title: Equivalent of #define in Java?
body: <p>I'm writing a library that needs to have some code if a particular library is included. Since this code is scattered all around the project, it would be nice if users didn't have to comment/uncomment everything themselves. </p>
  
  <p>In C, this would be easy enough with a <code>#define</code> in a header, and then code blocks surrounded with <code>#ifdefs</code>. Of course, Java doesn't have the C preprocessor... </p>
  
  <p>To clarify - several external libraries will be distributed with mine. I do not want to have to include them all to minimize my executable size. If a developer does include a library, I need to be able to use it, and if not, then it can just be ignored.</p>
  
  <p>What is the best way to do this in Java?</p>