Scala 101– Basic OOP : Writing a class

4 minute read

This is part 3 of my Scala tutorial – read the first part and the second part for a more general Scala intro. All the examples you see here were ran via the REPL ( that’s the Scala interpreter).

Scala’s take on OOP

Scala tends toward pure object oriented model:

### There is no such thing as “primitive”. In Java , You have Objects and Primitives. Scala , on the other hand , takes after other language like Python, Ruby, Smalltalk (and many others) in the sense that Everything is an object. Including the integer 1 and string “Hello, World”. For many , this seems a reasonable evolution from the Object/Primitive dichotomy used in Java. ### Functions and Closures are also Objects. This is something people with no background in functional programming sometimes find difficult to accept in the beginning. In Scala , a function is just another type of object , and as such it do anything an object can – Change , get sent as a parameter , be the return value of another function , and so forth. Even though the idea seems strange at first , you might recall that even in C/C++ you can pass a reference to a function , or use it as a return value from a function.

### Operators are methods – Like in C++

Let’s get technical:

Let’s build a simple class that will represent a fish. At first , a fish only has a name:

scala> class Fish(var name: String) {}
defined class Fish

Well , something looks a bit… off… , isn’t it? There’s no constructor , no fields , no nothing! And yet , it works:

scala> var jaws = new Fish("Jaws")
jaws: Fish = Fish@530f243b

scala> jaws.name
res2: String = Jaws

scala> jaws.name = "Rex"

scala> jaws.name
res3: String = Rex

So what happened here , exactly? What we’ve used here is the”Primary Constructor”. The variable we’ve passed is a property of the class , and you really don’t need any more setters and getters.

What if I want an immutable field? Just use “val” instead of “var” :

scala> class Fish(val name: String) {}
defined class Fish

scala> var jaws = new Fish("Jaws")
jaws: Fish = Fish@2876b359

scala> jaws.name
res4: String = Jaws

scala> jaws.name = "Rex"
<console>:7: error: reassignment to val
jaws.name = "Rex"
^

And private fields?

scala> class Fish(private val name: String) {}
defined class Fish

scala> var jaws = new Fish("Jaws")
jaws: Fish = Fish@15664f1a

scala> jaws.name
<console>:8: error: value name cannot be accessed in Fish
jaws.name
^

scala> jaws.name = "Rex"
<console>:7: error: value name cannot be accessed in Fish
jaws.name = "Rex"
^

What about fields that aren’t in the constructor?

scala> class Fish(val name: String) {
val kind : String = "Shark"
}
defined class Fish

scala> val f = new Fish("Jaws")
f: Fish = Fish@69066caf

scala> f.kind
res12: String = Shark

Well, that’s great , but I want more then one constructor!

scala> class Fish(val name: String) {
def this() = this("SomeName")
}
defined class Fish

scala> var jaws = new Fish()
jaws: Fish = Fish@1dd0eb0b

scala> jaws.name
res6: String = SomeName

And something to tease you – How do you create a private primary constructor?

scala> class Fish private (val name: String) {}
defined class Fish

scala> var jaws = new Fish("Jaws")
<console>:6: error: constructor Animal cannot be accessed in object $iw
var jaws = new Fish("Jaws")
^

Scala has a primary constructor and zero or more auxiliary constructors. The primary constructor is the entire body of the class.  So actually , every line written in the body of the class will be executed (not including those inside functions / methods , naturally). Note: In Scala , any auxiliary constructor must call another constructor of the same class as it’s first actions!

Let’s say we want a fish to print its name when it goes up:

 

scala> class Fish(var name: String) {
println(I am  + name)
}
defined class Fish

scala> var jaws = new Fish("Jaws")
I am Jaws

The code line “println (“I am “ + name)” , although it appears context-less , is actually part of the constructor.

Let’s sum everything up:

class Fish(var name: String, private var age : Int) {
   println (A new fish is born!);

   //This is accessible
   val kind : String = "Shark"

   //This is not accessible
   private val nickName = "Goldi"

   def this() = {
      //An auxiliary con'r MUST invoke another constructor as it's first action!
      this("Fishi", 0)
      println ("I'm an auxiliary constructor!")
   }

   def this(name: String) = this(name, 0);

   def swim() = println("Blo Blo")

   private def showUpperFin() = println("Dramatic music!")

   //If you override a function , you must declare it using the override keyword. Unless the
   //function is abstract , and then it's kind of obvious to the compiler
   override def toString = "My name is " + name

   //An operator is a method just like any other
   def + (that: Fish): Fish = return new Fish(this.name,this.age + that.age)
}

So what have we learned so far?

  • Scala has a Primary constructor (which is the entire body of the class) and auxiliary constructors - which serve just like Java constructors.
  • The default Scala scope is public
  • You can declare class parameters in the primary constructor
  • You can access and change non private class properties by accessing them directly

The next post will touch the getter/setters issue in Scala , limiting the scope of variables , using default values , and more . Stay tuned 😉