我什么时候在Java中使用package-private？ [英] When would I use package-private in Java?

问题描述

我喜欢任何语言的访问控制，但我发现在Java中我几乎从不（如果有的话）使用 package-private 访问修饰符（或缺少）。

我意识到内部类可以 private ， protected 或 package-private ，但外部类只能是 package-private 或 public 。为什么外部类可以 package-private 但不能 protected ？限制整个包所见的类/方法/字段有什么好处，而不是子类？

解决方案

我用 package-private 当我想要隐藏包外的用户（和其他类）的实现细节时的类和方法。

<例如，如果我有一个接口和一个工厂类来创建该接口的实例，我可能将实现类作为一个单独的文件，但将其标记为package-private，以便其他人不能使用它，也不会使JavaDoc混乱（如果javadoc设置为仅显示公共）。

如果密封jar文件，package-private方法也可以帮助限制谁可以访问这些方法。如果方法是公共的或受保护的，则子类仍然可以查看并调用该方法，即使它位于不同的包中。 （未密封的jar允许任何人在你的包中创建类，以便他们可以访问包私有或受保护的方法）

I love access control in any language, but I find that in Java I almost never (if ever) use the package-private access modifier (or lack thereof).

I realize that inner classes can be private, protected, or package-private, but outer classes can only be package-private or public. Why can an outer class be package-private but not protected? What is the benefit of restricting classes/methods/fields to be seen by the entire package, but not subclasses?

解决方案

I use package-private classes and methods when I want to hide implementation details from users (and other classes) outside the package.

For example if I have an interface and a factory class that creates instances of that interface, I may have the implementation class as a separate file but mark it package-private so others can not use it, nor will it clutter the JavaDoc (if javadoc set to only show public).

If you seal your jar file, package-private methods can also help restrict who can access these methods. If a method is public or protected, subclasses can still see and call that method even if it's in a different package. (Unsealed jars allow anyone to make classes in your packages so they will get access to package-private or protected methods)

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