Even though this question has an accepted answer, according to http://www.oracle.com/technetwork/java/javase/compatibility-417013.html#binary
The class file version for Java SE 7 is 51, as per the JVM Specification, because of the invokedynamic byte code introduced by JSR 292. Version 51 class files produced by the Java SE 7 compiler cannot be used in Java SE 6.