一：类转型
类转型：子类可以转父类，父类不可以转子类(除非父类对象本身就是子类）
1.父类
public class Human {
public void say() {
System.out.println("Hello World");
}
public void eat() {
System.out.println("eat food");
}
}
2.子类
public class Men extends Human{
public void plough() {
System.out.println("Men is ploughing");
}
public void eat() {
System.out.println("Men is eating");
}
}
3.实现转型
public class HumanTest {
public static void main(String args[]) {
Men m=new Men();
m.eat();
//子类转父类
Human hm=m;
hm.eat();
hm.say();
//hm.plough();//The method plough() is undefined for the type Human
//父类转子类（前提该父类是子类转过去的）
Men m2=(Men)hm;
m2.eat();
m2.plough();
}
}
Men is eating
Men is eating
Hello World
Men is eating
Men is ploughing
4.问题：编译时报错和运行时报错
class Pencil {
public void write (String content){
System.out.println( "Write",+content);
}
}
class RubberPencil extends Pencil{
public void write (String content){
System.out.println("Rubber Write"+content)；
}
public void erase (String content){
System.out.println( "Erase "+content);
}
}
执行下列代码的结果是哪项？
Pencil p=new Pencil();
(( RubberPencil) p).write("Hello");
强制转换是允许的，不过相当于会产生内存越界，导致运行时报错
二：多态
多态：子类转型为父类后，调用普通方法，依旧是子类的
1.父类
public class Human {
public void say() {
System.out.println("Hello World");
}
public void eat() {
System.out.println("eat food");
}
}
2.子类：对父类方法重写
public class Women extends Human{
public void weave() {
System.out.println("women is weaving");
}
public void say() {
System.out.println("Hello World,I am a women");
}
public void eat() {
System.out.println("Women eat food");
}
}
public class Men extends Human{
public void plough() {
System.out.println("Men is ploughing");
}
public void eat() {
System.out.println("Men is eating");
}
public void say() {
System.out.println("Hello World，I am a man");
}
}
public class Child extends Human{
public void eat() {
System.out.println("Child is eating");
}
public void say() {
System.out.println("Hello World，I am a Child");
}
}
3.多态实现
public class HumanTest {
public static void main(String args[]) {
Human[] HM= new Human[];
HM[]=new Men();
HM[]=new Women();
HM[]=new Child();
for(int i=;i<;i++) {
HM[i].eat();
HM[i].say();
}
}
}
Men is eating
Hello World，I am a man
Women eat food
Hello World,I am a women
Child is eating
Hello World，I am a Child
三：利用多态实现解耦
（一）父类实现解耦
public class HumanTest {
public static void main(String args[]) {
HumanSay(new Men());
HumanSay(new Women());
HumanSay(new Child());
HumanSay(new Child() {
public void say() {
System.out.println("Someone is eating");
}
});
}
public static void HumanSay(Human a) { //方法不是依赖某个具体子类，可以使用父类，抽象类，甚至接口来实现解耦
a.say();
}
public static void HumanEat(Human a) {
a.eat();
}
}
Hello World，I am a man
Hello World,I am a women
Hello World，I am a Child
Someone is eating
（二）接口实现：
1.接口
public interface Human {
public void say();
public void eat();
}
2.子类
public class Men implements Human{
public void plough() {
System.out.println("Men is ploughing");
}
public void eat() {
System.out.println("Men is eating");
}
public void say() {
System.out.println("Hello World，I am a man");
}
}
public class Women implements Human{
public class Child implements Human{
3.其他一样，结果相同
（三）抽象类实现
1.抽象类
public abstract class Human {
public abstract void say();
public void eat() {
System.out.println("Human is eating");
}
}
2.子类
public class Men extends Human{
public class Women extends Human{
public class Child extends Human{
3.调用方式一样，结果一样