Question

I didn’t imagine that I would encounter radically new syntax in Java anymore at this stage, but lo and behold, I just encountered something:

The exact context and what the code below should do is pretty irrelevant – it’s there just to give some kind of context.

I’m trying to synthetically create an event in IT Mill Toolkit, so I wrote this kind of line:

buttonClick(new Button.ClickEvent(button)); 

But, Eclipse gives me the following error message:

No enclosing instance of type Button is accessible. Must qualify the allocation with an enclosing instance of type Button (e.g. x.new A() where x is an instance of Button).

When I rewrite the line above as follows, it doesn’t complain anymore:

buttonClick(button.new ClickEvent(button)); // button instanceof Button 

So, my question is: What does the latter syntax mean, exactly, and why doesn’t the first snippet work? What is Java complaining about, and what’s it doing in the second version?

Background info: Both Button and Button.ClickEvent are non-abstract public classes.

Answer 1

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.