Where is final parameter stored in anonymous class instance?

Question

I have the following static factory method that creates a list view out of an int array:

public static List<Integer> newInstance(final int[] numbers) {
    return new AbstractList<Integer>() {

        @Override
        public Integer get(int index) {
            return numbers[index];
        }

        @Override
        public int size() {
            return numbers.length;
        }
    };
}


public static void main(String[] args) {
    int[] sequence = {10, 20, 30};
    List<Integer> list = ListFactory.newInstance(sequence);
    System.out.println("List is "+list);

}

In "Effective Java", Joshua Bloch mentioned this

as an Adapter that allows an int array to be viewed as a list of Integer instances.

However, I remember that Adapter uses composition and the instance of the anonymous list implementation should use the int[] as a member field.

Where exactly is the int[] input parameter stored if it's not a member field of the anonymous list implementation?

I would appreciate if anyone could provide some insights or some links to look for more information.

Answer 1

You can use javac -d . -XD-printflat ListFactory.java to see how the compiler understands the inner class. Actually, there are two Java classes in your example. The ListFactory (note how numbers are passed to the constructor of ListFactory$1):

public class ListFactory {

    public ListFactory() {
        super();
    }

    public static List newInstance(final int[] numbers) {
        return new ListFactory$1(numbers);
    }
}

and the representation of the anonymous implementation of AbstractList:

class ListFactory$1 extends AbstractList {
    /*synthetic*/ final int[] val$numbers;

    ListFactory$1(/*synthetic*/ final int[] val$numbers) {
        this.val$numbers = val$numbers;
        super();
    }

    @Override()
    public Integer get(int index) {
        return Integer.valueOf(val$numbers[index]);
    }

    @Override()
    public int size() {
        return val$numbers.length;
    }

    @Override()
    /*synthetic*/ public Object get(/*synthetic*/ int index) {
        return this.get(index);
    }
}

The methods and fields marked as synthetic are generated by the compiler and not accessible to you as a programmer, but are used during runtime to access the int array. And indeed there is a val$numbers field that holds a final reference to the int array.

By the way you can also notice boxing from int to Integer in Integer get(int index) and that to comply with the raw (non-generic) List interface an extra Object get(int index) method is generated that delegates to the type-safe Integer get(int index) implementation.