Scala instance variables best practice

Question

How would one translate the following Java code to Scala?

class ClassA {
    private int field1;
    private int field2;

    public ClassA() {
        field1 = 1;
        field2 = 2;
    }
}

I can see two options:

class ClassA(val field1: Int, val field2: Int) {
   ....
}

Or

class ClassA {
   val field1: Int = 1
   val field2: Int = 2
}

What is recommended, and why?

Neither version is equivalent, since both fields are private in the Java version and public in the Scala alternatives you've shown. Perhaps you'd care to be more specific about what you want to achieve? — Daniel C. Sobral
– Daniel C. Sobral, Commented Jan 31, 2011 at 16:58

Landei · Accepted Answer · 2011-01-31 17:10:50Z

20

There is no simple translation from Java to Scala, it depends on the context:

Are the variables mutable in Java? If yes (else they should be final in Java): Would it make sense to make them immutable in Scala?
Should the constructor stay public in Scala? Would be a factory (e.g. an apply method in the companion object) more apropriate?
Why are are the variables private? Do they have getters and/or setters?
When are the variables needed, would it make sense to make them lazy?
If the values are immutable and exposed, would they be useful in pattern matching? Would a case class be the right choice?
Could the variables be grouped (e.g. in a tuple) in order to simplify API and access?

You see, there are so many considerations. I'd suggest to learn about the possibilities of Scala, and to play around with different approaches, else you get stuck in the "Scala as better Java" trap longer than needed.

edited Jan 31, 2011 at 17:10

answered Jan 31, 2011 at 12:30

Landei

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Comments

ziggystar · Accepted Answer · 2014-10-28 08:49:02Z

10

This is the most direct translation to Scala:

class ClassA{
  private var field1 = 1
  private var field2 = 2
}

Note the usage of var instead of val. val is an immutable field, corresponding to public final in Java. Thus it cannot be changed later and providing a way to initialize such a field to the correct value for a given instance is important.

In order to decide what you want to use you should ask yourself the questions that are listed in Landei's answer.

edited Oct 28, 2014 at 8:49

answered Jan 31, 2011 at 14:14

ziggystar

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2 Comments

Submonoid Over a year ago

Not quite the direct translation - Java private variables are class private, not instance private, so you should drop the [this].

ziggystar Over a year ago

@Submonoid Thank you. Seems like I've always done it wrong. :)

eivindw · Accepted Answer · 2011-01-31 11:49:36Z

7

The biggest difference between the two is that the class parameter one can be used as constructor. If you want the constructor to have no parameters, as your Java example, then you need to use the second one, plus adding the private modifier as suggested by @Debilski.

Another option would be to use default parameters in the constructor. That way the fields could be changed if needed:

class ClassA (private val field1: Int = 1, private val field2: Int = 2)

// Using defaults
val a = new ClassA

// Setting new values
val b = new ClassA(3, 4)

answered Jan 31, 2011 at 11:49

eivindw

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2 Comments

Raphael Over a year ago

This is a nice solution, but is not equivalent to the OP's Java code.

eivindw Over a year ago

@Raphael That's true. But as the Java example given seems pretty pointless in itself, I thought maybe showing a few options could be interesting :)

Debilski · Accepted Answer · 2011-01-31 11:26:20Z

5

If you want to have private fields, why not declare them private?

class ClassA {
  private val field1: Int = 1
  private val field2: Int = 2
}

answered Jan 31, 2011 at 11:26

Debilski

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Comments

Nicolas · Accepted Answer · 2011-01-31 11:56:55Z

1

If you always want that field1 and field2 have the same value for each instance of the class A, I would suggest to put them in a companion module:

object A {
  private val field1 = 1
  private val field2 = 2
}

And then use them in class A:

class A {
  def complexInternalComputation = 2*A.a
}

or this way:

class A {
  import A._
  def complexInternalComputation = 2*a
}

answered Jan 31, 2011 at 11:56

Nicolas

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3 Comments

Raphael Over a year ago

This does not work; A.a can not be accessed from an instance of class A since it it private.

Nicolas Over a year ago

From the scala specification, §5.2:• The private modiﬁer can be used with any deﬁnition or declaration in a template. Such members can be accessed only from within the directly enclosing template and its companion module or companion class (§5.4).

Raphael Over a year ago

Ok then, I only tried in REPL, sorry. (Would remove the downvote, but it won't let me...)

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