java.lang.UnsupportedClassVersionError: Bad version number in .class file?

I also got the same error. Reason for that I was compiling the project using Maven. I had JAVA_HOME pointing to JDK7 and hence java 1.7 was being used for compilation and when running the project I was using JDK1.5. Changing the below entry in .classpath file or change in the eclipse as in the screenshot resolved the issue.

classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/J2SE-1.5

or change in the run configuarions of eclipse as

Have you tried doing a full "clean" and then rebuild in Eclipse (Project->Clean...)?

Are you able to compile and run with "javac" and "java" straight from the command line? Does that work properly?

If you right click on your project, go to "Properties" and then go to "Java Build Path", are there any suspicious entries under any of the tabs? This is essentially your CLASSPATH.

In the Eclipse preferences, you may also want to double check the "Installed JREs" section in the "Java" section and make sure it matches what you think it should.

You definitely have either a stale .class file laying around somewhere or you're getting a compile-time/run-time mismatch in the versions of Java you're using.

Did you compile with Eclipse? It uses a different compiler (not javac). That should not result in this error (if everything is configured properly), but you can try to compile it with javac instead.

If that fixed the problem, try to see if Eclipse has some incorrect compiler settings. Specifically have it target Java 5.

I've learned that error messages like this are usually right. When it couldn't POSSIBLY (in your mind) be what the error being reported says, you go hunting for a problem in another area...only to find out hours later that the original error message was indeed right.

Since you're using Eclipse, I think Thilo has it right The most likely reason you are getting this message is because one of your projects is compiling 1.6 classes. It doesn't matter if you only have a 1.5 JRE on the system, because Eclipse has its own compiler (not javac), and only needs a 1.5 JRE to compile 1.6 classes. It may be weird, and a setting needs to be unchecked to allow this, but I just managed to do it.

For the project in question, check the Project Properties (usually Alt+Enter), Java Compiler section. Here's an image of a project configured to compile 1.6, but with only a 1.5 JRE.