Java anonymous inner class calling enclosing type super types method

Question

I give you an example to set some context here, so I have two interfaces each inheriting the same parent interface and defining their own implementation of the parent interface's abstract method.

interface A
{
    Set a();
}

interface B extends A
{
    @Override
    default Set a()
    {
        return null;
    }
}

interface C extends A
{
    @Override
    default Set a()
    {
        return null;
    }
}

In an interface called D, The implementation creates an anonymous inner class which then needs to call the super types (B and C) a() implementation.

interface D extends B, C
{
    @Override
    default Set a()
    {
        return new HashSet()
        {
            {
                final int totalSize = D.this.B.super.a().size() + D.this.C.super.a().size();
            }
        };
    }
}

The problem is that the expressions D.this.B.super.a() and D.this.C.super.a() do not get compiled successfully, so what is up ?

Thank you by the way for your efforts.

"do not get compiled successfully" What is the error? – Andy Turner Oct 11 '18 at 19:32 — Andy Turner, Oct 11 '18 at 19:32

score 6 · Accepted Answer · answered Oct 11 '18 at 19:38

6

Java allows you to access specific interface implementations with the syntax:

InterfaceName.super.method()

However, inside the anonymous class to get the enclosing instance with D.this. It is not allowed to combine these syntaxes to first get the enclosing instance, then its super-interface implementations.

The solution here is to move the declaration of totalSize outside of the anonymous class so you can access the super-interface implementations. The anonymous class will still be able to refer to the local variable totalSize because it's final. (It would still be able to access it if it were effectively final -- not final but never reassigned.)

final int totalSize = B.super.a().size() + C.super.a().size();
return new HashSet()
{
     // Anonymous class implementation here.
     // You can refer to totalSize.
};

You may have omitted the generics for brevity, but in case you didn't, you will want to include type parameters for Set and HashSet.

answered Oct 11 '18 at 19:38

rgettman

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make sure that neither of `B.super.a()` and `C.super.a()` returns `null` – Andrew Tobilko Oct 11 '18 at 19:41
2

and don't use raw types – Andrew Tobilko Oct 11 '18 at 19:42
@AndrewTobilko chill dude, i used null as a return expression and raw types just for setting context purposes. – Selenophile Oct 11 '18 at 19:46
1

Nevertheless the points about checking for `null` and not using raw types are useful for other people who will view this post. – rgettman Oct 11 '18 at 19:49
@rgettman thank you for the response and feel free to edit my question so that it is helpful and easily readable to other viewers :) – Selenophile Oct 11 '18 at 19:51

Java anonymous inner class calling enclosing type super types method

1 Answers1