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

Question

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 !

Answer 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.

Answer 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; }