if you change the following: 
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    City("dublin")