一个类以public class 开头，类名必须和文件名一致，public class代表这个类是公共类，可以随意调用，public class后紧跟类名。类也叫做自定义类型，程序中不能出现相同名字的类。
类体中可以定义这个类的属性的变量，称之为成员变量。
public class Phone {
Screen screen;
Mainboard mainboard;
double price;
boolean hasFigurePrintUnlocker;
Phone prePhone;
}
使用new操作符可以创建一个类的实例/对象（instance/object）,new创建了一个对象之后，类中创建的变量都会被赋予与其类型相对应初始值。用点操作符来访问实体的属性。
public class MyPhoneMaker {
public static void main(String[] args) {
Phone phone = new Phone();
phone.hasFigurePrintUnlocker = true;
phone.price = 1999;
phone.screen = new Screen();
phone.screen.producer = "京东方";
phone.screen.size = 8.8;
CPU cpu = new CPU();
cpu.producer = "三星";
cpu.speed = 3.5;
Memory memory = new Memory();
memory.producer = "三星";
memory.capacity = 4;
Storage storage = new Storage();
storage.producer = "Intel";
storage.capacity = 128;
phone.mainboard = new Mainboard();
phone.mainboard.cpu = cpu;
phone.mainboard.memory = memory;
phone.mainboard.storage = storage;
phone.mainboard.model = "EA888";
phone.mainboard.year = 2019;
phone.prePhone = new Phone();
}
}
