Java 8 stream max() function argument type Comparator vs Comparable

Question

I wrote some simple code like below. This class works fine without any errors.

public class Test {
    public static void main(String[] args) {
        List&         


        

Answer 1

First trick: all instance methods actually take 1 additional implicit argument, the one you refer to as this in method body. E.g.:

public final class Integer extends Number implements Comparable {
    public int compareTo(/* Integer this, */ Integer anotherInteger) {
        return compare(this.value, anotherInteger.value);
    }
}

Integer a = 10, b = 100;
int compareResult            = a.compareTo(b);
// this actually 'compiles' to Integer#compareTo(this = a, anotherInteger = b)

Second trick: Java compiler can "transform" the signature of a method reference to some functional interface, if the number and types of arguments (including this) satisfy:

interface MyInterface {
    int foo(Integer bar, Integer baz);
}

Integer a = 100, b = 1000;

int result1 =                   ((Comparator) Integer::compareTo).compare(a, b);
int result2 = ((BiFunction) Integer::compareTo).apply(a, b);
int result3 =                           ((MyInterface) Integer::compareTo).foo(a, b);

// result1 == result2 == result3

As you can see class Integer implements none of Comparator, BiFunction or a random MyInterface, but that doesn't stop you from casting the Integer::compareTo method reference as those interfaces.