Why does the code not throw a `NullPointerException` when I use a method reference tied to a variable `dog` which I later assigned `null` to?

I am using Java 8.

```java
import java.util.function.Function;

class Dog {
    private int food = 10;

    public int eat(int num) {
        System.out.println("eat " + num);
        this.food -= num;
        return this.food;
    }
}

public class MethodRefrenceDemo {

    public static void main(String[] args) {
        Dog dog = new Dog();
        Function<Integer, Integer> function = dog::eat;

        dog = null;

        // I can still use the method reference
        System.out.println("still have " + function.apply(2));
    }
}

```