multiple classes in a single file : modifier private not allowed here

问题

I am not able to understand why this code doesn't compile:

class A {
    public static void main(String[] args) {
        System.out.println("hi");
    }
}

private class B {
    int a;
}

I am saving the contents in a file named A.java - and I get an error:

modifier private not allowed here // where I have defined class B

This happens both when I try B as private and protected. Can someone please explain me the reason behind this?

Thanks !

回答1:

From the Java Language specification:

The access modifiers protected and private pertain only to member classes within a directly enclosing class declaration

So yes, the private and the protected modifiers are not allowed for top level class declarations.

Top-level classes may be public or not, while private and protected are not allowed. If the class is declared public, then it can be referred to from any package. Otherwise it can only be referred to from the same package (namespace).

A private top level class wouldn't make much sense because it couldn't be referred to from any class. It would be unusable by definition. private is OK for member classes to make a class referable to only it's enclosing class.

A protected member class can be referred to from (1) any class of the same package and from (2) any subclass of the enclosing class. Mapping this concept to top level classes is difficult. The first case is covered by top level class with no access modifiers. The second case is not applicable for top level classes, because there is no enclosing class or something else from a different package with a special relation to this class (like a subclass). Because of this I think, protected is not allowed because it's underlying concept is not applicable for top level classes.

回答2:

Make the B nested of A, like this:

class A {
    public static void main(String[] args) {
        System.out.println("hi");
    }

    private class B {
        int a;
    }
}

Or move B to a separate file. Also you can stick with default access level, this way the class can be accessed only from within the package:

class A {
    public static void main(String[] args) {
        System.out.println("hi");
    } 
}

class B {
    int a;
}

回答3:

private and protected are meaningless to be allowed to a top level(not member) class/interface.

They are applicable only to the class members which can be variables, constants, constructors, methods, classes, and interfaces.

Why:

(1) private: What may be the meaning/purpose if we define a class as private. Its scope should be private to some area. default access is already package private. And nobody wants a class to be source file private, (Guessing the reason) it may not be a good programming practice to allow because java applications are finally organized in the form of packages, but not in terms of source files. Any source file should be part of some package, so in broad/final view each class/interface is part of some package, not just of some .java file. So not applicable.

(2) protected: If something is protected it should be available only within package and only to the sub classes in other packages. To extend a class in a different package, it should be available to all the classes in other packages, but protected says class should be available only to the classes extended it. It's a kind of deadlock situation. So not applicable.

Source: My readings and understanding

回答4:

Just have no private/protected modifier at all.

回答5:

B needs to be private to something. Place it within the definition of class A or create another file, B.java, and define it there, but then it cannot be private.

回答6:

If you don't use the public keyword for the class it will be private by default (visible only within the file).

There can be only one public class per .java file, all the others need to be private. So class A can be public and class B doesn't need any modifiers in your example. The public class name must match the .java file name (eg. A.java can only contain one public class called "A").

回答7:

A.java cannot contain two classes.

来源：https://stackoverflow.com/questions/3491131/multiple-classes-in-a-single-file-modifier-private-not-allowed-here