Nulling variable does not invalidate method reference

user_11576087 0
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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));
    }
}
method-reference nullpointerexception java
2 Answers

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.

user_12215710

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;
user_37649650
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