我什么时候会在 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).

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).

我意识到内部类可以是 private、protected 或 package-private，但外部类只能是 package-private 或 public.为什么外部类可以是 package-private 而不是 protected?限制类/方法/字段被整个包看到，而不是子类看到有什么好处?

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?

推荐答案

当我想对包外的用户(和其他类)隐藏实现细节时，我使用 package-private 类和方法.

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

例如，如果我有一个接口和一个创建该接口实例的工厂类，我可能将实现类作为一个单独的文件，但将其标记为包私有的，这样其他人就不能使用它，也不会弄乱 JavaDoc(如果 javadoc 设置为只显示 public).

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).

如果你密封你的 jar 文件，包私有方法也可以帮助限制谁可以访问这些方法.如果一个方法是公共的或受保护的，即使它在不同的包中，子类仍然可以看到和调用该方法.(未密封的 jar 允许任何人在您的包中创建类，以便他们可以访问包私有或受保护的方法)

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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