Java Instance Variable Accessibility [duplicate]

问题:

This question already has an answer here:

• What is the difference between public, protected, package-private and private in Java? 24 answers

What is the difference in the accessibility of the following variables in Java?

public class Joe {
    public int a;
    protected int b;
    private int b;
    int c;
}

I'm most interested in what the last one is doing.

回答1:

• public: read/writable for anyone
• protected: read/writable for instances of subclasses and from within the enclosing package
• private: read/writable for any instance of the class and inner or outer (enclosing) instance
• int c : package-private, read/writable for all classes inside same package

See the JLS for more details

EDIT: Added the comment for protected stating that access is granted from inside same package, you guys are totally right. Also added comment for private. I remember now... ;-)

回答2:

Sorry for answering corrections to one previous answer but I don't have enough reputation to modify directly...

• public - read/writable for anyone
• protected - read/writable for instances subclasses and all classes inside same package
• int c : package-private, read/writable for all classes inside same package
• private - read/writable for any member of that class itself and inner classes (if any)

It is better to order the access modifiers this way, from the broadest access (public) to the narrowest (private), knowing that when going from narrow to broad, you don't lose any possibilities.

That's particularly important for "protected", where it is often misunderstood that classes in the same package can also access protected members of a class (not only its subclasses).

回答3:

I try to avoid package level access completely (the last access you mention).

I like to keep classes self-contained. If another class needs access to something in my class it should be public (and it should by a method, not an attribute). Otherwise I feel you've broken encapsulation, as explained in Abstraction VS Information Hiding VS Encapsulation.

回答4:

And all of these are compile time protections, they can be readily overridden through reflection at runtime.