Lambda's and references to their container class

[Aleksey Shipilev]

Hi John,

You might want to read on lambda translation paper [1].

On 08/05/2012 12:44 PM, John Hendrikx wrote:
> Are they like anonymous inner classes created in a static context when they are 
> not accessing the parent or do they always keep a reference to the parent?

That depends on the translation strategy, chosen in runtime. In current
scheme, when runtime spins up an inner class when resolving indy,
non-capturing lambdas are not capturing the parent.

I should point out that the translation strategy might change even after
GA, if we figure out more clever (and probably VM-specific) way to
capture arguments.

> externalObject.addListener(new InvalidationListener() {
>    public void invalidated() {
>       System.out.println("invalidated");
>    }
> });
> 
> vs.
> 
> externalObject.addListener(() -> System.out.println("invalidated"));
> 
> Do both hold a reference to their containing instance?

In current scheme, first one does capture parent, second one is not.
Moreover, currently there is only one instance of non-capturing lambda
per call site. Non-capturing lambdas in this sense are more like static
inner class memoized in static final field.

> externalObject.addListener(() -> { someFieldOfParent = true; });
> 
> I'm assuming in this case it is forced to capture 'Parent.this' and keep 
> a reference around to the parent.

That's right, at this point we are forced to capture parent in one way
or the other.

-Aleksey.

[1] http://cr.openjdk.java.net/~briangoetz/lambda/lambda-translation.html