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?

Answer 1

1. Yes, it can.

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

3. 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)
   }