how scala treat companion object?

Question

I'm new to Scala with Java background.

In java when we want to share any field among different objects of class. we declare that field static.

class Car {
 static NO_Of_TYRES = 4;
 // some implementation.
public int getCarNoOftyres(){
       NO_Of_TYRES; // although it's not a good practice to use static without class name
                    //but we can directly access static member in same class  . 
  }


}

But in Scala we cannot declare static fields in class, we need to use object(companion object) for that. In scala we will do like this,

class Car {
 println(NO_Of_TYRES); // scala doesn't let us do that. gives error 
 println(Car.NO_Of_TYRES);// this is correct way. 

}

object Car {
 val NO_Of_TYRES: Int = 4;
}

I'm just curious, how scala treat companion objects? what different these two key-words (class and object) makes ? why does scala not letting us access NO_Of_TYRES directly in class?

You can import Car._ either at the file level or inside the class and use NO_OF_TYRES — Daenyth
as per my knowledge, Companaion Object must be in same file? still i need to import i don't think so? my question is, how scala treats keyword class and object? — Muneeb Nasir
I think the best answer for you is programmers.stackexchange.com/questions/179390/… Please take a look — Ngoc Nguyen

Mariusz Sakowski Mariusz Sakowski · Accepted Answer · 2015-05-29T13:59:43

Companion objects are singleton class instances (and definitions), just to recall singleton in java is more or less:

class Foo {
    private Foo() { }
    /* boilerplate to prevent cloning */
    private static Foo instance = new Foo();
    public static Foo getInstance() { return instance; }

    public int bar() { return 5; }
}

and then to call method bar of this object:

Foo.getInstance().bar();

Scala removed all this boilerplate and lets you create equivalent thing with just

object Foo { 
    def bar: Int = 5 
}

and to call it you only need

Foo.bar

now what's the difference between 'object' and 'companion object'? It's actually quite simple - companion object (so the object defined in the same file as a class and having the same name) has access to it's related class private fields and methods, and that's probably why scala authors decided that it should reside in the same file - so that references to private fields are in the same file as their declarations (which I think is always the case both in Java and Scala, unless using reflection magic)

how scala treat companion object?

4 Answers