I was wondering if it would be possible to have a field in a class that is not beeing mapped by SORM. I tried to add a #Transient annotation but ist was ignored.
I this part of SORM's paradigms to not do so?
It's impossible. However, since I don't see how anyone could benefit from such a feature, I suspect you're missing something essential about concepts utilized in SORM, such as operating on immutable data structures only, the idiomatic "absence" of nulls and stuff like that.
Related
This is a question independent from languages.
Conceptually, it's good to code for interfaces(contracts) instead of specific implementations. I've got no problem understanding merits about the practice.
However, when I really code in that practice, the users of my classes, from time to time need to cast the interfaces for specific needs of specific functions provided by specific classes that implement that interface.
I understand there must be something wrong, either on my side or on the user's side, as the interface should expose all methods/properties(in the case of c#) that can possibly be necessary.
The code base is huge, and the users are clients.
It won't be particularly easy to make changes on either side.
That makes me wonder some downsides about using interface as parameter and return type.
Can people please list demerits of the practice? And please, include any solution if you know how to work around it.
Thanks a lot for enlightening me.
EDIT:
To be a bit more specific:
Assume we have a class called DbInfoExtractor. It has a public method GetInfo, as follows:
public IInformation GetInfo(IInfoParam);
where IInformation is an interface implemented by specific classes like VideoInfo, AudioInfo, TextInfo, etc; IInfoParam is an interface implemented by specific classes like VidoeInfoParam, AudioInfoParam, TextInfoParam, etc;
Apparently, depending on the specific object passed into the method GetInfo, the DbInfoExtractor needs to take different actions, as it is reasonable to assume that for different types of information, the extractor considers different sets of aspects(e.g. {size, title, date} for video, {title, author} for text information, etc) as search keys and search for relevant information in different ways.
Here, I see two options to go on:
1, using if ... else ... to decide what actually to take depending on the type of the parameter the GetInfo method receives. This is certainly bad, as avoiding this situation is one the very reasons we use polymorphism.
2, We should call IInfoParam.TakeAction(), and each specific implementation of IInfoParam has its own TakeAction() method to actually search and find the corresponding information from the database.
This options seems better, but still quite bad, as it shouldn't be the parameter that takes action searching and finding the information; it should be the responsibility of DbInfoExtractor.
So how can I delegate the TakeAction back to DbInfoExtractor? (I actually wrote some code to do this, but it's neither standard nor elegant. Basically I make parameter classes nested classes in DbInfoExtractor, so that they can call various versions of TakeAction of DbInfoExtractor.)
Please enlighten me!
Thanks.
Thanks.
Why not
public IVideoInformation GetVideoInformation(VideoQuery);
public IAudioInformation GetAudioInformation(AudioQuery);
// etc.
It doesn't look like there's a need for polymorphism here.
The query types are Query Objects, if you need those. They probably don't need to be interfaces; they know nothing about the database. A simple list of parameters (maybe just ID) might be sufficient.
The question is what does the client have, and what do they want? That's your interface.
Switch statements and casting are a smell, and typically mean that you've violated the Liskov substitution principle.
I am looking for the correct design pattern to use in the following situation:
I have a process which runs and during this process I need to attach several properties to an object in the system. The object is of the same type but at runtime it might exhibit slightly different behaviour and therefore the way the properties are set can be different depending on the type.
Regardless of the type and behaviour of these objects I want to set the same properties on each.
I then need an object to parse these properties at another point in the process
What is the best way to approach this?
I would suggest you not try to pick a design pattern before coding. First, write the code. Then, start abstracting any redundant code, or code that varies. To understand abstracting code that varies, read Head First Design Patterns. At the very beginning of that book is an example of abstracting what varies, using the strategy pattern. The SimUDuck example there is one of the best-explained examples I've ever seen of the strategy pattern. It sounds like that's what you're asking about. However, your question doesn't have a concrete example of what you're trying to do, so giving a concrete example is difficult here.
Having said that, it sounds like you need good, ol' fashioned polymorphism here: you need to treat all objects the same way, and set the same properties, just with different values. To do this, create an interface and have all of your different types implement that interface. Then, in the calling/consuming code, deal with each of those concrete types as the interface.
If you try to pick a design pattern first, many times you'll end up finding that things change based on the details of the implementation, and your original guess at a design pattern ends up being the wrong fit. Then you end up coding to meet a design pattern and not solving the real problem. Write the code first, even if it's ugly. Get it working. Then find areas to abstract and it will naturally evolve into a design pattern on its own.
If i properly understand, you want add behaviours in runtime???
If yes, so i think - decorator (aka wrapper) design pattern can be good.
I have a question related to general OOP than specific to a language.
I was trying out a simple application (in java) and I was trying to model it like a real world scenario.
While re-factoring I realized that I came up with a simple object that just has one member and an overridden equals and hashcode.
My question is.... is it a bad oo practice to have such objects
(references to blogs etc would be welcome)
Short answer:
is it a bad oo practice to have such objects
Not necessarily, but it depends on the context.
Longer answer:
I have a question related to general OOP than specific to a language. I was trying out a simple application (in java) and I was trying to model it like a real world scenario.
There really isn't any rule stating that you should. In fact, I know of quite a few people who frown upon that statement, Uncle Bob Martin for one. It's more about modelling business processes than it is to model "real world scenarios". I've tried that in the past, and found there's no - or almost no - benefit to get from rigidly trying to model everything as it is in the real world. If anything, I think it makes your application more complex, and the more complex software becomes, the harder it becomes to maintain.
While re-factoring I realized that I came up with a simple object that just has one member and an overridden equals and hashcode.
Might be okay, as #Arseny already said, the ValueObject is a well-known way of working, although I usually don't end up with a lot of them when I write code. If more than a few of your objects doesn't have any behaviour, this might be an indication of a so-called Anemic Domain Model, which you have to be careful for (more complexity at no apparent benefit).
You can find out if you're "doing it wrong" (with variable values of "wrong", of course): just see what the collaborators are doing with your ValueObject, and see if there's anything there that resembles a calculation which actually belongs to the object itself.
However, if this is one of the few objects that doesn't contain any behaviour: well, yeah, that happens and you probably don't have to worry about it. We'd have to see some code to be conclusive in our anwers though.
For this case, no, because that's the only way to redefine the behavior of an object in a hashing data structure in Java.
For other cases there may be better and worse methods of doing things depending on whether they make sense, for example, if I want to change the order of objects in a queue, I'd prefer to implement a custom Comparator rather than inherit and override a compareTo method, especially if my new comparison routine is not "natural" for the objects.
Every design pattern has some cases that it's appropriate for and others that it's inappropriate for.
Normally, it would be considered a smell to have an object with no behaviour. The reason being that if it doesn't have any behaviour, then it isn't an object. When desiging your class you should be asking things like, "what is the class responsible for?". If it doesn't have any behaviour then this is a difficult questions to answer.
Rare exceptions to this being something like the Null Object pattern.
http://en.wikipedia.org/wiki/Null_Object_pattern
I may be that the member of your class should actually be a member of another class.
It may also be that your class has some functionality that you haven't discovered yet.
It may also be that you are putting too much importance on the concept when a primitive type would do.
There are a number of techniques for designing OO systems, here is one of the original:
http://en.wikipedia.org/wiki/Class-responsibility-collaboration_card
No it is not bad. There is Value Object pattern witch widely used and DTO pattern as well.
I use the Scanner class for reading multiple similar files. I would like to extend it to make sure they all use the same delimiter and I can also add methods like skipUntilYouFind(String thisHere) that all valid for them all.
I can make a utility-class that contain them, or embed the Scanner Class as a variable inside another class but this is more cumbersome.
I have found some reasons to declare a class final, but why is it done here?
Probably because extending it and overwriting some of it's methods would probably break it. And making it easier to overwrite methods would expose to much of the inner workings, so if in the future they decide to change those (for performance or some other reasons), it would be harder for them to change the class without breaking all the classes that extend it.
For example, consider the following method in the class:
public boolean nextBoolean() {
clearCaches();
return Boolean.parseBoolean(next(boolPattern()));
}
Say you want to overwrite this because you want to make 'awesome' evaluate to a 'true' boolean (for whatever reason). If you overwrite it, you can't call super.nextBoolean(), since that would consume the next token using the default logic. But if you don't call super.nextBoolean(), clearCaches() won't be called, possibly breaking the other not overwritten methods. You can't call clearCaches() because it's private. If they made it protected, but then realized that it's causing a performance problem, and wanted a new implementation that doesn't clear caches anymore, then they might break your overwritten implementation which would still be calling that.
So basically it's so they can easily change the hidden parts inside the class, which are quite complex, and protecting you from making a broken child class (or a class that could be easily be broken).
I suppose it is due to security reasons. This class reads user input, so that someone with bad intentions could extend it, modify it's behavior and you'd be screwed. If it is final, it is not that easy for the bad guy, because if he makes his own type of Scanner (not java.util.Scanner), the principles of Polymorphism would be broken. See the bad guy can be smart enough to write a bot/script which does this automatically on remote servers... He can even do it by dynamic classloading in compiled application.
I think that the link you provided explains it all.
In your case it seems like you should prefer composition instead of inheritance anyway. You are creating a utility that has some predefined behavior, and that can hide some (or all) of the details of the Scanner class.
I've seen many implementations that used inheritance in order to change a behavior. The end result was usually a monolithic design, and in some cases, a broken contract, and/or broken behavior.
I've read all the books about why to create a class and things like "look for the nouns in your requirements" but it doesn't seem to be enough. My classes seem to me to be messy. I would like to know if there are some sort of metrics or something that I can compare my classes to and see if there well designed. If not, who is the most respected OO guru where I can get the proper class design tips?
Creating classes that start clean and then get messy is a core part of OO, that's when you refactor. Many devs try to jump to the perfect class design from the get go, in my experience that's just not possible, instead you stumble around, solving the problem and then refactor. You can harvest, base classes and interfaces as the design emerges.
if you're familiar with database design, specifically the concept of normalization, then the answer is easy: a data-centric class should represent an entity in third normal form
if that is not helpful, try this instead:
a class is a collection of data elements and the methods that operate on them
a class should have a singular responsibility, i.e. it should represent one thing in your model; if it represents more than one thing then it should be more than one class.
all of the data elements in a class should be logically associated/related to each other; if they aren't, split it into two or more classes
all of the methods in a class should operate only on their input parameters and the class's data elements - see the Law of Demeter
that's about as far as i can go with general abstract advice (without writing a long essay); you might post one of your classes for critique if you need specific advice
Try to focus on behaviour instead of structure. Objects are 'living' entities with behaviour and responsibilities. You tell them to do things. Have a look at the CRC-card approach to help you model this way.
i think Object design is as much art as it is science. It takes time and practice to understand how to design clean & elegant classes. Perhaps if you can give an example of a simple class you've designed that you aren't happy with SO users can critique and give pointers. I'm not sure there are any general answers outside of what you've already read in the texts.
The most respected OO guru i personally know is StackOverflow. Put your classnames here and i reckon you'll get a goodly number of reviews.
Classes are typically used to model concepts of the problem domain. Once you have a well-defined problem (aka the set of use cases), you will be able to identify all participants. A subset of the participants will be intrinsic to the system you are designing. Start with one big black box as your system. Keep breaking it down, as and when you have more information. When you have a level where they can no longer be broken down (into concepts in your problem domain), you start getting your classes.
But then, this is a subjective view of a non-guru. I'd suggest a pinch of salt to the menu.
Metrics? Not so's that you'd trust them.
Are your classes doing the job of getting the program working and keeping it maintainable through multiple revisions?
If yes, you're doing ok.
If no, ask yourself why not, and then change what isn't working.