Java 8 Lambdas: Mapping a Stream to type Integer and then calling sum() won't compile
collections, java, java-8, lambda
Solution
I think they are changing the method
IntStream map(ToIntFunction<? super T> mapper)
to
IntStream mapToInt(ToIntFunction<? super T> mapper)
then your
persons.stream().mapToInt(Person::getAge).sum()
will always work, whether `getAge()` returns `int` or `Integer`.
See this thread: http://mail.openjdk.java.net/pipermail/lambda-libs-spec-experts/2013-March/001480.html
BTW you can post your questions to lambda-dev mailing list, they'd like to hear real world experiences.
Problem
I was playing around with Java8 lambda expressions. As an example I then tried to sum up the ages persons contained in a list: ``` import java.util.Arrays; import java.util.List; public class Person { public static void main(String[] args) { List<Person> persons = Arrays.asList(new Person("FooBar", 12), new Person("BarFoo", 16)); Integer sumOfAges = persons.stream().map(Person::getAge).sum(); System.out.println("summedUpAges: " + sumOfAges); } private final String name; private final Integer age; public Person(String name, Integer age) { this.name = name; this.age = age; } public String getName() { return name; } public Integer getAge() { return age; } } ``` When I try to compile this snippet with the following java compiler: ``` openjdk version "1.8.0-ea" OpenJDK Runtime Environment (build 1.8.0-ea-lambda-night OpenJDK 64-Bit Server VM (build 25.0-b21, mixed mode) ``` I get the following compile error: ``` java: cannot find symbol symbol: method sum() location: interface java.util.stream.Stream<java.lang.Integer> ``` But if I change the return value of the getAge() method from Integer to int, I get the expected result. But sometimes it is not possible or desired to change signatures on the fly. Is there any way to make this working when getAge() returns a Integer type? Thanks in advance