Java 8 stream's .min() and .max(): why does this compile?

Note: this question originates from a dead link which was a previous SO question, but here goes...

See this code (note: I do know that this code won't "work" and that Integer::compare should be used -- I just extracted it from the linked question):

final ArrayList <Integer> list 
    = IntStream.rangeClosed(1, 20).boxed().collect(Collectors.toList());

System.out.println(list.stream().max(Integer::max).get());
System.out.println(list.stream().min(Integer::min).get());

According to the javadoc of .min() and .max(), the argument of both should be a Comparator. Yet here the method references are to static methods of the Integer class.

So, why does this compile at all?

Jon Skeet
people
quotationmark

Comparator is a functional interface, and Integer::max complies with that interface (after autoboxing/unboxing is taken into consideration). It takes two int values and returns an int - just as you'd expect a Comparator<Integer> to (again, squinting to ignore the Integer/int difference).

However, I wouldn't expect it to do the right thing, given that Integer.max doesn't comply with the semantics of Comparator.compare. And indeed it doesn't really work in general. For example, make one small change:

for (int i = 1; i <= 20; i++)
    list.add(-i);

... and now the max value is -20 and the min value is -1.

Instead, both calls should use Integer::compare:

System.out.println(list.stream().max(Integer::compare).get());
System.out.println(list.stream().min(Integer::compare).get());

people

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