Calling Stream.map with a method reference returning an int

[Eirik Bjørsnøs]

I'm trying to understand why I can't use method references the way my
intuition wants it to work :-)

Given the following class:

public class Person {

    private int age;
    private String name;

    public Person(int age, String name) {
        this.age = age;
        this.name = name;
    }

    public int getAge() { return age;}

    public String getName() {return name;}

}

I can print a sorted list of names using this (elegant!) piece of Java 8
code: (given List<Person> list):


list.stream().map(Person::getName).sorted(String::compareTo).forEach(System.out::println);

Now, say I want to calculate the sum of ages. I can map the age using a
lambda expressions and then call IntStream.sum(), like this:

    long sum = list.stream().map(p->p.getAge()).sum();

However, when I try to replace the lambda expression "p -> p.getAge" with
the method reference "Person::getAge", I get a compiler error:

    long sum = list.stream().map(Person::getAge).sum();

java: cannot find symbol
  symbol:   method sum()
  location: interface java.util.stream.Stream<java.lang.Long>

In this case, calling Stream.map with Person::getAge is resolved to
Stream.map(Function<? super T, ? extends R> mapper) which returns a
Stream<Long>. While what I wanted was Stream.map(IntFunction<? super T>
mapper) which returns an IntStream.

So why is Person::getAge not considered an IntFunction? Could it? Should
it? Am I missing something?

I'm using build 1.8.0-ea-b74.

Eirik.