Should java give an error message about assigning a private variable from inner classes?

Question

I have some code like this:

class Outter {
    private Bar bar;

    [...]

    public update() {
        provider.doUpdate(new IListenner() {
            @Override
            public void onSuccess(Bar bar) {
                Outter.this.bar = bar;
                // Here, Outter.this.bar.equals(bar) is false.
            }
        }
    }

    [...]

}

I think this has to do with the way Java capture variables in my closure, capturing a bar reference instead of capturing the Outer instance reference. If I write:

Outter self = Outter.this;
self.bar = bar;

it works, forcing Java compiler to store a reference to my Outter instance instead of a reference to the bar field in its closure.

The right way to do this is probably to use a setBar(Bar bar) in Outter and call it from the inner class.

But as it don't work, why the Java compiler don't give an error message about it ? Like bar should be final when capturing the non-final reference ?

I compile this using Android Studio 1.2.1.1, not tried in other contexts, don't know how Android Studio actually compile Java. My JRE seems to be Java 1.7.0 from the openjdk-7-jre-headless Debian package version 7u79-2.5.5-1~deb8u1.

Answer 1

The behavior you're reporting sounds very strange. First, I can't reproduce it with neither ECJ nor with javac 7. (See http://ideone.com/B4CoQs)

The difference in bytecode between

Outter.this.bar = bar;

and

Outter self = Outter.this;
self.bar = bar;

is just an extra astore_2/aload_2, which basically corresponds to a no-op.

I encourage you to check your assumptions. Perhaps you have a buggy implementation of Bar.equals that causes the confusion.

Like bar should be final when capturing the non-final reference?

Fields aren't captured in that sense. Fields can be accessed from inner classes / methods without being final.