面试题：多态是编译时行为还是运行时行为?

package com.day0125_4;
import java.util.Random;
//面试题：多态是编译时行为还是运行时行为?
//运行时行为
//证明如下：
class Animal{
  protected void eat() {
    System.out.println("animal eat food");
  }
}
class Cat extends Animal{
  protected void eat() {
    System.out.println("cat eat fish");
  }
}
class Dog extends Animal{
  protected void eat() {
    System.out.println("dog eat bone");
  }
}
class Sheep extends Animal{
  protected void eat() {
    System.out.println("sheep eat grass");
  }
}
public class Test {
  public static Animal getInstance(int key) {
    switch(key) {
    case 0:
      return new Cat();
    case 1:
      return new Dog();
    default:
      return new Sheep();
    }
  }
  public static void main(String[] args) {  
    int key =new Random().nextInt(3);
    System.out.println(key);
    Animal animal=getInstance(key);
    animal.eat();
  }
}