Invoking toString via method reference in Java 8

问题:

What am I missing? Why do I have to use Object::toString below and not Integer::toString? Does it have anything to do with type erasure with generics?

Arrays.asList(1,2,3).stream().map(Integer::toString).forEach(System.out::println); //Won't compile

Arrays.asList(1,2,3).stream().map(Object::toString).forEach(System.out::println); //Compiles and runs fine

回答1:

This has nothing to do with type erasure.

Look at the error message :

(argument mismatch; invalid method reference
  reference to toString is ambiguous
    both method toString(int) in Integer and method toString() in Integer match)

The Integer class has two toString methods that match the functional interface expected by the map() method. One is static with an int argument, and the other is the toString() method that overrides Object's toString().

The compiler doesn't know if you want to execute this :

Arrays.asList(1,2,3).stream().map(i->Integer.toString(i)).forEach(System.out::println);

or this :

Arrays.asList(1,2,3).stream().map(i->i.toString()).forEach(System.out::println);