question: public static Map < String , List < String > > toMultimap ( Headers headers , String valueForNullKey ) { Map < String , List < String > > result = new TreeMap < > ( FIELD_NAME_COMPARATOR ) ; for ( int i = 0 , size = headers . size ( ) ; i < size ; i ++ ) { String fieldName = headers . name ( i ) ; String value = headers . value ( i ) ; List < String > allValues = new ArrayList < > ( ) ; List < String > otherValues = result . get ( fieldName ) ; if ( otherValues != null ) { allValues . addAll ( otherValues ) ; } allValues . add ( value ) ; result . put ( fieldName , Collections . unmodifiableList ( allValues ) ) ; } if ( valueForNullKey != null ) { result . put ( null , Collections . unmodifiableList ( Collections . singletonList ( valueForNullKey ) ) ) ; } return Collections . unmodifiableMap ( result ) ; }
target:  Returns an immutable map containing each field to its list of values.