Re: How to access a member constant by FQN

From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 05 Sep 2007 15:47:15 -0700

Message-ID:

<fbnbll$1g98$1@ihnp4.ucsd.edu>

Sideswipe wrote:

Hmm, ok, let me try this:

Class.forName("com.whatever.mycompany.MyConstants");
// MyConstants is now loaded and the static members initialized

MY_CONST is now an Integer instance floating around in the JVM. So, I
don't NEED a new instance of it, but I want to access it by a fully
qualified name.

When I do: Class.newInstance() <-- object is returned but if I am
given the string: "com.whatever.mycompany.MyConstants.MY_CONST" I am
not being given a FQN to a class, I am being given a FQN to a static
final member of a Class.

Another way to say it, how do I make an assignment to this instance
based on the FQN as a String. This is all runtime/reflection -- not
compile time so I have no way to know what will be referenced. If I
knew the Outter class I could use reflection to get the fields and
discover it by name, but I can't know that the String given to me
represents a class and not one of it's members.

If I understand the problem, you have a String
"com.whatever.mycompany.MyConstants.MY_CONST" and want to access the
corresponding field.

You first need to split it into the fully qualified class name and the
field name. Use Class.forName to get a reference to the Class object.
Use the Class's getField method to get a reference to the Field object
for "MY_CONST". Take a look at the API documentation for Field to see
how to do specific operations on the Field, such as getting a reference
to the Integer.

Almost, but not quite, always this sort of reflection tangle is a
mistake, and there is a simpler, cleaner solution. It may be worth
describing the problem you are trying to solve with all this.

Patricia