returns a function that always evaluates to the #constant_key . this is useful as a key extractor in group-by operations where no classification by key is desired . concode_field_sep String CONSTANT_KEY concode_field_sep DistributedFunction<T,T> wholeItem concode_elem_sep DistributedConsumer<T> noopConsumer concode_elem_sep DistributedPredicate<T> alwaysFalse concode_elem_sep DistributedFunction<Entry<K,V>,K> entryKey concode_elem_sep DistributedFunction<Entry<K,V>,V> entryValue concode_elem_sep DistributedPredicate<T> alwaysTrue
DistributedFunction < T , String > function ( ) { return t -> CONSTANT_KEY ; }