Integer::toString in Optional.map

Question

When using map with method reference in Java, I met following problem:

public class Dummy {

  public static void main(String[] args) {
    IntegerHolder ih = new IntegerHolder();
    Optional<IntegerHolder> iho = Optional.of(ih);
    iho.map(IntegerHolder::getInteger).map(Objects::toString);
    iho.map(IntegerHolder::getInteger).map((Integer ii) ->ii.toString());
    iho.map(IntegerHolder::getInteger).map(Integer::toString);// this line will not compile. The error is "non-static method toString() cannot be referenced from a static context" 
  }

  private static class IntegerHolder {
    private Integer i;
    Integer getInteger() {return i;}
 }
}

It looks to me that Integer::toString is same as the IntegerHolder::getInteger. Both are "Reference to an Instance Method of an Arbitrary Object of a Particular Type" I do not understand why one works, but the other does not. Could you please shed some light on this question? Thank you very much.

Answer 1

The error is very misleading, in java-11 for example the error would make a lot more sense:

reference to toString is ambiguous

both method toString(int) in Integer and method toString() in Integer match)

If you re-write this method via a lambda expression, you will see that both signatures can potentially match:

iho.map(IntegerHolder::getInteger).map((Integer ii) -> Integer.toString(ii));
iho.map(IntegerHolder::getInteger).map((Integer ii) -> ii.toString());

both of these can be re-written as a method reference, but in such a case, which method to call?