Polymorphism
What is polymorphism:
Same kind of things, showing different states at different moments
Conditions for polymorphism:
- There must be inheritance
- There must be a way to rewrite
- The reference of the parent class points to the object of the child class
Create animal parent class
import javaopp.Day05.InterFaceA;
import javaopp.Day05.InterFaceB;
public abstract class Animal {
int num=10;
abstract void eat();
abstract void sleep();
public static void testStatic(){
System.out.println("Animal Static");
}
public void test(){
System.out.println("Animal Test");
}
}
Create Dog subclass
public class Dog extends Animal {
int num=5;
@Override
public void eat() {
System.out.println("狗吃肉");
}
@Override
public void sleep() {
System.out.println("狗 to bed");
}
public void play(){
System.out.println("狗玩辣子");
}
}
Create Cat subclass
public class Cat extends Animal{
@Override
public void eat() {
System.out.println("喵喵鱼");
}
@Override
public void sleep() {
System.out.println("喵喵睡觉");
}
public void catchMouse(){
System.out.println("猫捉老鼠");
}
}
Test class
public class Test2 {
public static void main(String[] args) {
/*
向上类型准换
*/
Dog dog = new Dog();//多态体现在调用方法
CAt cat = new CAt();//父类的引用指向子类对象
Test2 t1 = new Test2();
t1.feedAnimal(dog);
t1.feedAnimal(cat);
}
//多态应用: 同一种事物,在不同时刻表现不同的状态
/*
向上类型转换的弊端:无法访问子类中的特有方法
*/
public void feedAnimal(Animal animal) {
animal.eat();
animal.sleep();
/*
向下转型,可以访问子类中的方法
*/
if(animal instanceof Dog){
Dog dog = (Dog) animal;
dog.play();
}
/*
animal instanceof CAt:在运行时检测判断animal 中实际包含是否是指定的类型
*/
if (animal instanceof CAt){
CAt cat = (CAt) animal;//向下转型:由父类类型转为子类类型
cat.catchMouse();
}
}
}
Reference type and reference type conversion
The premise is the inheritance relationship
Default transformation (upcast)
Animal dog = new Dog();
dog.eat();//编译看左边,运行期间看右边
Forced type conversion (downcast)
public static void main(String[] args) {
//向上转型 ,父类类型表示所有的子类
Animal dog = new Dog();
Animal cat = new Cat();
Test2 t1 = new Test2();
t1.feedAnmal(dog);
t1.feedAnmal(cat);
}
/*
多态缺点: 向上转型为父类类型,弊端:不能访问子类中特有的方法
*/
public void feedAnmal(Animal animal){
animal.eat();
//animal instanceof Dog 在运行时 检测判断 animal 中实际 包含是否是指定的类型
if(animal instanceof Dog){
Dog dog = (Dog)animal;//向下转型:由父类类型 转为 子类类型
dog.play();
}
if(animal instanceof Cat){
Cat cat = (Cat)animal;
cat.catchMouse();
}
}
}
final keyword
final can modify attributes, methods, classes, method parameters
class FinalDemo {
//final 修饰的变量变为常量,值不能改变
final static int num1 = 10;//直接初始化变量,建议用static修饰,在内存中就会有一份
//final int num;
/*
在创建的每一个对象中包含一个常量,必须在构造方法中赋值
*/
public FinalDemo() {
}
/*public FinalDemo(int a) {
num=a;
}*/
public final void test(final int a) {
//num=5;
System.out.println(a);
//a=1;
}
public void eat() {
System.out.println("aaaaa");
}
public static void main(String[] args) {
//FinalDemo f1 = new FinalDemo(5);
//FinalDemo f2 = new FinalDemo(10);
}
}
/*
public class Son extends FinalDemo
final 修饰的类无法被继承
* */
public class Son extends FinalDemo{
@Override
public void eat() {
super.eat();
}
/*
public void test(){}
被final修饰的方法,子类无法重写
*/
}