Question

I’ve noticed something funny Java does (or at least Netbeans) when I use classes implementing ArrayList and changing the generics type of the elements. I basically created an abstract class that extends ArrayList and some subclasses that are supposed to work with String objects (so something like ArrayList<String>). One of the things I did to try to achieve that was this:

public abstract class A extends ArrayList {
   ...
}
@Override
public abstract class B extends A {
   public Iterator<String> iterator() {
      return super.iterator();
   }
}

Another one was this:

public abstract class A extends ArrayList {
   ...
}
public abstract class B<String> extends A {
   @Override
   public Iterator<String> iterator() {
      return super.iterator();
   }
}

The first one overrides successfully the iterator() method assigning a String value to it. The other one somehow cancels out the type casting. The funny thing is that none of them works when it comes to for loops. This receives type Object instead of String.

for (String s : B) {
   ...
}

Do you have any idea why this happens and how can I fix it without implementing my own iterator?

Answer 1

Not sure what you are trying to do but if I understand correctly you want a class that extends ArrayList and has a Generic type of String… Perhaps you are looking for this:

public abstract class A<T> extends ArrayList<T> {
    ...
}
public abstract class B extends A<String> {
    ...
}

Then in your code, this:

B myList = ...;
for ( String s : myList ) {
    ...
} 

Will work just fine. Though I think you could come up with a much better solution. Do you have more specifics about your problem?