javac - Will JRE 1.4 support classes compiled with Java 1.5 & 1.6?

Question

Will code compiled using 1.5 and 1.6 run on a 1.4 JRE? We weren't sure which Java versions the 1.4 JRE supports.

We know that if the code in question implements 1.5 or 1.6 supported features then it definitely won't compile... and that there are some risks with "backwards compiling" but wasn't sure if the 1.4 JRE would refuse to even load the 1.5/1.6 compiled classes or not.

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Update: I confirmed you get a java.lang.UnsupportedClassVersionError exception if you run an 1.6 class file on JRE 1.4.

Answer 1

You can cross-compile. This document shows you how:

http://java.sun.com/javase/6/docs/technotes/tools/solaris/javac.html#crosscomp-example

You must specify the specific major version you're targeting (1.4, it sounds like).

Using this technique, your best bet is to always use the newest javac you can find! That way you have all the latest bug fixes and performance improvements, and it's perfectly safe.

EDIT: note that this does address the problem of library incompatibilities, which was discussed in several answers!