Method reference - invalid method reference - cannot be reference from static context

Question

I have following piece of code

StringJoiner joiner = new StringJoiner(", ");
joiner.add("Something");
Function<StringJoiner,Integer> lengthFunc =  StringJoiner::length;
Function<CharSequence,StringJoiner> addFunc = StringJoiner::add;

Last line cause an error

Error:(54, 53) java: invalid method reference
  non-static method add(java.lang.CharSequence) cannot be referenced from a static context

I understand that this method can't be used in static way and I should have something like :

Function<CharSequence,StringJoiner> addFunc = joiner::add;

instead. However I can't understand why third line, with StringJoiner::length; is for java compiler perfectly correct. Can someboedy explain me why is that ?

Answer 1

Function<StringJoiner,Integer> lengthFunc =  StringJoiner::length;

lengthFunc is a Function that takes a StringJoiner and returns an Integer. Therefore any instance method of StringJoiner that takes nothing and returns an Integer matches this interface. The instance of which the method is called will be the StringJoiner required by the Function<StringJoiner,Integer>.

On the other hand in

Function<CharSequence,StringJoiner> addFunc = StringJoiner::add

addFunc is a Function that takes a CharSequence and returns StringJoiner. No instance method of StringJoiner matches this interface, since the function doesn't have an input StringJoiner instance to apply the add method on.

You would need a BiFunction to match the signature of StringJoiner::add :

BiFunction<StringJoiner,CharSequence,StringJoiner> addFunc = StringJoiner::add;