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From Stefan Seelmann <seelm...@apache.org>
Subject Re: Immutable objects, what's best ?
Date Sun, 14 Mar 2010 13:19:51 GMT
Emmanuel Lecharny wrote:
> On 3/14/10 9:46 AM, Stefan Seelmann wrote:
>> Emmanuel Lecharny schrieb:
>>   
>>> Hi,
>>>
>>> we have many objects that we want to be immutable. What's the best
>>> solution
>>> to produce  those immutable objets ?
>>>
>>> - For DN, we would like to use valueOf(), and the DN() constructor,
>>> but no
>>> setter
>>> - For Entry, a constructor is not enough, as we may have to inject new
>>> attributes. We may need to have two different classes, one immutable,
>>> one
>>> mutable. The immutable class could be associated with a factory, or
>>> we can
>>> use a constructor with the list of attributes as a parameter.
>>> - For attributes, we have the same problem : we may have more than one
>>> value.
>>>
>>> DO any of you guys have a strong opinion ?
>>>      
>> Entry and Attribute object created by the user of the API shouldn't be
>> immutable. As user of the API I want to create an Entry object and add
>> attribute and values to it. So the API must provide setters.
>>    
> I came to the same conclusion, but with another idea : create two Entry
> objects, one for the client, one for the server. It's close to what we
> have on ADS, but the more I think about it, the more I find it complex
> and bothersome.
> Now, let's think about another option : what if we add a parameter in
> the constructor to create Immutable Entries ? Something like :
> Entry immutableEntry = new EntryImpl( DN, true ); // True => the entry
> is immutable ?
> 
>> So I think if Entry and Attribute are interfaces we just define the
>> getter methods.
>>    
> Smart ! That could do the trick, sure ! But is it better than the
> previous solution?
> 
> My idea was to forbid a setter to be called if the Immutable flag is
> set, a solution I find a bit more strong than hiding the setter though
> the interface, but it forces the user to add a flag to the constructor.
> 
> Again, what do you think is the best approach ?

I don't like it if a setXXX() method is provided and if I call it some
"YouIdiotAreNotAllowedToCallThisMethodException" is thrown.

So IMO having the interface which provides read-only methods is best.

And if using the EntryImpl you have all the setters and the setters work.

Then you can obtain an immutable Entry from that EntryImpl, see below.

>> The default implementations of those classes (e.g. ClientEntry and
>> ClientAttribute) additional have setters the user can use when
>> constructing the objects.
>>
>> The Entry objects returned from the API (e.g. from a search) should be
>> immutable to protect them from being casted by the user, e.g. by
>> wrapping a created ClientEntry into an ImmutableEntry implementation.
>>    
> Hmmm... Do you suggest that we should define 2 different classes ? (one
> immutable, one mutable). Wouldn't it be better to have one single
> implementation, with a limited interface, forcing the user to cast to be
> able to use the setters ? Or should we have a ClientEntry class
> extending a ServerEntry class ?

I wouldn't call it ClientEntry and ServerEntry, sorry that I used that
name before ;-)

In the public API I would define an default implementation (not sure
about the name, we discussed some names before) which is mutable. This
implementation is used by the user because I don't think he wants to use
an immutable implementation.

The immutable implementation just works like the java.util.Collections
class, there are static methods like unmodifyableSet(Set set). So we can
have some Utils class which wraps the default mutable class into an
immutable. Or the default Entry implmentation could provide a method
"toImmutableEntry()". The immutable implementation shouldn't be part of
the public API.

wdyt?

Kind Regards,
Stefan


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