I'm trying to write a class to implement KryoSerializer so that I can serialize objects for use with Spark. The issue I'm having is that while all of the classes implement a public interface, all of the implementing classes are private. Kryo doesn't seem to want to allow me to define a serializer for either a private out-of-package class or an interface.
The way this issue manifests is that when I attempt to define the KryoSerializer class, I get an error that class [implementation] in package graph cannot be accessed in package [same package].
What I'm hoping someone can help with, is a strategy for solving this issue.
I understand the reasons why Kryo wants to serialize and deserialize concrete objects. But, in this case, since I am defining my own KryoSerializer anyway, it actually would make more sense to define serialization for the interface. Is there a way to trick Kryo into doing the right thing?
(The reason this will work, is that there is a related Object that has functions to take an instance with the interface and write or read from a stream. My serializer would essentially wrap those functions while adding some serialization format version information.)
One possibility I thought of is the reflection trick. When deserializing a class with private/final members, we sometimes use reflection to make the private member accessible and writable, set the value, then set it back to private/final. I'm not sure if its possible to do that for a private class in another packager, but even if it is, it seems rather ugly and inefficient.
Another possibility would be to define new classes that extend the private classes, along with a set of implicits to convert among them. That would also be rather ugly though, for a few reasons, and there are quite a few private classes at issue.
Can anyone suggest an approach? Advise regarding pitfalls I should avoid?
Related
I am new to Swift and OOP. For example, I have a class that manages the system-wide configurations.
class system_conf {
init()
getValue1()
getValue2()
...
setValue1()
setValue2()
...
reloadValues()
activateX()
activeteY()
...
}
This class should have only one instance and many other classes will use it. What's the recommended way for this case?
Should I pass around this instance?
Should I consider to use Singleton?
Should I use static functions directly?
Should I create a global instance, so every other class can access it directly?
or?
It seems your class is a configuration class. If you intend to pass it to a bunch of classes, you should wonder if you need to write unit tests for them.
If so, assuming you are either using a singleton or static methods or a global var, take a moment to think about how you would mock this configuration class for each of your tests. It's not easy, is it?
If your class is a kind of mediator, a global var or static methods are fine (or any other alternative you suggested). However, in your case, it would be better to pass your object in any initializer/constructor of each class using it. Then, testing would definitely be easier. Also, passing it via an interface is even better: you can mock it super easily (mock up libraries mostly work with interfaces only).
So there is no unique answer to your question. It is just a matter of compromises and scaling. If your app is small, any of the method you listed above is perfectly fine. However, if you app tends to get bigger, a proxy solution would be better for maintainability and testability.
If you fancy reading, you should glance at this article from Misko Hevery, especially this chapter.
In c# we have the protected accessor which allows class members to be visible on inherited clases but not for the rest.
In Swift this doesn't exist so I wonder what's a correct approach for something like this:
I want to have a variable (internal behavior) and and a public method using this variable on a base class. This variable will be used also on inherited clases.
Options I see
Forget about base class and implement variable and methods everywhere I need it. WRONG, duplicated code
Implement inheritance by composition. I'd create a class containing common methods and this will be used by composition instead of inheritance. LESS WRONG but still repeating code that could be avoided with inheritance
Implement inheritance and make variable internal on base class. WRONG since exposes things without any justification except allowing visibility on inherited clases.
Implementation Details for Base Class
I want to have a NSOperationQueue instance and and a public method to cancel queued operations. I add new operations to this queue from inherited classes.
In Swift the correct answer is almost always protocols and extensions. It is almost never inheritance. Sometimes Cocoa stands in our way, because there are classes in Cocoa more often than protocols, but the goal is almost always protocols and extensions. Subclassing is our last choice.
Your particular case is confusing because NSOperationQueue already has a public method to cancel queued operations (cancelAllOperations). If you want to protect the queue from outside access (prevent callers from using addOperation directly for instance), then you should put the queue inside another type (i.e. composition), and forward what you want to the queue. More details on the specific problem you're solving would allow us to help suggest other Swift-like solutions.
If in the end you need something that looks like protected or friend, the correct solution is private. Put your subclass or your friend in the same file with the target, and mark the private thing private. Alternately, put the things that need to work together in a framework, and mark the attribute internal. The Swift Blog provides a good explanation of why this is an intentional choice.
I've written a Scala trait, named Cache[A,B], to provide a caching API. The Cache has the following methods, asyncGet(), asyncPut(), asyncPutIfAbsent(), asyncRemove().
I'm going to have a few static methods, such as getOrElseUpdate(key: A)(op: => B). I don't want methods like this as abstract defs in the Cache trait because I don't want each Cache implementation to have to provide an implementation for it, when it can be written once using the async*() methods.
In looking at Google Guava and parts of the Java library, they place public static functions in a class that is the plural of the interface name, so "Caches" would be the name I would use.
I like this naming scheme actually, even though I could use a Cache companion object. In looking at much of my code, many of my companion objects contain private val's or def's, so users of my API then need to look through the companion object to see what they can use from there, or anything for that matter.
By having a object named "Caches" is consistent with Java and also makes it clear that there's only public functions in there. I'm leaning towards using "object Caches" instead of "object Cache".
So what do people think?
Scala's traits are not just a different name for Java's interfaces. They may have concrete (implemented) members, both values (val and var) and methods. So if there's a unified / generalized / shared implementation of a method, it can be placed in a trait and need not be replicated or factored into a separate class.
I think the mistake starts with "going to have a few static methods". Why have static methods? If you explain why you need static methods, it will help figure out what the design should be.
Similar question but not quite the same thing
I was thinking that with extension methods in the same namespace as the interface you could get a similar effect to multiple inheritance in that you don't need to have duplicate code implementing the same interface the same way in 10 different classes.
What are some of the downsides of doing this? I think the pros are pretty obvious, it's the cons that usually come back to bite you later on.
One of the cons I see is that the extension methods can't be virtual, so you need to be sure that you actually do want them implemented the same way for every instance.
The problem that I see with building interface capability via extension methods is that you are no longer actually implementing the interface and so can't use the object as the interface type.
Say I have a method that takes an object of type IBar. If I implement the IBar interface on class Foo via extension methods, then Foo doesn't derive from IBar and can't be used interchangeably with it (Liskov Substitution principle). Sure, I get the behavior that I want added to Foo, but I lose the most important aspect of creating interfaces in the first place -- being able to define an abstract contract that can be implemented in a variety of ways by various classes so that dependent classes need not know about concrete implementations.
If I needed multiple inheritance (and so far I've lived without it) badly enough, I think I'd use composition instead to minimize the amount of code duplication.
A decent way to think about this is that instance methods are something done by the object, while extension methods are something done to the object. I am fairly certain the Framework Design Guidelines say you should implement an instance method whenever possible.
An interface declares "I care about using this functionality, but not how it is accomplished." That leaves implementers the freedom to choose the how. It decouples the intent, a public API, from the mechanism, a class with concrete code.
As this is the main benefit of interfaces, implementing them entirely as extension methods seems to defeat their purpose. Even IEnumerable<T> has an instance method.
Edit: Also, objects are meant to act on the data they contain. Extension methods can only see an object's public API (as they are just static methods); you would have to expose all of an object's state to make it work (an OO no-no).
I am looking at nServiceBus and came over this interface
namespace NServiceBus
{
public interface IMessage
{
}
}
What is the use of an empty interface?
Usually it's to signal usage of a class. You can implement IMessage to signal that your class is a message. Other code can then use reflection to see if your objects are meant to be used as messages and act accordingly.
This is something that was used in Java a lot before they had annotations. In .Net it's cleaner to use attributes for this.
#Stimpy77 Thanks! I hadn't thought of it that way.
I hope you'll allow me to rephrase your comment in a more general way.
Annotations and attributes have to be checked at runtime using reflection. Empty interfaces can be checked at compile-time using the type-system in the compiler. This brings no overhead at runtime at all so it is faster.
Also known as a Marker Interface:
http://en.wikipedia.org/wiki/Marker_interface_pattern
In java Serializable is the perfect example for this. It defines no methods but every class that "implements" it has to make sure, that it is really serializable and holds no reference to things that cannot be serialized, like database connections, open files etc.
In Java, empty interfaces were usually used for "tagging" classes - these days annotations would normally be used.
It's just a way of adding a bit of metadata to a class saying, "This class is suitable for <this> kind of use" even when no common members will be involved.
Normally it's similar to attributes. Using attributes is a preferred to empty interfaces (at least as much as FxCop is aware). However .NET itself uses some of these interfaces like IRequiresSessionState and IReadOnlySessionState. I think there is performance loss in metadata lookup when you use attributes that made them use interfaces instead.
An empty interface acts simply as a placeholder for a data type no better specified in its interface behaviour.
In Java, the mechanism of the interface extension represents a good example of use. For example, let's say that we've the following
interface one {}
interface two {}
interface three extends one, two {}
Interface three will inherit the behaviour of 'one' and 'two', and so
class four implements three { ... }
has to specify the two methods, being of type 'three'.
As you can see, from the above example, empty interface can be seen also as a point of multiple inheritance (not allowed in Java).
Hoping this helps to clarify with a further viewpoint.
They're called "Mark Interfaces" and are meant to signal instances of the marked classes.
For example... in C++ is a common practice to mark as "ICollectible" objects so they can be stored in generic non typed collections.
So like someone over says, they're to signal some object supported behavior, like ability to be collected, serialized, etc.
Been working with NServiceBus for the past year. While I wouldn't speak for Udi Dahan my understanding is that this interface is indeed used as a marker primarily.
Though I'd suggest you ask the man himself if he'd had thoughts of leaving this for future extension. My bet is no, as the mantra seems to be to keep messages very simple or at least practically platform agnostic.
Others answer well on the more general reasons for empty interfaces.
I'd say its used for "future" reference or if you want to share some objects, meaning you could have 10 classes each implementing this interface.
And have them sent to a function for work on them, but if the interface is empty, I'd say its just "pre"-work.
Empty interfaces are used to document that the classes that implement a given interface have a certain behaviour
For example in java the Cloneable interface in Java is an empty interface. When a class implements the Cloneable interface you know that you can call run the clone() on it.
Empty interfaces are used to mark the class, at run time type check can be performed using the interfaces.
For example
An application of marker interfaces from the Java programming language is the Serializable interface. A class implements this interface to indicate that its non-transient data members can be written to an ObjectOutputStream. The ObjectOutputStream private method writeObject() contains a series of instanceof tests to determine writeability, one of which looks for the Serializable interface. If any of these tests fails, the method throws a NotSerializableException.
An empty interface can be used to classify classes under a specific purpose. (Marker Interface)
Example : Database Entities
public interface IEntity {
}
public class Question implements IEntity {
// Implementation Goes Here
}
public class Answer implements IEntity {
// Implementation Goes Here
}
For Instance, If you will be using Generic Repository(ex. IEntityRepository), using generic constraints, you can prevent the classes that do not implement the IEntity interface from being sent by the developers.