Re: Reference to a class that does not exist

Steven Simpson <ss@domain.invalid>
Sat, 09 Jun 2007 12:38:55 +0100
Brendan Guild wrote:

I am wondering when it is safe to mention a class that I know will be
impossible to load. Specifically, I am writing a Java library and
there is another library that has some very useful features, but I
don't expect that other library to always be available.

[snip uncertainty about JLS]

I want to do this in a way that is guaranteed to be correct by Java,
not just a way that happens to work at the whim of the JVM that I am

You're concerned that the link error might be allowed to occur so early
as to prevent an application, which is using your library, from running,

Assuming it is deemed to be a problem...

Define an interface for doing some operation provided by the optional

interface EdgeDetector {
  Image detectEdges(Image source, int threshold);

Create an implementation that actually uses the optional library:

class SomeLibEdgeDetector implements EdgeDetector { ... }

In your library, when you need to do this operation (or to determine
whether it can be done), identify the class by String:

EdgeDetector ed = null;
try {
  Class edc = Class.forName("SomeLibEdgeDetector");
  ed = edc.newInstance();
} catch ( various exceptions... ) {
  // Ignore.

If the optional library isn't available, ed should be null. I'm
assuming that at least one of the calls in the try block will throw
something straight-away, but even if not, you could still catch the
error when you make the call.

ss at comp dot lancs dot ac dot uk |

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