Can Java 10 compiled classes run on 9? [duplicate]

Question

If I understand correctly the binary form of Java 10 is 100% identical to Java 9 so it shouldn't make any problems.

Is my assumption correct?

Answer 1

This has nothing to do with Java 10; the rules are the same as they've always been.

In every version of Java, the classfile version number has been incremented. You can run older classfiles on a newer Java runtime (you can run files compiled twenty years ago under Java 1.0 on Java 10!), but it has never been true that you can run newer classfiles on an older runtime. This hasn't changed with Java 10.

However, you can compile classes such that they run on older versions, if you don't use newer features introduced since that version. This used to be accomplished by the -source and -target switches:

// Run this on Java 8
javac -source 6 -target 6 Foo.java

and you'll get classfiles that will run on Java 6 and later. (The source version cannot be greater than the target version.) This was recently replaced with the more comprehensive --release option:

javac --release 9

which implies not only -source 9 and -target 9, but the compiler will also prevent you from using JDK functionality introduced after 9.