Java using generics with lists and interfaces

Question

Ok, so here is my problem:

I have a list containing interfaces - List a - and a list of interfaces that extend that interface: Li

Answer 1

Just to explain why Java does not permit this:

• A List is a list in which you can put any object implementing Record, and every object you get out will implement Record.
• A List is a list in which you can put any object implementing SubRecord, and every object you get out will implement SubRecord.

If it would be allowed to simply use a List as a List, then the following would be allowed:

List subrecords = new ArrayList();
List records = subrecords;
records.add(new Record()); // no problem here

SubRecord sr = subrecords.get(0); // bang!

You see, this would not be typesafe. A List (or any opject of a parametrized class, in fact) can not typesafely change its parameter type.

In your case, I see these solutions:

• Use List from start. (You can add SubRecord objects to this without problems.)
  • as a variation of this, you can use List for the method which adds stuff. List is a subtype of this.

• copy the list:

  List records = new ArrayList(subrecords);
  

---

To exand a bit on th variation:

void addSubrecords(List subrecords) {
    ...
}


List records = new ArrayList();
addSubrecords(records);
recordkeeper.records = records;