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Unit Testable convention for Service "Helper Classes" in DDD pattern

I'm fairly new to Java and joining a project that leverages the DDD pattern (supposedly). I come from a strong python background and am fairly anal about unit test driven design. That said, one of the challenges of moving to Java is the testability of Service layers.
Our REST-like project stack is laid out as follows:
ServiceHandlers which handles request/response, etc and calls specific implementations of IService (eg. DocumentService)
DocumentService - handles auditing, permission checking, etc with methods such as makeOwner(session, user, doc)
Currently, something like DocumentService has repository dependencies injected via guice. In a public method like DocumentService.makeOwner, we want to ensure the session user is an admin as well as check if the target user is already an owner (leveraging the injected repositories). This results in some dupe code - one for both users involved to resolve the user and ensure membership, permissions, etc etc. To eliminate this redundant code, I want make a sort of super simpleisOwner(user, doc) call that I can concisely mock out for various test scenarios (such as throwing the exception when the user can't be resolved, etc). Here is where my googling fails me.
If I put this in the same class as DocumentService, I can't mock it while testing makeOwner in the same class (due to Mockito limitations) even though it somewhat feels like it should go here (option1).
If I put it in a lower class like DocumentHelpers, it feels slightly funny but I can easily mock it out. Also, DocumentHelpers needs the injected repository as well, which is fine with guice. (option 2)
I should add that there are numerous spots of this nature in our infant code base that are untestable currently because methods are non-statically calling helper-like methods in the same *Service class not used by the upper ServiceHandler class. However, at this stage, I can't tell if this is poor design or just fine.
So I ask more experienced Java developers:
Does introducing "Service Helpers" seem like a valid solution?
Is this counter to DDD principals?
If not, is there are more DDD-friendly naming convention for this aside from "Helpers"?
3 bits to add:
My googling has mostly come up with debates over "helpers" as static utility methods for stateless operations like date formatting, which doesn't fit my issue.
I don't want to use PowerMock since it breaks code coverage and is pretty ugly to use.
In python I'd probably call the "Service Helper" layer described above as internal_api, but that seems to have a different meaning in Java, especially since I need the classes to be public to unit test them.
Any guidance is appreciated.

That the user who initiates the action must be an admin looks like an application-level access control concern. DDD doesn't have much of an opinion about how you should do that. For testability and separation of concerns purposes, it might be a better idea to have some kind of separate non-static class than a method in the same service or a static helper though.
Checking that the future owner is already an owner (if I understand correctly) might be a different animal. It could be an invariant in your domain. If so, the preferred way is to rely on an Aggregate to enforce that rule. However, it's not clear from your description whether Document is an aggregate and if it or another aggregate contains the data needed to tell if a user is owner.
Alternatively, you could verify the rule at the Application layer level but it means that your domain model could go inconsistent if the state change is triggered by something else than that Application layer.

As I learn more about DDD, my question doesn't seem to be all that DDD related and more just about general hierarchy of the code structure and interactions of the layers. We ended up going with a separate DocumentServiceHelpers class that could be mocked out. This contains methods like isOwner that we can mock to return true or false as needed to test our DocumentService handling more easily. Thanks to everyone for playing along.

How can I apply oo design patterns in this situation?

Situation: Suppose we're designing the UI of Windows 9 using Java API. We need to build up 3 classes main, BuildInWindow and ApplicationWindow.
main - the window for rendering the system UI (i.e. the start botton & wallpaper page)
BuildInWindow- windows for rendering buildt-in apps (e.g. IE)
ApplicationWindow- windows for rendering apps from third party (e.g. eclipse)
all of them have to implement 3 Java API interfaces, WindowFocusListener, WindowListener and WindowStateListener and have the methods onExit() and onCrushing().
onExit() performs when the system/built-in app/ third-party app is shut down normally
onCrushing() captures any system/application crush and send system state back to server
This is the original design:
http://i.stack.imgur.com/JAJiY.png
I have some ideas of how to design it in a OO manner, but I am not sure if that's the right way. Here's my thoughts:
Create an abstract class with method onExit() and onCrushing(). Since the code of onExit()would vary from 3 classes, it should be an abstract method & onCrushing()would be same fo all classes, so it would be an concrete method
tHE MAIN WINdow should use singleton design to ensure user only create one instance of main.
Use the facade design to save the trouble of implementing 3 interfaces to three classes
My question is I don't really understand facade design, so I am not sure if it can be applied in this case. Also I am not really sure if onExit() would be different for 3 classes and onCrushing() would perform the same function.
I tried my best to explain the question clearly...if you don't understand free free to comment. Thank you very much!

I've left some questions in a comment linked to your question but here's some guidance for you:
You shouldn't create an abstract class on the basis of both BuildInwindow and ApplicationWindow both having to have methods #onExit and #onCrushing if they are not to share any implementation. Abstract classes are most useful where there is a common implementation. An interface containing these methods would be sufficient. That said, your two windows may share other functionality and, if so, it could be shared through a common superclass (abstract if it relies on subclass implementation detail). You may find the Template Method pattern useful for managing the overall window mechanism with specific tailoring for different window types. You may also find the Factory Method means of instance creation (for your window classes) will help separate the object creation and set-up from the creation mechanism.
A single shared instance would seem sensible and a singleton would serve this purpose (so long as you're able to handle termination, etc). Alternatively, your application may just launch a single Main instance - you may even just hide the constructor through package access to ensure no others are created.
The facade pattern just serves to simplify a complex interface. It mainly does this by rolling calls to collaborating instances together under a single (coarser) interface. This wouldn't normally be a done to hide which interfaces a class supports. Indeed, publishing which interfaces a class extends is important to API users. You could roll the three interfaces into a single interface for "convenience" but I think this is unnecessary. If you do settle on a common superclass then that would "extend" the three interfaces (if all subclasses were expected to support them). It may also implement some default implementation of these interfaces (again, watch access modifiers to ensure those you intend to be can be overridden while others may be final).
Edit: Guidance
You just have to identify the classes and relationships:
I suggest you just grab some paper and draw. You already have your nouns and verbs (you can otherwise go noun and verb spotting to identify classes and methods on them).
So, why not draw a simple diagram containing all the info (A, B, C, Main, etc) and draw the relationships between them. This is your start point. You may have some confusion when working out how Main links to the window classes (given there are two kinds). Just write a note on it and move on to clarify the rest of the picture.
Next, refine your diagram to start moving common features into a single place (abstraction). You know this exists with regards to your interfaces and the methods you suggest but you may need to decide which (if any) have any common functionality. Then decide if interfaces satisfies your needs (methods are common but implementations are different) or if the implementation itself is the same and so a parent superclass may be useful (this addresses abstraction [who is responsible for what], encapsulation [individual implementations at the appropriate level] and polymorphism [which classes support common methods]). Note that, even if you settle on an superclass, you'd be wise to back it with an interface (it makes introduction of sibling or replacement classes easier in time - think maintenance).
Next, work on the issues you found. Has your draft design clarified any of them? For instance, your Main needs to know about its windows but - what type are they? So, has any of your refinement made this clearer?
Do any patterns present themselves? for this you need to already have a feel for design patterns I'm afraid so buy and absorb the GoF Design Patterns book. It'll put you in good stead for spotting patterns as you go. I'd also recommend reading this specific book before taking on any others as it's technology agnostic (and some other books arebloated with tech-specific workarounds). Perhaps study the two patterns I pointed out and see if they fit your requirement.
On the whole though, your ideas seem to be going in the right direction.

Java Interfaces Methodology: Should every class implement an interface?

I've been programming in Java for a few courses in the University and I have the following question:
Is it methodologically accepted that every class should implement an interface? Is it considered bad practice not to do so? Can you describe a situation where it's not a good idea to use interfaces?
Edit: Personally, I like the notion of using Interfaces for everything as a methodology and habit, even if it's not clearly beneficial. Eclipse automatically created a class file with all the methods, so it doesn't waste any time anyway.

You don't need to create an interface if you are not going to use it.
Typically you need an interface when:
Your program will provide several implementations for your component. For example, a default implementation which is part of your code, and a mock implementation which is used in a JUnit test. Some tools automate creating a mock implementation, like for instance EasyMock.
You want to use dependency injection for this class, with a framework such as Spring or the JBoss Micro-Container. In this case it is a good idea to specify the dependencies from one class with other classes using an interface.

Following the YAGNI principle a class should implement an interface if you really need it. Otherwise what do you gain from it?
Edit: Interfaces provide a sort of abstraction. They are particularly useful if you want to interchange between different implementations(many classes implementing the same interface). If it is just a single class, then there is no gain.

No, it's not necessary for every class to implement an interface. Use interfaces only if they make your code cleaner and easier to write.
If your program has no current need for to have more than 1 implementation for a given class, then you don't need an interface. For example, in a simple chess program I wrote, I only need 1 type of Board object. A chess board is a chess board is a chess board. Making a Board interface and implementing that would have just required more code to write and maintain.
It's so easy to switch to an interface if you eventually need it.

Every class does implement an interface (i.e. contract) insofar as it provides a non-private API. Whether you should choose to represent the interface separately as a Java interface depends on whether the implementation is "a concept that varies".
If you are absolutely certain that there is only one reasonable implementation then there is no need for an interface. Otherwise an interface will allow you to change the implementation without changing client code.
Some people will shout "YAGNI", assuming that you have complete control over changing the code should you discover a new requirement later on. Other people will be justly afraid that they will need to change the unchangeable - a published API.
If you don't implement an interface (and use some kind of factory for object creation) then certain kinds of changes will force you to break the Open-Closed Principle. In some situations this is commercially acceptable, in others it isn't.
Can you describe a situation where it's not a good idea to use interfaces?
In some languages (e.g. C++, C#, but not Java) you can get a performance benefit if your class contains no virtual methods.
In small programs, or applications without published APIs, then you might see a small cost to maintaining separate interfaces.
If you see a significant increase in complexity due to separating interface and implementation then you are probably not using interfaces as contracts. Interfaces reduce complexity. From the consumer's perspective, components become commodities that fulfil the terms of a contract instead of entities that have sophisticated implementation details in their own right.

Creating an interface for every class is unnecessary. Some commonly cited reasons include mocking (unneeded with modern mocking frameworks like Mockito) and for dependency injection (e.g. Spring, also unneeded in modern implementations).
Create an interface if you need one, especially to formally document public interfaces. There are a couple of nifty edge cases (e.g. marker interfaces).
For what it's worth, on a recent project we used interfaces for everything (both DI and mocking were cited as reasons) and it turned out to be a complete waste and added a lot of complexity - it was just as easy to add an interface when actually needed to mock something out in the rare cases it was needed. In the end, I'm sure someone will wind up going in and deleting all of the extraneous interfaces some weekend.
I do notice that C programmers first moving to Java tend to like lots of interfaces ("it's like headers"). The current version of Eclipse supports this, by allowing control-click navigation to generate a pop-up asking for interface or implementation.

To answer the OP's question in a very blunt way: no, not all classes need to implement an interface. Like for all design questions, this boils down to one's best judgment. Here are a few rule of thumbs I normally follow:
Purely functional objects probably
don't need to (e.g. Pattern,
CharMatcher – even though the
latter does implement Predicate, it
is secondary to its core function)
Pure data holders probably don't need
to (e.g. LogRecord, Locale)
If you can
envision a different implementation
of a given functionality (say, in-memory
Cache vs. disk-based Cache), try to
isolate the functionality into an interface. But don't go too far trying to predict the future either.
For testing purposes, it's
very convenient when classes that do
I/O or start threads are easily mockable, so
that users don't pay a penalty when
running their tests.
There's nothing
worse than a interface that leaks its
underlying implementation. Pay attention where you draw the line and make sure your interface's Javadoc is neutral in that way. If it's not, you probably don't need an interface.
Generally
speaking, it is preferable for
classes meant for public consumption
outside your package/project to
implement interfaces so that your
users are less coupled to your
implementation du jour.
Note that you can probably find counter-examples for each of the bullets in that list. Interfaces are very powerful, so they need to be used and created with care, especially if you're providing external APIs (watch this video to convince yourself). If you're too quick in putting an interface in front of everything, you'll probably end up leaking your single implementation, and you are only making things more complicated for the people following you. If you don't use them enough, you might end up with a codebase that is equally hard to maintain because everything is statically bound and very hard to change. The non-exhaustive list above is where I try to draw the line.

I've found that it is beneficial to define the public methods of a class in a corresponding interface and when defining references to other classes strictly use an interface reference. This allows for easy inversion of control, and it also facilitates unit testing with mocking and stubbing. It also gives you the liberty of replacing the implementation with some other class that implements that interface, so if you are into TDD it may make things easier (or more contrived if you are a critic of TDD)

Interfaces are the way to get an polymorphism. So if You have only one implementation, one class of particularly type, You don't need an interface.

A good way of learning what are considered good methodologies, especially when it comes to code structure design, is to look at freely available code. With Java, the obvious example is to take a look at the JDK system libraries.
You will find many examples of classes that do not implement any interfaces, or that are meant to be used directly, such as java.util.StringTokenizer.

If you use Service Provider Interface pattern in your application interfaces are harder to extend than abstract classes. If you add method to interface, all service providers must be rewritten. But if you add non-abstract method to the abstract class, none of the service providers must be rewritten.
Interfaces also make programming harder if only small part of the interface methods usually have meaningfull implementation.

When I design a new system from scratch I use a component oriented approach, each component (10 or more classes) provide an interface, this allows me (sometimes) to reuse them.
When designing a Tool (Or a simple system) I think this must not necessarily be an extensible framework I introduce interfaces when I need a second implementation as an option.
I saw some products which exposed nearly every functionality by an interface, it took simply too much time to understand unnecessary complexity.

An interface is like a contract between a service provider (server) and the user of such a service (client).
If we are developing a Webservice and we are exposing the rest routes
via controller classes, controller classes can implement interfaces
and those interfaces act as the agreement between web service and the
other applications which use this web service.
Java interfaces like Serializable, Clonnable and Remote
used to indicate something to compiler or JVM.When JVM sees a class
that implement these interfaces, it performs some operation on the to
support Serialization, cloning or Remote Method Invocation. If your class needs these features, then you will have to implement these interfaces.

Using Interface is about to make your application framework resilient to change. Since as I mentioned here (Multiple Inheritance Debates II: according to Stroustrup) multiple inheritance was cancelled in java and c# which I regret, one should always use Interface because you never know what the future will be.

OK to put my public interfaces into their own package

Would it be OK to put my public interfaces into their own package (for my organisation only).
for example
com.example.myprogram - contains all normal code
com.example.myprogram.public - contains public accessible interfaces
com.example.myprogram.abstract - contains abstract classes
Is this a good or a bad thing to do, are there any disadvantages?

I wouldn't like this practice at all. You should group classes, both abstract and concrete, and interfaces according to functionality.
Look at the Java API as an example. Did Sun separate the Collections interfaces from implementations? No. Sun's practices aren't always the best guide, but in this case I agree.
Don't do it.

I can suggest you 2 common ways:
If you really think that your interfaces can have more implementations in future (i.e. you're working on API) then move them to a separate module and create there special package with name 'core', for example. (com.example.myprogram.core). Implementations should be in correspondent packages (like com.example.myprogram.firstimpl).
If you have only 1 implementation then let all your interfaces be in com.example.myprogram package and all concrete classes in com.example.myprogram.impl package.

I can't see that as being bad practice, however you might wanna consider as an alternative organizing your stuff per logical functionality rather than syntactic definition, so that all code for a given unit of functionality interfaces/abstract classes/normal code goes in the same package. This is one of the principles of modular programming.
Said so, putting all the interfaces (but only those) in a separated package might be necessary depending on the size of the project, and might eve become almost necessary if you have a pure component based plugin architecture (so that other module know only about interfaces and the actual implementation is somehow dynamically injected).

Public interfaces are a formal contract between system modules or systems. Because of that, it makes sense to isolate them from the remainder of the code, to make them stand out.
For example, in a system I've worked on, all public interfaces between the server and client components of the system have been placed in a special system module (called, no surprise, "api"). This has a number of desirable effects, among which these:
- semantically, you know where to look if you need any kind of information on how communication should take place
- you can version the api module separately, which is especially useful when you don't want a moving target, i.e. you sign a contract to deliver an application which will support "the api v.1.1" rather than constantly playing catch while someone else changes the interface and requires you to adapt your side
That doesn't mean you shouldn't organize them further in sub-packages to distinguish what they are for. :)
In summary, you are doing the right thing by separating the interfaces from the rest of the code base, although depending on your specific needs, you might do well to take it a step further and isolate the interfaces in a separate system module.

Java getter/setter style question

I have a question about Java style. I've been programming Java for years, but primarily for my own purposes, where I didn't have to worry much about style, but I've just not got a job where I have to use it professionally. I'm asking because I'm about to have people really go over my code for the first time and I want to look like I know what I'm doing. Heh.
I'm developing a library that other people will make use of at my work. The way that other code will use my library is essentially to instantiate the main class and maybe call a method or two in that. They won't have to make use of any of my data structures, or any of the classes I use in the background to get things done. I will probably be the primary person who maintains this library, but other people are going to probably look at the code every once in a while.
So when I wrote this library, I just used the default no modifier access level for most of my fields, and even went so far as to have other classes occasionally read and possibly write from/to those fields directly. Since this is within my package this seemed like an OK way to do things, given that those fields aren't going to be visible from outside of the package, and it seemed to be unnecessary to make things private and provide getters and setters. No one but me is going to be writing code inside my package, this is closed source, etc.
My question is: is this going to look like bad style to other Java programmers? Should I provide getters and setters even when I know exactly what will be getting and setting my fields and I'm not worried about someone else writing something that will break my code?

Even within your closed-source package, encapsulation is a good idea.
Imagine that a bunch of classes within your package are accessing a particular property, and you realize that you need to, say, cache that property, or log all access to it, or switch from an actual stored value to a value you generate on-the-fly. You'd have to change a lot of classes that really shouldn't have to change. You're exposing the internal workings of a class to other classes that shouldn't need to know about those inner workings.

I would adhere to a common style (and in this case provide setters/getters). Why ?
it's good practise for when you work with other people or provide libraries for 3rd parties
a lot of Java frameworks assume getter/setter conventions and are tooled to look for these/expose them/interrogate them. If you don't do this, then your Java objects are closed off from these frameworks and libraries.
if you use setters/getters, you can easily refactor what's behind them. Just using the fields directly limits your ability to do this.
It's really tempting to adopt a 'just for me' approach, but a lot of conventions are there since stuff leverages off them, and/or are good practise for a reason. I would try and follow these as much as possible.

I don't think a good language should have ANY level of access except private--I'm not sure I see the benefit.
On the other hand, also be careful about getters and setters at all--they have a lot of pitfalls:
They tend to encourage bad OO design (You generally want to ask your object to do something for you, not act on it's attributes)
This bad OO design causes code related to your object to be spread around different objects and often leads to duplication.
setters make your object mutable (something that is always nice to avoid if you can)
setters and getters expose your internal structures (if you have a getter for an int, it's difficult to later change that to a double--you have to touch every place it was accessed and make sure it can handle a double without overflowing/causing an error, if you had just asked your object to manipulate the value in the first place, the only changes would be internal to your object.

Most Java developers will prefer to see getters and setters.
No one may be developing code in your package, but others are consuming it. By exposing an explicitly public interface, you can guarantee that external consumers use your interface as you expect.
If you expose a class' internal implementation publicly:
It isn't possible to prevent consumers from using the class inappropriately
There is lost control over entry/exit points; any public field may be mutated at any time
Increase coupling between the internal implementation and the external consumers
Maintaining getters and setters may take a little more time, but offers a lot more safety plus:
You can refactor your code any time, as drastically as you want, so long as you don't break your public API (getters, setters, and public methods)
Unit testing well-encapsulated classes is easier - you test the public interface and that's it (just your inputs/outputs to the "black box")
Inheritance, composition, and interface designs are all going to make more sense and be easier to design with decoupled classes
Decide you need to add some validation to a mutator before it's set? One good place is within a setter.
It's up to you to decide if the benefits are worth the added time.

I wouldn't care much about the style per se (or any kind of dogma for that matter), but rather the convenience in maintainability that comes with a set of getter/setter methods. If you (or someone else) later needed to change the behavior associated with a change of one of those attributes (log the changes, make it thread-safe, sanitize input, etc.), but have already directly modified them in lots of other places in your code, you will have wished you used getter and setter methods instead.

I would be very loath to go into a code review with anything but private fields, with the possible exception of a protected field for the benefit of a subclass. It won't make you look good.
Sure, I think from the vantage point of a Java expert, you can justify the deviation from style, but since this is your first professional job using Java, you aren't really in that position.
So to answer your question directly: "Is this going to look like bad style?" Yes, it will.
Was your decision reasonable? Only if you are really confident that this code won't go anywhere. In a typical shop, there may be chances to reuse code, factor things out into utility classes, etc. This code won't be a candidate without significant changes. Some of those changes can be automated with IDEs, and are basically low risk, but if your library is at the point where it is stable, tested and used in production, encapsulating that later will be regarded as a bigger risk than was needed.

Since you're the only one writing code in your closed-source package/library, I don't think you should worry too much about style - just do what works best for you.
However, for me, I try to avoid directly accessing fields because it can make the code more difficult to maintain and read - even if I'm the sole maintainer.

Style is a matter of convention. There is no right answer as long as it is consistent.
I'm not a fan of camel, but in the Java world, camelStyle rules supreme and all member variables should be private.
getData();
setData(int i);
Follow the Official Java code convention by Sun (cough Oracle) and you should be fine.
http://java.sun.com/docs/codeconv/

To be brief, you said "I'm asking because I'm about to have people really go over my code for the first time and I want to look like I know what I'm doing". So, change your code, because it does make it look like you do not know what you are doing.
The fact that you have raised it shows that you are aware that it will probably look bad (this is a good thing), and it does. As has been mentioned, you are breaking fundamentals of OO design for expediency. This simply results in fragile, and typically unmaintainable code.

Even though it's painful, coding up properties with getters and setters is a big win if you're ever going to use your objects in a context like JSP (the Expression Language in particular), OGNL, or another template language. If your objects follow the good old Bean conventions, then a whole lot of things will "just work" later on.

I find getters and setters are better way to program and its not about only a matter of coding convention. No one knows the future, so we can write a simple string phonenumber today but tomorrow we might have to put "-" between the area code and the number, in that case, if we have a getPhonenumber() method defined, we can do such beautifications very easily.
So I would imagine, we always should follow this style of coding for better extensibility.

Breaking encapsulation is a bad idea. All fields should be private. Otherwise the class can not itself ensure that its own invariants are kept, because some other class may accidentally modify the fields in a wrong way.
Having getters and setters for all fields is also a bad idea. A field with getter and setter is almost the same as a public field - it exposes the implementation details of the class and increases coupling. Code using those getters and setters easily violates OO principles and the code becomes procedural.
The code should be written so that it follows Tell Don't Ask. You can practice it for example by doing an Object Calisthenics exercise.

Sometimes I use public final properties w/o get/setter for short-living objects which just carry some data (and will never do anything else by design).
Once on that, I'd really love if Java had implied getters and setters created using a property keyword...

Using encapsulation is a good idea even for closed source as JacobM already pointed out. But if your code is acting as library for other application, you can not stop the other application from accessing the classes that are defined for internal use. In other words, you can not(?) enforce restriction that a public class X can be used only by classes in my application.
This is where I like Eclipse plugin architecture where you can say what packages in my plugin can dependent plugins access during runtime. JSR 277 aimed at bringing this kind of modular features to JDK but it is dead now. Read more about it here,
http://neilbartlett.name/blog/2008/12/08/hope-fear-and-project-jigsaw/
Now the only option seems to be OSGi.

While I am well aware about the common pressure to use getters and setters everywhere regardless the case, and the code review process leaves me no choice, I am still not convinced in the universal usefulness of this idea.
The reason, for the data carrying classes, over ten years of development it has never been for me a single case where I would write anything different from set the variable in the setter and read the variable in the getter while lots of time has been spent on generating, understanding and maintaining this cargo cult code that seems not making any sense.
The data class is a structure or record, not a class. It does not do anything itself. Other classes are making changes to it. It should not be any functionality there at all, leave alone the functionality in the setters or getters. Java probably needs a separate keyword for the multi-field data record that has no methods.
From the other side, the process seems gone so far now that probably makes a lot of sense to put getters and setters just from beginning, even first time in the new team. It is important not to conflict with the team.