... but what is modifying? and what is
Extending?
My explain for modifying is that just changes the code and does not change signiture of the class method.
Extending means change signiture of the class, Add method or
subclass from interface like IData?
please help me!
Well modifying could involve the changing of signatures, the overloading of methods, the removal or addition of methods, etc. Some of these things would not be wise for the most part but still come under the definition of modification.
Extending involves inheritance. In other words, you create a subclass of Data. You then add new methods, overload and override methods and add new state.
Hope this helps,
Michael Morris
Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction. - Ernst F. Schumacher