Related
when programming in Java I practically always, just out of habit, write something like this:
public List<String> foo() {
return new ArrayList<String>();
}
Most of the time without even thinking about it. Now, the question is: should I always specify the interface as the return type? Or is it advisable to use the actual implementation of the interface, and if so, under what circumstances?
It is obvious that using the interface has a lot of advantages (that's why it's there). In most cases it doesn't really matter what concrete implementation is used by a library function. But maybe there are cases where it does matter. For instance, if I know that I will primarily access the data in the list randomly, a LinkedList would be bad. But if my library function only returns the interface, I simply don't know. To be on the safe side I might even need to copy the list explicitly over to an ArrayList:
List bar = foo();
List myList = bar instanceof LinkedList ? new ArrayList(bar) : bar;
but that just seems horrible and my coworkers would probably lynch me in the cafeteria. And rightfully so.
What do you guys think? What are your guidelines, when do you tend towards the abstract solution, and when do you reveal details of your implementation for potential performance gains?
Return the appropriate interface to hide implementation details. Your clients should only care about what your object offers, not how you implemented it. If you start with a private ArrayList, and decide later on that something else (e.g., LinkedLisk, skip list, etc.) is more appropriate you can change the implementation without affecting clients if you return the interface. The moment you return a concrete type the opportunity is lost.
For instance, if I know that I will
primarily access the data in the list
randomly, a LinkedList would be bad.
But if my library function only
returns the interface, I simply don't
know. To be on the safe side I might
even need to copy the list explicitly
over to an ArrayList.
As everybody else has mentioned, you just mustn't care about how the library has implemented the functionality, to reduce coupling and increasing maintainability of the library.
If you, as a library client, can demonstrate that the implementation is performing badly for your use case, you can then contact the person in charge and discuss about the best path to follow (a new method for this case or just changing the implementation).
That said, your example reeks of premature optimization.
If the method is or can be critical, it might mention the implementation details in the documentation.
Without being able to justify it with reams of CS quotes (I'm self taught), I've always gone by the mantra of "Accept the least derived, return the most derived," when designing classes and it has stood me well over the years.
I guess that means in terms of interface versus concrete return is that if you are trying to reduce dependencies and/or decouple, returning the interface is generally more useful. However, if the concrete class implements more than that interface, it is usually more useful to the callers of your method to get the concrete class back (i.e. the "most derived") rather than aribtrarily restrict them to a subset of that returned object's functionality - unless you actually need to restrict them. Then again, you could also just increase the coverage of the interface. Needless restrictions like this I compare to thoughtless sealing of classes; you never know. Just to talk a bit about the former part of that mantra (for other readers), accepting the least derived also gives maximum flexibility for callers of your method.
-Oisin
Sorry to disagree, but I think the basic rule is as follows:
For input arguments use the most generic.
For output values, the most specific.
So, in this case you want to declare the implementation as:
public ArrayList<String> foo() {
return new ArrayList<String>();
}
Rationale:
The input case is already known and explained by everyone: use the interface, period. However, the output case can look counter-intuitive.
You want to return the implementation because you want the client to have the most information about what is receiving. In this case, more knowledge is more power.
Example 1: the client wants to get the 5th element:
return Collection: must iterate until 5th element vs return List:
return List: list.get(4)
Example 2: the client wants to remove the 5th element:
return List: must create a new list without the specified element (list.remove() is optional).
return ArrayList: arrayList.remove(4)
So it's a big truth that using interfaces is great because it promotes reusability, reduces coupling, improves maintainability and makes people happy ... but only when used as input.
So, again, the rule can be stated as:
Be flexible for what you offer.
Be informative with what you deliver.
So, next time, please return the implementation.
In OO programming, we want to encapsulate as much as possible the data. Hide as much as possible the actual implementation, abstracting the types as high as possible.
In this context, I would answer only return what is meaningful. Does it makes sense at all for the return value to be the concrete class? Aka in your example, ask yourself: will anyone use a LinkedList-specific method on the return value of foo?
If no, just use the higher-level Interface. It's much more flexible, and allows you to change the backend
If yes, ask yourself: can't I refactor my code to return the higher-level interface? :)
The more abstract is your code, the less changes your are required to do when changing a backend. It's as simple as that.
If, on the other hand, you end up casting the return values to the concrete class, well that's a strong sign that you should probably return instead the concrete class. Your users/teammates should not have to know about more or less implicit contracts: if you need to use the concrete methods, just return the concrete class, for clarity.
In a nutshell: code abstract, but explicitly :)
In general, for a public facing interface such as APIs, returning the interface (such as List) over the concrete implementation (such as ArrayList) would be better.
The use of a ArrayList or LinkedList is an implementation detail of the library that should be considered for the most common use case of that library. And of course, internally, having private methods handing off LinkedLists wouldn't necessarily be a bad thing, if it provides facilities that would make the processing easier.
There is no reason that a concrete class shouldn't be used in the implementation, unless there is a good reason to believe that some other List class would be used later on. But then again, changing the implementation details shouldn't be as painful as long as the public facing portion is well-designed.
The library itself should be a black box to its consumers, so they don't really have to worry about what's going on internally. That also means that the library should be designed so that it is designed to be used in the way it is intended.
It doesn't matter all that much whether an API method returns an interface or a concrete class; despite what everyone here says, you almost never change the implementiation class once the code is written.
What's far more important: always use minimum-scope interfaces for your method parameters! That way, clients have maximal freedom and can use classes your code doesn't even know about.
When an API method returns ArrayList, I have absolutely no qualms with that, but when it demands an ArrayList (or, all to common, Vector) parameter, I consider hunting down the programmer and hurting him, because it means that I can't use Arrays.asList(), Collections.singletonList() or Collections.EMPTY_LIST.
As a rule, I only pass back internal implementations if I am in some private, inner workings of a library, and even so only sparingly. For everything that is public and likely to be called from the outside of my module I use interfaces, and also the Factory pattern.
Using interfaces in such a way has proven to be a very reliable way to write reusable code.
The main question has been answered already and you should always use the interface. I however would just like to comment on
It is obvious that using the interface has a lot of advantages (that's why it's there). In most cases it doesn't really matter what concrete implementation is used by a library function. But maybe there are cases where it does matter. For instance, if I know that I will primarily access the data in the list randomly, a LinkedList would be bad. But if my library function only returns the interface, I simply don't know. To be on the safe side I might even need to copy the list explicitly over to an ArrayList.
If you are returning a data structure that you know has poor random access performance -- O(n) and typically a LOT of data -- there are other interfaces you should be specifying instead of List, like Iterable so that anyone using the library will be fully aware that only sequential access is available.
Picking the right type to return isn't just about interface versus concrete implementation, it is also about selecting the right interface.
You use interface to abstract away from the actual implementation. The interface is basically just a blueprint for what your implementation can do.
Interfaces are good design because they allow you to change implementation details without having to fear that any of its consumers are directly affected, as long as you implementation still does what your interface says it does.
To work with interfaces you would instantiate them like this:
IParser parser = new Parser();
Now IParser would be your interface, and Parser would be your implementation. Now when you work with the parser object from above, you will work against the interface (IParser), which in turn will work against your implementation (Parser).
That means that you can change the inner workings of Parser as much as you want, it will never affect code that works against your IParser parser interface.
In general use the interface in all cases if you have no need of the functionality of the concrete class. Note that for lists, Java has added a RandomAccess marker class primarily to distinguish a common case where an algorithm may need to know if get(i) is constant time or not.
For uses of code, Michael above is right that being as generic as possible in the method parameters is often even more important. This is especially true when testing such a method.
You'll find (or have found) that as you return interfaces, they permeate through your code. e.g. you return an interface from method A and you have to then pass an interface to method B.
What you're doing is programming by contract, albeit in a limited fashion.
This gives you enormous scope to change implementations under the covers (provided these new objects fulfill the existing contracts/expected behaviours).
Given all of this, you have benefits in terms of choosing your implementation, and how you can substitute behaviours (including testing - using mocking, for example). In case you hadn't guessed, I'm all in favour of this and try to reduce to (or introduce) interfaces wherever possible.
when programming in Java I practically always, just out of habit, write something like this:
public List<String> foo() {
return new ArrayList<String>();
}
Most of the time without even thinking about it. Now, the question is: should I always specify the interface as the return type? Or is it advisable to use the actual implementation of the interface, and if so, under what circumstances?
It is obvious that using the interface has a lot of advantages (that's why it's there). In most cases it doesn't really matter what concrete implementation is used by a library function. But maybe there are cases where it does matter. For instance, if I know that I will primarily access the data in the list randomly, a LinkedList would be bad. But if my library function only returns the interface, I simply don't know. To be on the safe side I might even need to copy the list explicitly over to an ArrayList:
List bar = foo();
List myList = bar instanceof LinkedList ? new ArrayList(bar) : bar;
but that just seems horrible and my coworkers would probably lynch me in the cafeteria. And rightfully so.
What do you guys think? What are your guidelines, when do you tend towards the abstract solution, and when do you reveal details of your implementation for potential performance gains?
Return the appropriate interface to hide implementation details. Your clients should only care about what your object offers, not how you implemented it. If you start with a private ArrayList, and decide later on that something else (e.g., LinkedLisk, skip list, etc.) is more appropriate you can change the implementation without affecting clients if you return the interface. The moment you return a concrete type the opportunity is lost.
For instance, if I know that I will
primarily access the data in the list
randomly, a LinkedList would be bad.
But if my library function only
returns the interface, I simply don't
know. To be on the safe side I might
even need to copy the list explicitly
over to an ArrayList.
As everybody else has mentioned, you just mustn't care about how the library has implemented the functionality, to reduce coupling and increasing maintainability of the library.
If you, as a library client, can demonstrate that the implementation is performing badly for your use case, you can then contact the person in charge and discuss about the best path to follow (a new method for this case or just changing the implementation).
That said, your example reeks of premature optimization.
If the method is or can be critical, it might mention the implementation details in the documentation.
Without being able to justify it with reams of CS quotes (I'm self taught), I've always gone by the mantra of "Accept the least derived, return the most derived," when designing classes and it has stood me well over the years.
I guess that means in terms of interface versus concrete return is that if you are trying to reduce dependencies and/or decouple, returning the interface is generally more useful. However, if the concrete class implements more than that interface, it is usually more useful to the callers of your method to get the concrete class back (i.e. the "most derived") rather than aribtrarily restrict them to a subset of that returned object's functionality - unless you actually need to restrict them. Then again, you could also just increase the coverage of the interface. Needless restrictions like this I compare to thoughtless sealing of classes; you never know. Just to talk a bit about the former part of that mantra (for other readers), accepting the least derived also gives maximum flexibility for callers of your method.
-Oisin
Sorry to disagree, but I think the basic rule is as follows:
For input arguments use the most generic.
For output values, the most specific.
So, in this case you want to declare the implementation as:
public ArrayList<String> foo() {
return new ArrayList<String>();
}
Rationale:
The input case is already known and explained by everyone: use the interface, period. However, the output case can look counter-intuitive.
You want to return the implementation because you want the client to have the most information about what is receiving. In this case, more knowledge is more power.
Example 1: the client wants to get the 5th element:
return Collection: must iterate until 5th element vs return List:
return List: list.get(4)
Example 2: the client wants to remove the 5th element:
return List: must create a new list without the specified element (list.remove() is optional).
return ArrayList: arrayList.remove(4)
So it's a big truth that using interfaces is great because it promotes reusability, reduces coupling, improves maintainability and makes people happy ... but only when used as input.
So, again, the rule can be stated as:
Be flexible for what you offer.
Be informative with what you deliver.
So, next time, please return the implementation.
In OO programming, we want to encapsulate as much as possible the data. Hide as much as possible the actual implementation, abstracting the types as high as possible.
In this context, I would answer only return what is meaningful. Does it makes sense at all for the return value to be the concrete class? Aka in your example, ask yourself: will anyone use a LinkedList-specific method on the return value of foo?
If no, just use the higher-level Interface. It's much more flexible, and allows you to change the backend
If yes, ask yourself: can't I refactor my code to return the higher-level interface? :)
The more abstract is your code, the less changes your are required to do when changing a backend. It's as simple as that.
If, on the other hand, you end up casting the return values to the concrete class, well that's a strong sign that you should probably return instead the concrete class. Your users/teammates should not have to know about more or less implicit contracts: if you need to use the concrete methods, just return the concrete class, for clarity.
In a nutshell: code abstract, but explicitly :)
In general, for a public facing interface such as APIs, returning the interface (such as List) over the concrete implementation (such as ArrayList) would be better.
The use of a ArrayList or LinkedList is an implementation detail of the library that should be considered for the most common use case of that library. And of course, internally, having private methods handing off LinkedLists wouldn't necessarily be a bad thing, if it provides facilities that would make the processing easier.
There is no reason that a concrete class shouldn't be used in the implementation, unless there is a good reason to believe that some other List class would be used later on. But then again, changing the implementation details shouldn't be as painful as long as the public facing portion is well-designed.
The library itself should be a black box to its consumers, so they don't really have to worry about what's going on internally. That also means that the library should be designed so that it is designed to be used in the way it is intended.
It doesn't matter all that much whether an API method returns an interface or a concrete class; despite what everyone here says, you almost never change the implementiation class once the code is written.
What's far more important: always use minimum-scope interfaces for your method parameters! That way, clients have maximal freedom and can use classes your code doesn't even know about.
When an API method returns ArrayList, I have absolutely no qualms with that, but when it demands an ArrayList (or, all to common, Vector) parameter, I consider hunting down the programmer and hurting him, because it means that I can't use Arrays.asList(), Collections.singletonList() or Collections.EMPTY_LIST.
As a rule, I only pass back internal implementations if I am in some private, inner workings of a library, and even so only sparingly. For everything that is public and likely to be called from the outside of my module I use interfaces, and also the Factory pattern.
Using interfaces in such a way has proven to be a very reliable way to write reusable code.
The main question has been answered already and you should always use the interface. I however would just like to comment on
It is obvious that using the interface has a lot of advantages (that's why it's there). In most cases it doesn't really matter what concrete implementation is used by a library function. But maybe there are cases where it does matter. For instance, if I know that I will primarily access the data in the list randomly, a LinkedList would be bad. But if my library function only returns the interface, I simply don't know. To be on the safe side I might even need to copy the list explicitly over to an ArrayList.
If you are returning a data structure that you know has poor random access performance -- O(n) and typically a LOT of data -- there are other interfaces you should be specifying instead of List, like Iterable so that anyone using the library will be fully aware that only sequential access is available.
Picking the right type to return isn't just about interface versus concrete implementation, it is also about selecting the right interface.
You use interface to abstract away from the actual implementation. The interface is basically just a blueprint for what your implementation can do.
Interfaces are good design because they allow you to change implementation details without having to fear that any of its consumers are directly affected, as long as you implementation still does what your interface says it does.
To work with interfaces you would instantiate them like this:
IParser parser = new Parser();
Now IParser would be your interface, and Parser would be your implementation. Now when you work with the parser object from above, you will work against the interface (IParser), which in turn will work against your implementation (Parser).
That means that you can change the inner workings of Parser as much as you want, it will never affect code that works against your IParser parser interface.
In general use the interface in all cases if you have no need of the functionality of the concrete class. Note that for lists, Java has added a RandomAccess marker class primarily to distinguish a common case where an algorithm may need to know if get(i) is constant time or not.
For uses of code, Michael above is right that being as generic as possible in the method parameters is often even more important. This is especially true when testing such a method.
You'll find (or have found) that as you return interfaces, they permeate through your code. e.g. you return an interface from method A and you have to then pass an interface to method B.
What you're doing is programming by contract, albeit in a limited fashion.
This gives you enormous scope to change implementations under the covers (provided these new objects fulfill the existing contracts/expected behaviours).
Given all of this, you have benefits in terms of choosing your implementation, and how you can substitute behaviours (including testing - using mocking, for example). In case you hadn't guessed, I'm all in favour of this and try to reduce to (or introduce) interfaces wherever possible.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Coding to interfaces?
I am reading the Collections tutorial from Java and it strongly recommends that codes referring to Collections be implemented using its interface type and not its actual implementation type. e.g:
Set<String> s = new HashSet<String>();
It says that it will give me flexibility to change implementations should I decide to change it later on.
Set<String> s = new TreeSet<String>();
Other than flexibility, are there any other benefits to implementing a collection using its interface type?
Yes, when using the interface class, you will only have access to the most default methods. These methods guarantee to be intuitive. When using the implementation class, you might see more methods that might confuse you or be abused.
For example: a collection interface will have a method that returns the number of elements, lets say: size(). But the implementation class might also provide a capacity() method that tells you how big the underlying array is.
But as the tutorial tells you, the most important reason is that you can change the implementation without any effort. Changing the implementation might be interesting for performance optimization is very specific cases.
I think here it's more than just Collections, it's about polymorphism: better to use the interface as the declared type, because you may change implementation and/or concrete class to be bound at runtime. (discussion here could be longer - there are plenty of documents/tutorials about this - Java basics)
The interface type often contains fewer methods than an actual implementation, making it tempting to use the later because it allows access to them.
However, this then ties your hands in a significant way. For example, if you decide to expose a return type of Vector from a public method of a class that uses a vector, but later realize that your module would be better served with a LinkedList, you now have some problems -- this will break anything which uses the method returning a vector.
On the other hand, if in the first place you had used a return type of List then there would be no problem -- you could switch the internal Vector to a LinkedList, or implement your own thing that fulfills interface List. In my experience, this is a common event (of course, if you make it difficult or impossible, then it will happen less).
So, unless you have a specific reason to do otherwise (eg, you need to provide access to methods only available with vectors), always use the generic interface type for your return value.
I realize this is still about flexibility but it is not clear from your post if you understand how important that is. If you are asking for a better reason to use interfaces, ie. "is it okay to use the specific implementation if I don't care
flexibility", ie. "I don't care about flexibility and want to use specific types, is that okay?" the answer is you should care about flexibility ;) As others have said, this is an important fundamental of good java programming.
After comming from Ruby world, I'm having little problems doing TDD in Java. The biggest issue is when I have application that is just communicating with external API.
Say I want to just fetch some data from Google Calendar, or 5 tweets from some Twitter user and display it.
In Ruby, I don't have any problems, because I can monkey-patch the API library in tests directly, but I have no such option in Java.
If I think about this in terms of MVC, my model objects are directly accessing the API through some library. The question is, is this bad design? Should I always wrap any API library in some interface, so I can mock/stub it in Java?
Because when I think about this, the only purpose of that interface would be to simulate (please don't kill me for saying this) the monkey-patch. Meaning that any time I use any external resource, I have to wrap each layer in interface that can be stubbed out.
# do I have to abstract everything just to do this in Java?
Twitter.stub!(:search)
Now you might say that I should always abstract away the interface, so I can change the underlying layer to anything else. But if I'm writing twitter app, I'm not going to change it to RSS reader.
Yes, I can add for example Facebook and then it would make sense to have interface. But when there is no other resource that can be substituted for the one I'm using, than I still have to wrap everything in interfaces to make it testable.
Am I missing something, or is this just a way to test in the Java world?
Using interfaces is just generally good practice in Java. Some languages have multiple inheritance, others have duck typing, Java has interfaces. It's a key feature of the language, it lets me use
different aspects of a class in different contexts and
different implementations of the same contract without changing client code.
So interfaces are a concept you should embrace in general, and then you would reap the benefits in situations like this where you could substitute your services by mock objects.
One of the most important books about Java best practices is Effective Java by Joshua Bloch. I would highly suggest you to read it. In this context the most important part is Item 52: Refer to objects by their interfaces. Quote:
More generally, you should favor the use of interfaces rather than
classes to refer to objects. If appropriate interface types exist, then parameters, return values, variables, and fields should all be declared using interface
types. The only time you really need to refer to an object’s class is when you’re
creating it with a constructor.
And if you take things even further (e.g. when using dependency injection), you aren't even calling the constructor.
One of the key problems of switching languages is that you have to switch the way of thinking too. You can't program language x effectively while thinking in language y. You can't program C effectively without using pointers, Ruby not without duck typing and Java not without Interfaces.
Wrapping the external API is the way I would do this.
So, as you already said, you would have an interface and two classes: the real one and the dummy implementation.
Yes, it may seem unreasonable from the perspective of some services indeed being specific, like Twitter. But, this way your build process doesn't depend on external resources. Depending on external libraries isn't all that bad, but having your tests depend on actual data present or not present out there on the web can mess up the build process.
The easiest way is to wrap the API service with your interface/class pair and use that throughout your code.
I understand that what you want are Mock objects.
As you described it, one of the ways one can generate "test versions" of objects is by implementing a common interface and using it.
However, what you are missing is to simply extend the class (provided that it is not declared final) and override the methods that you want to mock. (NB: the possibility of doing that is the reason why it is considered bad form for a library to declare its classes final - it can make testing considerably harder.)
There is a number of Java libraries that aim in facilitating the use of Mock objects - you can look at Mockito or EasyMock.
Mockito is more handy and like your ruby mocks.
You can "monkey-patch" an API in Java. The Java language itself does not provide specific means to do it, but the JVM and the standard libraries do. In Ruby, developers can use the Mocha library for that. In Java, you can use the JMockit library (which I created because of limitations in older mocking tools).
Here is an example JMockit test, equivalent to the test_should_calculate_value_of_unshipped_orders test available in Mocha documentation:
#Test
public void shouldCalculateValueOfUnshippedOrders()
{
final Order anOrder = new Order();
final List<Order> orders = asList(anOrder, new Order(), new Order());
new NonStrictExpectations(Order.class)
{{
Order.findAll(); result = orders;
anOrder.getTotalCost(); result = 10;
}};
assertEquals(30, Order.unshippedValue());
}
This question already has answers here:
What does it mean to "program to an interface"?
(33 answers)
Closed 6 years ago.
This is a real beginner question (I'm still learning the Java basics).
I can (sort of) understand why methods would return a List<String> rather than an ArrayList<String>, or why they would accept a List parameter rather than an ArrayList. If it makes no difference to the method (i.e., if no special methods from ArrayList are required), this would make the method more flexible, and easier to use for callers. The same thing goes for other collection types, like Set or Map.
What I don't understand: it appears to be common practice to create local variables like this:
List<String> list = new ArrayList<String>();
While this form is less frequent:
ArrayList<String> list = new ArrayList<String>();
What's the advantage here?
All I can see is a minor disadvantage: a separate "import" line for java.util.List has to be added. Technically, "import java.util.*" could be used, but I don't see that very often either, probably because the "import" lines are added automatically by some IDE.
When you read
List<String> list = new ArrayList<String>();
you get the idea that all you care about is being a List<String> and you put less emphasis on the actual implementation. Also, you restrict yourself to members declared by List<String> and not the particular implementation. You don't care if your data is stored in a linear array or some fancy data structure, as long as it looks like a List<String>.
On the other hand, reading the second line gives you the idea that the code cares about the variable being ArrayList<String>. By writing this, you are implicitly saying (to future readers) that you shouldn't blindly change actual object type because the rest of the code relies on the fact that it is really an ArrayList<String>.
Using the interface allows you to quickly change the underlying implementation of the List/Map/Set/etc.
It's not about saving keystrokes, it's about changing implementation quickly. Ideally, you shouldn't be exposing the underlying specific methods of the implementation and just use the interface required.
I would suggest thinking about this from the other end around. Usually you want a List or a Set or any other Collection type - and you really do not care in your code how exactly this is implemented. Hence your code just works with a List and do whatever it needs to do (also phrased as "always code to interfaces").
When you create the List, you need to decide what actual implementation you want. For most purposes ArrayList is "good enough", but your code really doesn't care. By sticking to using the interface you convey this to the future reader.
For instance I have a habit of having debug code in my main method which dumps the system properties to System.out - it is usually much nicer to have them sorted. The easiest way is to simply let "Map map = new TreeMap(properties);" and THEN iterate through them, as TreeMap returns the keys sorted.
When you learn more about Java, you will also see that interfaces are very helpful in testing and mocking, since you can create objects with behaviour specified at runtime conforming to a given interface. An advanced (but simple) example can be seen at http://www.exampledepot.com/egs/java.lang.reflect/ProxyClass.html
if later you want to change implementation of the list and use for example LinkedList(maybe for better performance) you dont have to change the whole code(and API if its library). if order doesnt matter you should return Collection so later on you can easily change it to Set if you would need items to be sorted.
The best explanation I can come up with (because I don't program in Java as frequently as in other languages) is that it make it easier to change the "back-end" list type while maintaining the same code/interface everything else is relying on. If you declare it as a more specific type first, then later decide you want a different kind... if something happens to use an ArrayList-specific method, that's extra work.
Of course, if you actually need ArrayList-specific behavior, you'd go with the specific variable type instead.
The point is to identify the behavior you want/need and then use the interface that provides that behavior. The is the type for your variable. Then, use the implementation that meets your other needs - efficiency, etc. This is what you create with "new". This duality is one of the major ideas behind OOD. The issue is not particularly significant when you are dealing with local variables, but it rarely hurts to follow good coding practices all the time.
Basically this comes from people who have to run large projects, possibly other reasons - you hear it all the time. Why, I don't actually know. If you have need of an array list, or Hash Map or Hash Set or whatever else I see no point in eliminating methods by casting to an interface.
Let us say for example, recently I learned how to use and implemented HashSet as a principle data structure. Suppose, for whatever reason, I went to work on a team. Would not that person need to know that the data was keyed on hashing approaches rather than being ordered by some basis? The back-end approach noted by Twisol works in C/C++ where you can expose the headers and sell a library thus, if someone knows how to do that in Java I would imagine they would use JNI - at which point is seems simpler to me to use C/C++ where you can expose the headers and build libs using established tools for that purpose.
By the time you can get someone who can install a jar file in the extensions dir it would seem to me that entity could be jus short steps away - I dropped several crypto libs in the extensions directory, that was handy, but I would really like to see a clear, concise basis elucidated. I imagine they do that all the time.
At this point it sounds to me like classic obfuscation, but beware: You have some coding to do before the issue is of consequence.