I know this is the wrong section, but this is the best place I could think of.

In the leaked prerelease for Minecraft Raspberry Pi edition, the two jars for Java api are compiled with Java 7, and running on OpenJDK 6 results in:

$ java -cp McPiDemos.jar pi.demo.LoopDemo
Exception in thread "main" java.lang.UnsupportedClassVersionError: pi/demo/LoopDemo : Unsupported major.minor version 51.0
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:634)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
Could not find the main class: pi.demo.LoopDemo. Program will exit.

Trying to compile the source with Java 6 results in the following error:

$ javac -cp .:../src-api/ -d ../bindemo pi/demo/LoopDemo.java
../src-api/pi/EventFactory.java:16: illegal start of type
List<BlockHitEvent> events = new ArrayList<>();
^
../src-api/pi/Connection.java:84: cannot find symbol
symbol : method close(java.io.BufferedReader,java.io.BufferedWriter,java.net.Socket)
location: class pi.Connection
close(in, out, socket);
^
2 errors

Since some people are still running Java 6, it would make sense to build the Java API for Java 6.