title: NoClassDefFoundError on an interface, not a class
body: <p>I have an issue with <code>NoClassDefFoundError</code>. I am using interfaces, and no class definition should be available:</p>
  
  <pre><code>package code;
  public interface Constants {...}
  </code></pre>
  
  <p>The class implementing this interface compiles without any errors, and a JAR file has been built, but at runtime it gives me an error.</p>
  
  <pre><code>import ...;
  import code.*;
  public class MultiDoc extends LanguageAnalyser implements Constants{}
  </code></pre>
  
  <p><code>Constants</code> contains only a list of constants.</p>
  
  <p>I read some posts pointing to CLASSPATH as a cause of this problem, but I have the <code>code</code> package in my CLASSPATH. If I didn't have it, it would produce a compilation error. So, the problem should be something else.</p>
  
  <p>The runtime error is:</p>
  
  <pre><code>java.lang.NoClassDefFoundError: code/Constants
  </code></pre>
  
  <p>What's the solution?</p>