在Java中将超类扩展到子类时出错 - 多态(Error when extending superclass to subclass in Java - polymorphism)

 我正在解决java旅程中的多态问题，我正在Eclipse中尝试这些示例代码。 我只是复制了代码，但我真的想知道为什么我的代码不能编译。 
 
 所以我有三个文件：Example.java，Animals.java和Dog.java。 
 
 Example.java包含main方法： 
 
public class Example {
    public static void main(String[] args) {
        Animal myDog = new Dog();
    }
}
 
 然后我为Animal和Dog创建了另外两个类，如下所示 
 
 动物类 
 
public class Animal {

    public void makeNoise() {
    }

    public void eat(){
    }

    public void sleep() {   
    }

    public void roam() {

    }

}
 
 狗类 
 
public class Dog {

}
 
 我理解它的方式是我可以使Dog成为扩展超类Animal的子类。 但为什么我的代码不能编译？ 
 
 我错过了这些代码中的内容吗？ 

I'm currently tackling polymorphism on my java journey and I'm trying out these example codes in Eclipse. I just copied the codes but I'm really wondering why my codes wouldn't compile.
 
So I got three files: Example.java, Animals.java and Dog.java.
 
Example.java contains the main method:
 
public class Example {
    public static void main(String[] args) {
        Animal myDog = new Dog();
    }
}
 
Then I created two other classes for Animal and Dog as follows
 
Animal Class
 
public class Animal {

    public void makeNoise() {
    }

    public void eat(){
    }

    public void sleep() {   
    }

    public void roam() {

    }

}
 
Dog Class
 
public class Dog {

}
 
The way I understand it is that I can make Dog my subclass that extends the superclass Animal. But why wouldn't my codes compile?
 
Am I missing something in these codes? 

原文：https://stackoverflow.com/questions/44406767