Java8的IntSummaryStatistics编译错误

Question

import java.util.Arrays;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.stream.Collectors;`

I came across this error in Java8 (in Eclipse Oxygen) :

List<Integer> nums = Arrays.asList(1,2,3);
IntSummaryStatistics stats2 = nums.stream().collect(Collectors.summarizingInt(x->x));`

System.out.println(stats2);// Works!

Now instead of using stats2, i directly pass the entire RHS to println:

System.out.println(nums.stream().collect(Collectors.summarizingInt(x->x)));// Compile Error

Error: The method println(char) is ambiguous for the type PrintStream

However, if I add a toString(), it works.

System.out.println(nums.stream().collect(Collectors.summarizingInt(x->x)).toString());

So I am wondering why it's behaving like this!

Generally when we pass an object to System.out.println() , it's toString() method would be called (internally). So why do I need to explicitly call the toString() method?

Also usually we can substitute the RHS to another method, like :

double x = Math.sqrt(5);
System.out.println(x); // works
System.out.println(Math.sqrt(5)); //RHS substitution works

Any help in understanding this would be great.

Added the exact code.

package java8.examples;

import java.util.Arrays;
import java.util.IntSummaryStatistics;
import java.util.List;
import java.util.stream.Collectors;

public class SampleProblem {

    public static void main(String[] args) {
        List<Integer> nums = Arrays.asList(1,2,3);
        IntSummaryStatistics stats = nums.stream().collect(Collectors.summarizingInt(x->x));
        System.out.println(stats);
        System.out.println(nums.stream().collect(Collectors.summarizingInt(x->x))); //Produces compile error
    }

}

Answer 1

It looks like a Eclipse issue. There is a open bug for Eclipse Oxygen warning this ambiguity in a method. https://bugs.eclipse.org/bugs/show_bug.cgi?id=527083

In your case, a workaround, it would be solved by casting:

System.out.println((IntSummaryStatistics)nums.stream().collect(Collectors.summarizingInt(x->x)));