是否可以从 Java 中的超类调用子类的实例变量？

Question

I've got a superclass Animal that is extended by Cat, Dog & Lion. These three all have a field String sound representing their sound. The method playSound(int i, String s) plays the sound s, i amount of times. Could it be possible to have a general method in the Animal superclass that takes in the field of the subclass calling it? Or do I have to make a separate, overriding method in each class that calls super with their own "sound" variable?

Answer 1

The superclass can't access fields in the subclass. The usual approach here is to make an abstract getter:

protected abstract Sound getSound();

public void playSound(int times) {
    for(int i=0; i<times; i++) {
        getSound().play();
    }
}

Answer 2

The super has no access to the sub's fields, nor should it -- super classes should have no dependencies on their children -- but thanks to polymorphism, it doesn't need to. All you need to do is give super a makeSound() method, or something similar, and then have the sub's override the method, each playing its own sound.

re,

Or do I have to make a separate, overriding method in each class that calls super with their own "sound" variable?

I don't understand what you mean by "that calls super...". You simply have the sub call its own sound within the makeSound() method override.

Answer 3

As far as I know, no, it's not possible. You can try making comparisons in the superclass like

if (this intanaceof of Cat)
play cat sound

But that would kind of defeat the purpose of polymorphism. I don't see why you would want to do it this way though. I'd do something like this:

abstract class Animal{

Sound [] sounds;
int soundI;

Animal(int soundI){
this.soundI = soundI
}

void playSound(){
sounds[i].play();
}

}

class Cat extends Animal{

Cat(){
super(5);
}

}