当一个成员是私有类实例时，该类如何看到另一个类的公共成员

Question

I am a C++ programmer trying to understand best practices of the Java language. Please tailor your responses accordingly :)

I worked out an example and I don't understand why/how the behavior I observed is possible.

//HelloWorld.java

public class HelloWorld{
    public static void main(String []args){
        Newfile nf = new Newfile();
        nf.setTest(5);
        System.out.println(Integer.toString(nf.test.i));
        System.out.println("Hello World");
    }
}

//Newfile.java

class TEST {
    public int i;
}

public class Newfile{    
    public TEST test = new TEST();
    public int setTest(int i) {
        return test.i = i;
    }
}

// THIS WORKED 
/*
   5
   Hello World
*/

Since Test is private (well I presume) then how can HelloWorld even manipulate the contents of TEST (ie the i member)?

Now the strange part, when I moved TEST to a private inner class of Newfile it now behave as I would've expected the above experiment to behave and I got:

i in Newfile.TEST is defined in an inaccessible class or interface

Answer 1

Since TEST is private (well I presume)

Nope. It's actually package-private .

From Java Tutorials :

If a class has no modifier (the default, also known as package-private), it is visible only within its own package

---

EDIT

Both your HelloWorld and your TEST classes are members of the same package (in your case, since you don't appear to have specified the package explicitly, they are members of the unnamed package). Therefore, even though TEST is package-private, it is accessible from HelloWorld .

Now the strange part, when I moved TEST to a private inner class of Newfile it now behave as I would've expected the above experiment to behave and I got:

 i in Newfile.TEST is defined in an inaccessible class or interface 

By making TEST a private inner class of Newfile you make it accessible only to members of Newfile class itself, so, naturally, it's no longer accessible from HelloWorld in this case.