Setting Java generics of iterators

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?

In general you should [favor composition over inheritance](http://stackoverflow.com/questions/49002/prefer-composition-over-inheritance) ([Effective Java, Item 16](http://books.google.com/books?id=ka2VUBqHiWkC&pg=PA81&lpg=PA81&dq=favor+composition+over+inheritance+effective+java&source=bl&ots=yYEfOms-O4&sig=AzKrW2db1sgCyjTP_u1F2B8P_cY&hl=en&ei=rQ64TubcAuH74QTF_qjkAw&sa=X&oi=book_result&ct=result&resnum=5&ved=0CDkQ6AEwBA#v=onepage&q&f=false)). Why are you extending `ArrayList` in the first place? — Matt Ball, Nov 07 '11 at 17:00
You mean that in that for loop s is of type Object instead of String? — Tudor, Nov 07 '11 at 17:02

score 4 · Accepted Answer · answered Nov 07 '11 at 17:05

4

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?

answered Nov 07 '11 at 17:05

Lucas

14,227
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Thanks, that's what I was trying to do. What would you recommend using instead, though? – Aleksandar Savkov Nov 23 '11 at 13:38
@Aleksandar, I guess I just dont understand what the _abstract_ base class (`A` in this example) is used for. Can you expand on this? – Lucas Nov 28 '11 at 16:16

Anish Dasappan · Answer 2 · 2011-11-07T17:13:37.427

1

Use composition instead of Inheritance

    public class A implements Iterable<String>{
        List<String> myList = new ArrayList<String>();
        //do operations on myList
        public Iterator<String> iterator() {
             return myList.iterator();
        }
    }

edited Nov 07 '11 at 17:13

answered Nov 07 '11 at 17:04

Anish Dasappan

415
2
9

score 0 · Answer 3 · answered Nov 07 '11 at 17:06

If you extend generic class you should care about generics. I mean that your declaration should look like

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

if you want to use strings or

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

if you want your class to be generic.

In both cases you do not have to override iterator() method: you can invoke its from super class and it will return you "good" iterator. Your declaration is equivalent to

public abstract class A extends ArrayList<Object> {
   ...
}

This is the reason for "strange" behavior.

BTW may I ask you why are you extending ArrayList? It really sounds strange.

score 0 · Answer 4 · answered Nov 08 '11 at 05:40

OMG this is terrible:

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

String is not the class String, rather, you are declaring a new type variable called String (like T) that shadows the class String

Setting Java generics of iterators

4 Answers4