the pmd plugin is set to jdk 1.4 by default. if you use 1.5 java code (or 1.6 etc) it will fail.
it can be configured like this:


           <plugin>
               <artifactId>maven-pmd-plugin</artifactId>
               <configuration>
                   <targetJdk>1.5</targetJdk>
               </configuration>
           </plugin>

http://maven.apache.org/plugins/maven-pmd-plugin/usage.html