Hi,

"I''ve read up on inner classes, but for the life of me, I don''t see why they''re needed.
Could anyone explain to me why I would ever need an inner class? The examples I''ve come across (Thinking in Java 2nd Ed) don''t really convince me they''re necessary."

First of all there is normally no mechanism that will ever really physically force you to choose inner classes, but sometimes it makes sense. Lets assume you write a class which needs some helper class that needs knowledge about the outer class attributes and methods, then you would normally use inner classes for example in JAVA. (Be careful to compare JAVA and C++ at this point, because they have a different behaviour. While JAVA inner classes can easily access everything that is wrapped around them, this is not automatically true for inner C++ classes.)

An example is that you may write a linked list class, then you will have a class that describes a node in your list. It makes sense to use an inner class for the node representation and do not give the implementation of this inner class to the outside. Doing so will prevent you from chaos if you decide to change the inner class (node) implementation later on. All a user of your list is interested in, is to iterate over your list and manipulate the list and its contents. This should be done with safe iterators and using abstract interfaces. Your list class should have the standard methods like create, delete, changeValue, etc. so that there is no need to explicitly create a node outside in the list users code and then add it to the inner guts of your list class. All new instances of nodes should be implicitly created inside your list class by using the public methods of your list class. Using the appropriate keywords (private, protected) for the constructors of the inner class will make it impossible to instance such a inner class from the outside. The reason to do so is "encapsulation of data" which grants the consistence of data.

cu

Peter

I do it just for tidyness... If I want a class to be accessed only within that class, then it makes sense to contain it. It also prevents any later misunderstandings about what owns what, and where certain classes should be used. Though certainly some of my inner classes get messy... try this

CLandMap
|- CRegion
_ |- CLayer
_ | |- PointRef_t
_ | |- TriangleRef_t
_ |- RegionSize_t

Keeps me honest though


Edited by - ludemann8 on June 8, 2001 10:00:15 AM

I use java annoymous inner classes for things like menus and window close events. Since each MenuItem should do something different you either need to be able to parameterize some ActionListener subclass using Method objects (which are so ugly) or you can simply use inner classes.

For example I want a menu item that when selected calls a method:

MenuItem file1 = new MenuItem("save");
file1.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent e)
{Global.save();}
});

so I''m creating a class that doesn''t actually have a name, and I don''t even need to use the class keyword. Since the definition follows new ActionListener() it automatically implements ActionListener, so I override the method and that''s it. As for the named inner classes, I haven''t really figured them out yet so I can''t help you with that.