Re: Generics and ArrayList.toArray(T[] a)

From:

"Daniel Pitts" <googlegroupie@coloraura.com>

Newsgroups:

comp.lang.java.programmer

Date:

11 Jan 2007 12:54:20 -0800

Message-ID:

<1168548860.085712.94190@77g2000hsv.googlegroups.com>

Ian Wilson wrote:

I don't see how to elegantly satisfy toArray(T[] a) in an abstract class.

The code below works but I'd rather avoid the need for the setType()
which instantiates an T[] for .toArray(). Obviously I cant instantiate
an T[] in the abstract class.

Is there a better way?

----------------------------------------------------
import java.util.ArrayList;

public class ArrayListProblem {
     public static void main(String[] args) {
         FooModel model = new FooModel();
         String[] fooCodes = model.getCodes();
         for(int i = 0; i<fooCodes.length; i++) {
             System.out.println(fooCodes[i]);
         }
     }
}

abstract class AbstractCodeModel<T> {
     private ArrayList<T> codes = new ArrayList<T>();
     private T[] type;

     void addCode(T item) {
         codes.add(item);
     }

     void setType(T[] t) {
         this.type = t;
     }

     public T[] getCodes() {
         return codes.toArray(type);
     }
}

class FooModel extends AbstractCodeModel<String> {
     FooModel() {
         setType(new String[0]);
         addCode("A");
         addCode("B");
     }
}
--------------------------------------------------------
The above is greatly simplified, in reality the abstract class contains
lots of code that would otherwise be duplicated in what are now its
subclasses.

Try this instead:
abstract class AbstractCodeModel<T> {
      private List<T> codes = new ArrayList<T>();

      void addCode(T item) {
          codes.add(item);
      }

      public List<T> getCodes() {
          return new ArrayList<T>(codes);
      }
}

Why deal with an array at all? Primative obsession is a bad thing.
You could also use:
return java.util.Collections.unmodifiableList(codes);
If you wanted an unmodifiable view into codes.

If the clients really want an array, they can get it from
getCodes().toArray(...);