模块之间只通过API通信，其他的隐藏起来。
降低耦合，独立地开发，测试，优化

Access control

• private—The member is accessible only from the top-level class where it is declared.
• package-private—The member is accessible from any class in the package where it is declared. Technically known as default access, this is the access lev- el you get if no access modifier is specified.
• protected—The member is accessible from subclasses of the class where it is declared (subject to a few restrictions [JLS, 6.6.2]) and from any class in the package where it is declared.
• public—The member is accessible from anywhere.

ps:

1. 子类的访问权限不能低于父类
2. 不能为了测试， 而将类／接口变成API，可以将测试和类定义在同一个包中，访问protected元素
3. 类中不允许有 public mutable/static fields， 但是final可以为public。final域全为大写字母，单词用下划线隔开。final域不可以是可变对象的引用，例如数组。所以类具有public static final的数组或者返回它的访问方法都是错误的。
   修改方案：
   （1）使public array变成private, 增加一个public immutable list:

private static final Thing[] PRIVATE_VALUES = { ... };
public static final List<Thing> VALUES =
  Collections.unmodifiableList(Arrays.asList(PRIVATE_VALUES));

（2）使array变成private，定义一个public方法，访问array的备份：

  private static final Thing[] PRIVATE_VALUES = { ... };
   public static final Thing[] values() {
       return PRIVATE_VALUES.clone();
   }

summary

尽可能减少public数目，field除了final以外，都不能是public的。并且final的引用对象都是不可变的。