为什么子类对象在Java中调用父类字段变量？

Question

This is my code, pretty simple and demonstrative:

JVM entrance:

class Demo {
    public static void main(String[] args) {
        new Dog().start();
    }
}

Parent class:

public class Animal {
    int num = 100;
    public void call() {
        System.out.println(this.num);
    }

}

Child class:

 public class Dog extends Animal{
        int num = 10000;
        public void start() {
            this.call();
        }
    }

Console output: 100

Why it's 100, not 10000? How to comprehend this?

Answer 1

Your instance has two fields: Animal::num and Dog::num . Animal::call() only knows about Animal::num , which is 100.

It is not usually helpful to declare a field in a subclass with the same name as a field in the superclass. Fields are not subject to overriding; and shadowing a name only leads to confusion.

---

Suppose that instead of declaring a new num field in Dog , you set the existing num field to a new value.

class Dog extends Animal {
    public Dog() {
        num = 10000;
    }
    public void start() {
        this.call();
    }
}

Now if you run new Dog().start() , you will find that 10000 is printed. The instance has only one num field, declared in Animal , and set to 10000 inside Dog .