Java类对象id号的自动生成，以Book为例，bookId自动生成案例

1：创建Book类

package com.dareway.test4;
public class Book {
private int no;
private String name;
private double price;
private static int num = 3;
private static int count = 0;
public Book(){
count++;
this.no = count;
}
public Book(String name, double price) {
super();