Strange syntax for instantiating an inner class

Inner classes (like Button.ClickEvent) need a reference to an instance of the outer class (Button).

That syntax creates a new instance of Button.ClickEvent with its outer class reference set to the value of button.

Here’s an example – ignore the lack of encapsulation etc, it’s just for the purposes of demonstration:

class Outer
{
    String name;

    class Inner
    {
        void sayHi()
        {
            System.out.println("Outer name = " + name);
        }
    }
}

public class Test
{
    public static void main(String[] args)
    {
        Outer outer = new Outer();
        outer.name = "Fred";

        Outer.Inner inner = outer.new Inner();
        inner.sayHi();
    }
}

See section 8.1.3 of the spec for more about inner classes and enclosing instances.