private关键字
概念：私有的，一种权限修饰符，用来修饰类的成员
特点：被修饰的成员只能在本类中访问
用法：
- 1. private 数据类型 变量名；
- 2. private 返回值类型 方法名(参数列表){}
public关键字
公共的，访问权限修饰符，用来修饰类，成员变量，成员方法等，被修饰的内容可以在任意类中访问
this关键字
表示本类对象的引用，本质是一个对象
- 特点
// 每一个普通方法都有一个this，谁调用该方法，this就指向谁
- 用法
// this.属性名
// this.方法名（参数）
When nothing seems to help, I go look at a stonecutter hammering away at his rock, perhaps a hundred times without as much as a crack showing in it. Yet at the hundred and first blow it will split in two, and I know it was not that blow that did it, but all that had gone before. -- Jacob Riis