anonymous classes

[cyril vidal]

Dear all,
I can't figure out something about anonymous classes.
As far as I have understood by now, anonymous classes represent a specified kind of local (to method) inner classes.
Hence, here is what we can read in Complete Java 2 Certification Study Guide by Simon Roberts (page 205):

Some classes that you define inside a method do not need a name . A class defined in this way without a name is called an anonymous class
...
This means that anonymous inner classes are unique to method scopes; you cannot have anonymity with a member class

This is OK.
But in dan chisholm's mock exam devoted to anonymous classes, we can find the following example that compiles and runs wells (prints 5):

 
abstract class A {
  private int x = 4;
  private int y = 2;
  public A(int i1) {x=i1;}
  public int x() {return x;}
  public void x(int x) {this.x = x;}
  public int y() {return y;}
  public void y(int y) {this.y = y;}
  public void incY() {y++;}
  public abstract int math();
}
class B {
  [B]static A a1 = new A(2) {
    {incY();}
    public int math() {return x()+y();}
  };[/B]
  public static void main(String[] args) {
    System.out.print(a1.math());
  }
}
 

Can't we say here that the following code:
static A a1 = new A(2) {
{incY();}
public int math() {return x()+y();}
};
defines an anonymous class as a member class of class B? And that the restrictions that are supposed to be applied (actually not qualify them as static) to anonymous inner class do not work any more? That's why a1 is declared as static...
I think I'm wrong about something, but I can't grasp about what exactly...
I would be very grateful for any assistance,
Thanks in advance,
Cyril.

[Alton Hernandez]

Hi Cyril,
It is the variable a1(a reference type) which is the member of class B. It just so happen that it is initialized to point to an object created thru anonymous class.

Originally posted by cyril vidal:

As far as I have understood by now, anonymous classes represent a specified kind of local (to method) inner classes.

Anonymous class doesn't have to be defined inside a method.
[ July 05, 2003: Message edited by: Alton Hernandez ]

[cyril vidal]

Hi Alton,
thanks for your response.
So, the following line extracted from the book isn't correct, or, at least, ambiguous?

Some classes that you define inside a method do not need a name . A class defined in this way without a name is called an anonymous class...
This means that anonymous inner classes are unique to method scopes

And what about the assertion telling that anonymous classes can't be static?
I've understood that in the example, it was a reference to an object (so an instance and not the class by itself) but in the case of anonymous classes, can we have something else than an instance, either of this following form:
new Xxxx() {/* class body */}
or this one:
Xxxx anXxxx = new Xxxx() {/* class body */};
For me, telling of an anonymous class or telling of an instance of anonymous class is the same (this instance may be referenced or not).
Am I wrong on this point?

[Alton Hernandez]

So, the following line extracted from the book isn't correct, or, at least, ambiguous?

I don't have a copy of that book but I believe that the author is just talking about where you would normally use an anonymous class.

..can we have something else than an instance, either of this following form:
new Xxxx() {/* class body */}
or this one:
Xxxx anXxxx = new Xxxx() {/* class body */};
For me, telling of an anonymous class or telling of an instance of anonymous class is the same (this instance may be referenced or not).

When you 'define' an anonymous class, you start with the new statement. This tells you what makes anonymous class different from the others. It is instantiated at the location where it is defined. But the definition and instantation of the class are still 2 different thing.

And what about the assertion telling that anonymous classes can't be static?

As per JLS 8.1.2, an inner class is a nested class that is NOT explicitly or implicitly declared static, and an anonymous class is considered as an inner class.
[ July 05, 2003: Message edited by: Alton Hernandez ]

[Jose Botella]

I have tried to compile such a class with a static member but the compiler complains:
"inner classes cannot have static declarations"