Arma3Sync arbitrarily restricts usage to Java 7, and does not permit JRE 8 to run it, even though it is backwards compatible. Java 8 has officially been released to general availability, so this should definitely be fixed. It's a trivial fix, no doubt, and slightly more proper detection of the Java version can be done like this:
if(Double.parseDouble(System.getProperty("java.specification.version")) >= 1.7) {
// do stuff
}
Note that this will still break for Java 10 (whenever that happens), which is why I say it's only slightly more proper.