What are the necessary conditions for polymorphism to exist in Java?
Three necessary conditions for the existence of polymorphism:
(Recommended learning: java introductory tutorial)
-
Inheritance
Override
The parent class reference points to the child class object
When When calling a method using polymorphism, first check whether the method exists in the parent class. If not, a compilation error occurs; if so, call the method of the same name in the subclass.
Benefits of polymorphism:
It allows the program to be well expanded and can handle objects of all classes universally.
(Video tutorial: java video tutorial)
Test code:
public class Test { public static void main(String[] args) { show(new Cat()); // 以 Cat 对象调用 show 方法 show(new Dog()); // 以 Dog 对象调用 show 方法 Animal a = new Cat(); // 向上转型 a.eat(); // 调用的是 Cat 的 eat Cat c = (Cat)a; // 向下转型 c.work(); // 调用的是 Cat 的 work } public static void show(Animal a) { a.eat(); // 类型判断 if (a instanceof Cat) { // 猫做的事情 Cat c = (Cat)a; c.work(); } else if (a instanceof Dog) { // 狗做的事情 Dog c = (Dog)a; c.work(); } } } abstract class Animal { abstract void eat(); } class Cat extends Animal { public void eat() { System.out.println("吃鱼"); } public void work() { System.out.println("抓老鼠"); } } class Dog extends Animal { public void eat() { System.out.println("吃骨头"); } public void work() { System.out.println("看家"); } }
Output result:
吃鱼 抓老鼠 吃骨头 看家 吃鱼 抓老鼠
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