How to extend a class from super package?

Question

Say I have a dir a\
in dir a\ there is a dir b\
in b\ there is a class called DerivedClass
in a\ there is a class called ExtendedClass
how do I extend ExtendedClass from DerivedClass?

Answer 1

package a.b;
import a.ExtendedClass;
public class DerivedClass extends ExtendedClass{
    //define class members
This is interesting. But the question is, how does java know where is package b??     By the classpath variable - the classpath is set to one directory above 'a'. When you say
     a.ExtendedClass, since the classpath is set to the immediate parent of a, it searches the child directories
     for a directory called 'a' and upon finding it, searches for a .class file 'ExtendedClass'.
     Similarly when you say a.b.DerivedClass, the search begins in the parent directory for a directory called a and then
     a directory 'b' inside it on finding which it searches for DerivedClass.class
Alternatively you could have defined DerivedClass to be in package 'b'.     
     package b; //note the difference. Its b and not a.b
     import a.ExtendedClass;
     public class DerivedClass extends ExtendedClass{
         //define class members
     }in which case, you should set the classpath to
1. a's parent directory
2. a itself
The parent directory cp is still required to locate a.ExtendedClass
The fully qualified name of DerivedClass is b.DerivedClass
Thus if #1 alone were present, since 'b' is not directly inside the parent directory,
b.DerivedClass would never be found
Hence the #2 is required-the search should start from the directory 'a' (as directory 'b' is inside 'a').
therefore, a package is a directory.No, package is a very java specific thing and though logically package names should match directory names, they
arent the same.
cheers,
ram.