继承类的构建过程是向外扩散的，子类初始化了，父类一定也初始化了
class Art{
Art(){
System.out.println("Art");
}
}
class Drawing extends Art{
Drawing(){
System.out.println("Drawing");
}
}
public class Cartoon extends Drawing {
Cartoon(){
System.out.println("Cartoon");
}
public static void main(String[] args) {
Art cartoon = new Cartoon();
}
}
输出为：
Art
Drawing
Cartoon