Lambda this reference in java

Question

I want to convert an anonymous class to a lambda expression. But this anonymous class uses the this keyword.

For example, I wrote this simple Observer/Observable pattern:

import java.util.ArrayList;
import java.util.Collection;

public static class Observable {
    private final Collection<Observer> notifiables = new ArrayList<>();

    public Observable() { }

    public void addObserver(Observer notifiable) { notifiables.add(notifiable); }
    public void removeObserver(Observer notifiable) { notifiables.add(notifiable); }

    public void change() {
        notifiables.forEach(notifiable -> notifiable.changed(this));
    }
}

public interface Observer {
    void changed(Observable notifier);
}

and this sample code with an anonymous class (use the this keyword):

public class Main {

    public static void main(String[] args) {
        Observable observable = new Observable();
        observable.addObserver(new Observer() {
            @Override
            public void changed(Observable notifier) {
                notifier.removeObserver(this);
            }
        });
        observable.change();
    }
}

but when I convert it to a lambda expression:

public class Main {

    public static void main(String[] args) {
        Observable observable = new Observable();
        observable.addObserver(notifier -> { notifier.removeObserver(this); });
        observable.change();
    }
}

I get this compilation error:

Cannot use this in a static context and in a non `static` context



public class Main {
    public void main(String[] args) {
        method();
    }

    private void method() {
        Observable observable = new Observable();
        observable.addObserver(notifier -> {
                notifier.removeObserver(this);
        });
        observable.change();
    }
}

The compilation error is:

The method removeObserver(Main.Observer) in the type Main.Observable is not applicable for the arguments (Main)

Is there a way to reference the lambda object with this?

Answer 1

You can't reference to this in a lambda expression. The semantic of this has been changed to reference the instance of the surrounding class only, from within the lambda. There is no way to reference to the lambda expression's this from inside the lambda.

The problem is that you use this in the main() method. The main method is static and there is no reference to an object that represents this.

When you use this inside an instance of an inner class you are referencing to the instance of the inner class. A lambda expression is not an inner class, this is not referencing to the instance of the lambda expression. It is referencing to the instance of the class you define the lambda expression in. In your case it would be a instance of Main. But since your are in a static method, there is no instance.

This is what your second compilation error is telling you. You hand over an instance of Main to your method. But your method signature requires an instance of Observer.

Update:

The Java Language Specification 15.27.2 says:

Unlike code appearing in anonymous class declarations, the meaning of names and the this and super keywords appearing in a lambda body, along with the accessibility of referenced declarations, are the same as in the surrounding context (except that lambda parameters introduce new names).

The transparency of this (both explicit and implicit) in the body of a lambda expression - that is, treating it the same as in the surrounding context - allows more flexibility for implementations, and prevents the meaning of unqualified names in the body from being dependent on overload resolution.

Practically speaking, it is unusual for a lambda expression to need to talk about itself (either to call itself recursively or to invoke its other methods), while it is more common to want to use names to refer to things in the enclosing class that would otherwise be shadowed (this, toString()). If it is necessary for a lambda expression to refer to itself (as if via this), a method reference or an anonymous inner class should be used instead.

Answer 2

Workaround 1

Your change() method throws ConcurrentModificationException anyway.

public class Main {
    public static void main(String[] args) {
        Observable observable = new Observable();
        final Observer[] a = new Observer[1];
        final Observer o = er -> er.removeObserver(a[0]); // !!
        a[0] = o;
        observable.addObserver(o);
        observable.change();
    }
}
public class Observable {
    private final java.util.Collection<Observer> n
        = java.util.new ArrayList<>();
    public void addObserver(Observer notifiable) {
        n.add(notifiable);
    }
    public void removeObserver(Observer notifiable) {
        n.add(notifiable);
    }
    public void change() {
        for (final Observer o : n.toArray(new Observer[n.size()])) {
            o.changed(this);
        }
    }
}
public interface Observer {
    void changed(Observable notifier);
}

Workaround 2

I changed the changed(Observable) to changed(Observable, Observer) so that an observer can handle itself.

public class Main {
    public static void main(String[] args) {
        Observable observable = new Observable();
        final Observer o = (er, ee) -> er.removeObserver(ee); // !!
        observable.addObserver(o);
        observable.change();
    }
}
public class Observable {
    private final java.util.Collection<Observer> n
        = new java.util.ArrayList<>();
    public void addObserver(Observer notifiable) {
        n.add(notifiable);
    }
    public void removeObserver(Observer notifiable) {
        n.add(notifiable);
    }
    public void change() {
        for (final Observer o : n.toArray(new Observer[n.size()])) {
            o.changed(this, o);
        }
    }
}
public interface Observer {
    void changed(Observable notifier, Observer notifiee);
}