A future reserved word does not yet have a special meaning, unlike a keyword, but is still reserved by the standard and thus is not a valid identifier. The stated reason for this is that the words "are used as keywords in proposed extensions and are therefore reserved to allow for the possibility of future adoption of those extensions."

abstract enum int short
boolean export interface static
byte extends long super
char final native synchronized
class float package throws
const goto private transient
debugger implements protected volatile
double import public