On 1/5/2017 1:16 PM, Rony G. Flatscher wrote:
The rule for using the Java bridge for the Rexx programmers has always been
very simple: you are
only allowed to use public Java classes, public Java fields and public Java
methods as these are
guaranteed to be available to everyone at all times.
This is what happens in this case too: the public Java class java.awt.Toolkit
gets loaded, its
public class method getDefaultToolkit() gets invoked and a java.awt.Toolkit
object gets returned by
that method.
The returned Toolkit object is known from the documentation to allow access to
all documented public
methods, so its class (or one of its superclasses) must have the public
abstract method
getScreenDimension() implemented (being a subclass of java.awt.Toolkit it must
implement all
abstract members otherwise no instance could be created from it), therefore
getting that Method
object and invoking it.
There has been no reason since the beginning of Java to know from a
programmer's point of view how
the implementation of the class took place for the returned object (e.g. which
package serves as its
home) as long as it was a subclass of that public Java class and as long as we
have adhered to its
its public members only.
The j.l.r.Method object on which you call invoke() should not be
obtained by inspecting the methods of the implementation class given by
getDefaultToolkit().getClass(). Implementation classes (i.e. classes in
non-exported packages) cannot be instantiated, nor their members
manipulated, by code outside their module.
The j.l.r.Method object on which you call invoke() should be obtained by
inspecting the methods of the "public Java class" java.awt.Toolkit. The
first argument that you pass to invoke(), indicating the receiver, can
still be instanceof the implementation class.
(Also, I would expect to be able to access any additional public members of
such a subclass, irrespectible of its implementation, just from knowing that in
Java one can always
access public classes and public members.)
Nope, because access to members is gated by access to the enclosing
class. If the enclosing class is inaccessible then its members are
inaccessible.
Alex
