Java OOP三大特征之封装继承与多态详解

class A{
    A(){
        System.out.println("A");
    }
}
class B extends A{
    B(){
        super();//如果程序原没写构造方法，编译器会自己提供一个这样的不带参数的构造方法
    }
}
public class Test{
    public static void main(String[] args){
        new B();
    }
}

class A{
    A(){
        System.out.print("A");
    }
}
class B extends A{
    B(){  //程序会自动补上super,最终打印AB
        System.out.print("B");
    }
}
public class Test{
    public static void main(String[] args){
        new B();
    }
}

class A{
    A(int a){
        System.out.println("A");
    }
}
class B extends A{
    //若父类构造方法带参数，子类不会自动补构造方法
    //程序编译失败
}
public class Test{
    public static void main(String[] args){
        new B();
    }
}

class Character{
    public int data1;
    public int data2;
    static{
     System.out.println("父类的静态内部类初始化完成！");
    }
    {
        System.out.println("父类的实例内部类初始化完成！");
    }
    public Character(){
        System.out.println("父类的构造方法初始化完成！");
    }
}
class Art extends Character{
    public int data3;
    public int data4;
    static{
        System.out.println("子类的静态内部类初始化完成！");
    }
    {
        System.out.println("子类的实例内部类初始化完成！");
    }
    public Art(){
        super();
        System.out.println("子类的构造方法初始化完成！");
    }
}
public class Test{
    public static void main(String[] args){
        Art art = new Art();
    }
}

//人类
class Human{
    private String name;
    Human(String name){
        setName(name);
    }
    public void action(){
        System.out.println(name + "看到一架钢琴~");
    }
    public void setName(String name){
        this.name = name;
    }
    public String getName(){
        return this.name;
    }
}
//美术生
class Art extends Human{
    Art(String name){
        super(name);
    }
    public void action(){
        System.out.println(getName() + "开始绘画钢琴~");
    }
}
//音乐生
class Music extends Human{
    Music(String name){
        super(name);
    }
    public void action(){
        System.out.println(getName() + "开始弹钢琴~");
    }
}
public class Test{
    public static void function(Human human){
        human.action();
    }
    public static void main(String[] args){
        Human human1 = new Art("美术生");
        Human human2 = new Music("音乐生");
        function(human1);
        function(human2);
    }
}

public class Test{
    public static void function(Human human){
        human.action();
    }
    public static void main(String[] args){
        Human human = new Human("音乐生");
        Music music = (Music)human;
    }
}

public class Test{
    public static void function(Human human){
        human.action();
    }
    public static void main(String[] args){
        Human human = new Music("音乐生");//这样写便是有些人是音乐生
        if(human instanceof Music) {//保证安全性，向下转型
            Music music = (Music) human;
        };
    }
}