Method reference in Java 8

Question

public class Car {

    private int maxSpeed;

    public Car(int maxSpeed) {
        this.maxSpeed = maxSpeed;
    }

    public int getMaxSpeed() {
        return maxSpeed;
    }
}

We can sort a list of cars by,

    Car carX = new Car(155);
    Car carY = new Car(140);

    List<Car> cars = new ArrayList<>();
    cars.add(carX);
    cars.add(carY);

    cars.sort(Comparator.comparing(Car::getMaxSpeed));

If we see the signature of the method Comparator.comparing, the input parameter type is Function<? super T, ? extends U>

In the above example, how is Car::getMaxSpeed being cast to Function<? super T, ? extends U> while the following does not compile?

  Function<Void, Integer> function = Car::getMaxSpeed;

Answer 1

The assignment:

Function<Void, Integer> function = carX::getMaxSpeed;

does not compile because it's a Supplier<Integer>, not a Function.

So then, why does this compile?:

Comparator.comparing(Car::getMaxSpeed)

Java 8 allows an instance method reference that is a Supplier<U> to be provided where a Function<T, U> is expected, and the compiler effectively converts the getter method into a function.

To find out why this is possible, let's look at how we invoke a getter method using reflection:

System.out.println(Car.class.getMethod("getMaxSpeed").invoke(carX)); // "155"

When calling invoke() on an instance method, we pass the instance to the invoke() method of the getter's Method - there's an implied parameter of the instance type. When looked at it this way, we see that under the hood a getter is really implemented as a Function<T, U> via the invoke() method.