Update: CSCF appears to have speedily resolved this problem. Plain old javac should now work. You can ignore the rest of this post.

For whatever reason, while the default JVM on the edlab is Java 7 compatible:

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$ java -version
java version "1.7.0_65"
OpenJDK Runtime Environment (IcedTea 2.5.1) (7u65-2.5.1-4ubuntu1~0.12.04.2)
OpenJDK Server VM (build 24.65-b04, mixed mode)

the default Java compiler is only Java 6:

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$ javac -version
javac 1.6.0_32

I’ve asked CSCF to update the javac default, but in the meantime, you can use, for example,

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/usr/lib/jvm/java-7-openjdk-i386/bin/javac Validator.java

to compile your code.