Java Gotchas

• If you build a Java JAR file with a recent version of the JDK (say 1.6) and then try to run the JAR file on another PC that has an older version of the JRE, you get a cryptic message like "Could not find the main class". All you have to do in this case is to build the java code on your development PC with a command like: java -target 1.5 mycode.java, and then make a new JAR file with the updated class files.
  
  Why the designers of the JRE couldn't make it pop up a message like "Java code built with newer version of Java" I don't know...