Import statement and directory structure

Question

First of all, sorry for such a long post, I believe part of it is because I am unsure of the concept of importing in Java. Secondly, Thanks to anyone who can ultimately enlighten me to the concept of import. I did ask this question before in the "erorr and error handling" forum, and the people who have helped me there did a great job. But, I believe I require a little more clarification and thus have decided to post here.
Anyhow, my question..
Could someone explain to me the concept of the import statement, or direct me to a webpage with sort of explanation for newbies? For some reason, I am having a hard time grasping the concept.
As I understand it, the import statement in Java, is very similar to the namespace keyword in C. That is to say, import doesn't actually "import" any source code, the way that the #include statement does in C.
So I suppose what my question is, say I have a java class file like below:
//filename: sentence.java
//located: c:\school\csc365
package csc365;
class sentence
//some variables here..
//some constructor here..
//some methods here..
And some sample program like the one below which implements the above..
//filename: test.java
//located: c:\school\csc365
import csc365.*;
import java.io.*;
class test.java
//creates some sentence object
//uses the object's methods
//some other things.
As I understand it, the test.java file should not compile because the csc365 package is not in the correct directory. (assuming of course, the classpath is like c:\school\csc365;c:\school )
But, ... where then should the sentence.java be located? In a subdirectory of c:\school called csc365 (i.e c:\school\csc365\) ?
And thus that would mean the test.java file could be located anywhere on the hard drive?
I suppose, I just need a little clarification on the correlation between a package's "name" (i.e package csc365; ) and its corresponding directory's name, and also how the javac compiler searches the classpath for java classes.
..So, theoretically if I were to set the classpath to look in every conceivable directory(provided the directory names were all unique) of the harddrive, then I could compile a test.java anywhere?
As a note: I have been able to get the test.java file to compile, by leaving out the import statement in the test.java file, and also leaving out the package statement for the sentence class, but I assume this is because the files are defaulted to the same package?

Answer 1

Hi Mary,
No, import isn't analogous to C++ namespace - Java package is closer to the namespace mark.
import is just a convenience for the programmer. You can go your whole Java career without ever writing an import statement if you wish. All that means is that you'll have to type out the fully-resolved class name every time you want to use a class that's in a package other than java.lang. Example:
// NOTE: No import statements
public class Family
   // NOTE: fully-resolved class names
   private java.util.List children = new java.util.ArrayList();
}If you use the import statement, you can save yourself from typing:
import java.util.ArrayList;
import java.util.List;
public class Family
   // NOTE: fully-resolved class names
   private List children = new ArrayList();
}import isn't the same as class loader. It does not bring in any source code at all.
import comes into play when you're compiling or running your code. Java will check to make sure that any "shorthand" class names you give it live in one of the packages you've imported. If it can't find a matching fully-resolved class name, it'll give you a message like "Symbol not found" or something like that.
I arrange Java source in a directory structure that matches the package structure in the .class files.
If I've got a Java source file like this:
package foo.bar;
public class Baz
   public static void main(String [] args)
        Baz baz = new Baz();
        System.out.println(baz);
   public String toString()
       return "I am a Baz";
}I'll store it in a directory structure like this:
root
+---classes
+---src
      +---foo
           +---bar
                +---Baz.javaWhen I compile, I go to root and compile by typing this:
javac -d classes foo/bar/*.javaI can run the code from root by typing:
java -classpath classes foo.bar.BazI hope this wasn't patronizing or beneath you. I don't mean to be insulting. - MOD