Default Package vs Default (implicit) Class Access Modifier

[Chris Phillip]

All -

I am currently studying for the OCA exam, and I'm using the OCA/OCP Java SE 7 Programmer I & II Study Guide book, and I'm also testing my knowledge with Enthuware.

I've only made it through Chapter 1 in the book, but I really thought I had a VERY good understanding of the topics discussed, which included the use of Access Modifiers.

In the book, on pages 19 - 20, it say "A class declaration with the public keyword gives all classes from all packages access to the public class.  In other words, all classes in the Java Universe (JU) have access to a public class.  Don't forget, though, that if a public class you're trying to use is in a different package from the class you're writing, you'll still need to import the public class.".

Based on that statement, my understanding was the following:

Class Access Modifier	Package	Can a different class in same package access this?	Can a different class in a different package access this?
none/default	none/default	Y	N
public	none/default	Y	Y

So then I ran into the following question on Enthuware that contradicts that rule.

Given the following contents of two java source files:

package util.log4j;
public class Logger  { 
  public void log(String msg){
      System.out.println(msg);
  } 
}

and

package util;
public class TestClass {
    public static void main(String[] args) throws Exception {
        Logger logger = new Logger();
        logger.log("hello");
    }
}

What changes, when made independently, will enable the code to compile and run?

1)  Replace Logger logger = new Logger(); with:
log4j.Logger logger = new log4j.Logger();

2)  Replace package util.log4j; with 
package util;

3)  Replace Logger logger = new Logger(); with:
util.log4j.Logger logger = new util.log4j.Logger();

4)  Remove package util.log4j; from Logger.

5)  Add import log4j; to TestClass.

I chose answers 2, 3 and 4.  Answer 4 was incorrect, with their explanation "Remember that you can never access a class that is defined in the default package (i.e. the package with no name) from a class in any other package. So if you remove the package statement from Logger, you can't access it from util package, which is where TestClass is.".

I was really surprised that my answer was wrong, and even more surprised by Enthuware's explanation.

I know that you can explicitly define a public class in the default package, obviously without the use of the package statement, but Enthuware said that a different public class in a different package can't access the public class in the default package.

Based on that, it seems that the following is more correct:

Class Access Modifier	Package	Can a different class in the same package access this?	Can a different class in a different package access this?
none/default	none/default	Y	N
public	none/default	Y	N
public	non-default	Y	Y

So what I THINK the book should have said is "for public classes is "A class declaration with the public keyword in a non-default package gives all classes from all non-default packages access to the public class.  In other words, all classes in the Java Universe (JU) have access to a public class.  Don't forget, though, that if a public class you're trying to use is in a different package from the class you're writing, you'll still need to import the public class.".

I'm also wondering if the rules I thought I knew for protected methods will need to be adjusted in my mind due to this.  I know protected methods can be inherited even if they're in different packages, but maybe that doesn't hold true if the class containing the protected methods is in the default package.

Thoughts?  Is my second table more correct than the first?  

[Stephan van Hulst]

Take another look at the what the book says:

OCA/OCP Java SE 7 Programmer I & II Study Guide wrote:Don't forget, though, that if a public class you're trying to use is in a different package from the class you're writing, you'll still need to import the public class.

That means you have to import the Logger class. Since you can't import from the default package, you can't use a class in the default package from outside of it. This has nothing to do with access specifiers.

[Chris Phillip]

Stephan van Hulst wrote:Take another look at the what the book says:

OCA/OCP Java SE 7 Programmer I & II Study Guide wrote:Don't forget, though, that if a public class you're trying to use is in a different package from the class you're writing, you'll still need to import the public class.

That means you have to import the Logger class. Since you can't import from the default package, you can't use a class in the default package from outside of it. This has nothing to do with access specifiers.

Yeah, I actually thought about that and was going to try to edit my post to say that, but I have no edit post options.  I do think that the wording is a bit tricky, especially since chapter 1 hasn't said anything about not being able to import the default package.  I'm glad it was just a misunderstanding, otherwise I'd be concerned about my knowledge of protected members too.