How to implement this Java mechanism in Scala?

Question

In Java, the abstract class's static member will be inherited all through its subclasses. like

abstract class TopThing{
  public TopThing next;
  public TopThing(TopThing tt){
        this.next = tt
  }

  private static HashTable<String, TopThing> someTable = new HashTable<String,TopThing>();
  protected void add(String name) {
      someTable.put(name, this);
  public static Parent forName(String name) {
      return someTable.get(name)    ;
  }
}

class SomeSpecific extends TopThing {
    public final String name;
    public SomeSpecific (String name, TopThing tt) {
       super(tt);
       this.name = name;
       this.add(name);
}

This is I am first time writing in Scala, the only way I know to achieve the above is using companion object, but it seems does not work for this case

Can companion object store the private static table? (it seems no...)
If I can declare the table in the companion object, can it be referenced from companion class? ( it seems no...)
In the add method, how can the subclass's instance referred and be inserted into the table? (the question is also about this in add method)
What's a good practice of this in Scala?

Alexey Romanov · Accepted Answer · 2012-11-03 08:25:08Z

6

Yes, it can.
Yes, you reference it as TopThing.someTable (or just someTable after import TopThing._).

Same as in Java: this. So

abstract class TopThing(val next: TopThing) {
  protected def add(name: String) {
    TopThing.someTable += (name -> this)
  }
}

object TopThing {
  private val someTable = collection.mutable.Map[String,TopThing]()

  def forName(name: String) = someTable.get(name)
}

class SomeSpecific(val name: String, next: TopThing) extends TopThing(next) {
  this.add(name)
}

answered Nov 3, 2012 at 8:25

Alexey Romanov

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Comments

Tim Green · Accepted Answer · 2012-11-03 08:51:09Z

0

For question 1: you can use private[namespace]

For question 2: no, but you can use import

class Xxx {
  import Xxx._
  ...
}

For question 3: I don't known how to answer it.

For question 4:

trait TopThing {
  val someTable: HashTable[String, TopThing] 
}
class Xxx extend TopThing {
  val someTable = Xxx.someTable  
}

edited Nov 3, 2012 at 8:51

answered Nov 3, 2012 at 7:05

Tim Green

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2 Comments

som-snytt Over a year ago

"I don't known how to answer it." I'd love to have this on a quality t-shirt. Preferably with a lolcat. Also, I would +1 for answering bullet #4, except that someTable should be theTable, I think, as in the other response.

DaoWen Over a year ago

Putting the table in a trait will create one per instance which is not at all the same thing as the Java code's static table.

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