I found the following 4 places where we use JAVA 6 API calls, while it should be java 5 compliant.
org.zkoss.zul.GroupsModelArray<D, H, F, E> line 311
Array.newInstance(classD, gd.length, 0) does not exist in java 5
org.zkoss.zul.impl.NumberInputElement line 213 (already handled via exception)
org.zkoss.lang.Objects line 438 uses generics... while it is not generic in java 5
org.zkoss.xel.fn.CommonFns line 525 uses String.isEmpty