在JavaBean中，通常将属性设置为private，然后生成与属性对应的public setXXX(),public getXXX()。那么为什么要这么设计呢？
| 同一个类 | 同一个包 | 同一个包下的子类 | 不同包下的子类 | 不同包的非子类 | |
|---|---|---|---|---|---|
| public | + | + | + | + | |
| protected | + | + | + | ||
| private | + |
根据上表可以得知，将属性设置为private限制了外界对这个属性的直接访问操作，那么设置对应的get、set方法并设为public的意义就相当于给外界提供了访问的方法，而不是直接将属性暴露。
引用Stackoverflow的答案：
- Encapsulation of behavior associated with getting or setting the property this allows additional functionality (like validation) to be added more easily later.
- Hiding the internal representation of the property while exposing a property using an alternative representation.
- Insulating your public interface from change - allowing the public interface to remain constant while the implementation changes without affecting existing consumers.
- Controlling the lifetime and memory management (disposal) semantics of the property - particularly important in non-managed memory environments (like C++ or Objective-C).
- Providing a debugging interception point for when a property changes at runtime - debugging when and where a property changed to a particular value can be quite difficult without this in some languages.
- Improved interoperability with libraries that are designed to operate against property getter/setters - Mocking, Serialization, and WPF come to mind.
- Allowing inheritors to change the semantics of how the property behaves and is exposed by overriding the getter/setter methods.
- Allowing the getter/setter to be passed around as lambda expressions rather than values.
- Getters and setters can allow different access levels - for example the get may be public, but the set could be protected.
自己目前所理解的：
- 实现java域变量的封装性和安全性
- 可以对属性进行验证操作，比如age属性，在set中可以添加验证，如果输入的age小于0则抛出异常
- 可以在debug的时候用到