Nulling variable does not invalidate method reference [duplicate]

问题

This question already has answers here:

Function pointer to String method in Java (3 answers)

Closed 6 months ago.

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.

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));
    }
}

回答1:

The dog::eat method reference captures the instance referenced by dog, so when you call function.apply(2), the eat method is executed for that instance. It doesn't matter that the dog variable no longer references that instance.

回答2:

The variable dog used at the lambda expression is visible only at the scope of the lambda expression since its definition and nullifying the dog will not affect the method reference dog::eat.

An example without usage of dog with the same functionality:

Function<Integer, Integer> function = new Dog()::eat;

来源：https://stackoverflow.com/questions/56457483/nulling-variable-does-not-invalidate-method-reference