If by "syntax compatibility" he means code that compiles in one version does not compile is another, that's definitely true. Name collisions with the new default methods in Java 8 are one source. If your Comparator-implementing class has a method "void reverseOrder()", that will no longer compile.

Here's a couple of recent examples the Guava authors have fixed. Fortunately they were in internals and not their public API, so users won't care.

https://code.google.com/p/guava-libraries/source/detail?r=7d... https://code.google.com/p/guava-libraries/source/detail?r=25...