Why should one extend the RefType class?

[CaledoniaProject]

The following class is declared to filter certain class,

class Context extends RefType {
  Context() {
    this.hasQualifiedName("javax.naming", "Context")
    or
    this.hasQualifiedName("javax.naming", "InitialContext")
  }
}

So why does it extend the RefType class instead of any other base class?

[smowton]

It's just a type that provides hasQualifiedName. It would be equally valid to use ClassOrInterface, a subtype of RefType that includes other reference-types (e.g. arrays), or Class or Interface depending on which one Context and InitialContext happen to be, but RefType suffices so the author hasn't bothered to restrict it further.

It's worth noting that if the types Context and InitialContext happen to match the requirements of a subtype, then Context will be compatible with that subtype even if it doesn't extend it. For example, Java defines

class NestedType extends RefType {
  NestedType() { enclInReftype(this, _) }
  ...
}

Where enclInRefType is a database relation (something populated by the extractor) which means that this occurs syntactically inside some other RefType, e.g. an inner class. Supposing for a moment that Context really was a Java nested class, even though Context directly extends RefType, we will be able to do things like from Context c where c.(NestedType).getEnclosingType().hasName("MyOuterClass") .... That is, we were able to cast Context, which extends RefType, to a NestedType even though it does not extend NestedType. The cast "worked" (that is, we got any results) because it matched NestedType's characteristic predicate enclInReftype(this, _).