When reading about singletons, I have found this explanation as a reason to use singleton:
since these object methods are not changing the internal class state, we
can create this class as a singleton.
What does this really mean ? When you consider that some method is not changing internal class state ? If it is a getter ? Can someone provide code examples for class that uses methods that are not changing its internal state, and therefore can be used as a singleton, and class that should not be a singleton ?
Usually, when people are explaining singleton pattern, they use DB connection class as an example. And that makes sense to me, because I know that I want to have only one db connection during one application instance. But what if I want to provide an option to force using the new connection when I instantiate DB connection class? If I have some setter method, or constructor parameter that forces my class to open new connection, is that class still a subject to be a singleton ?
I am using PHP, but may understand examples written in JAVA, C#...
This is the article reference. You can ctrl+f search for "internal". Basically, autor is explaining why FileStorage class is a good candidate to be a singleton. I do not understand this sentance
"These operations do not change the internal class state, so we can
create its instance once and use it multiple times."
and therefore I do not understand when to use singletons.
In their example, they have some FileStorage class :
class FileStorage
{
public function __contruct($root) {
// whatever
}
public function read() {
// whatever
}
public function write($content) {
// whatever
}
}
And they say that this class can be a singleton since its methods read() and write() do not chage internal class structure. What does that mean ? They are not setters and class is automatically singleton ?
The quote reads:
These operations do not change the internal class state, so we can create its instance once and use it multiple times.
This means that the object in question has no interesting internal state that could be changed; it’s just a collection of methods (that could probably be static). If the object has no internal state, you don’t have to create multiple instances of it, you can keep reusing a single one. Therefore you can configure the dependency injection container to treat the object as a singleton.
This is a performance optimization only. You could create a fresh instance of the class each time it’s needed. And it would be better – until the object creation becomes a measurable bottleneck.
Related
Let's say I have a few controllers. Each controller can at some point create new objects which will need to be stored on the server. For example I can have a RecipeCreationViewController which manages a form. When this form is submitted, a new Recipe object is created and needs to be saved on the server.
What's the best way to design the classes to minimize complexity and coupling while keeping the code as clean and readable as possible?
Singleton
Normally I would create a singleton NetworkAdapter that each controller can access directly in order to save objects.
Example:
[[[NetworkAdapter] sharedAdapter] saveObject:myRecipe];
But I've realized that having classes call singletons on their own makes for coupled code which is hard to debug since the access to the singleton is hidden in the implementation and not obvious from the interface.
Direct Reference
The alternative is to have each controller hold a reference to the NetworkAdapter and have this be passed in by the class that creates the controller.
For example:
[self.networkAdapter saveObject:myRecipe];
Delegation
The other approach that came to mind is delegation. The NetworkAdapter can implement a "RemoteStorageDelegate" protocol and each controller can have a remoteStorageDelegate which it can call methods like saveObject: on. The advantage being that the controllers don't know about the details of a NetworkAdapter, only that the object that implements the protocol knows how to save objects.
For example:
[self.remoteStorageDelegate saveObject:myRecipe];
Direct in Model
Yet another approach would be to have the model handle saving to the network directly. I'm not sure if this is a good idea though.
For example:
[myRecipe save];
What do you think of these? Are there any other patterns that make more sense for this?
I would also stick with Dependency Injection in your case. If you want to read about that you will easily find good articles in the web, e.g. on Wikipedia. There are also links to DI frameworks in Objective C.
Basically, you can use DI if you have two or more components, which must interact but shouldn't know each other directly in code. I'll elaborate your example a bit, but in C#/Java style because I don't know Objective C syntax. Let's say you have
class NetworkAdapter implements NetworkAdapterInterface {
void save(object o) { ... }
}
with the interface
interface NetworkAdapterInterface {
void save(object o);
}
Now you want to call that adapter in a controller like
class Controller {
NetworkAdapterInterface networkAdapter;
Controller() {
}
void setAdapter(NetworkAdapterInterface adapter) {
this.networkAdapter = adapter;
}
void work() {
this.networkAdapter.save(new object());
}
}
Calling the Setter is where now the magic of DI can happen (called Setter Injection; there is also e.g. Constructor Injection). That means that you haven't a single code line where you call the Setter yourself, but let it do the DI framework. Very loose coupled!
Now how does it work? Typically with a common DI framework you can define the actual mappings between components in a central code place or in a XML file. Image you have
<DI>
<component="NetworkAdapterInterface" class="NetworkAdapter" lifecycle="singleton" />
</DI>
This could tell the DI framework to automatically inject a NetworkAdapter in every Setter for NetworkAdapterInterface it finds in your code. In order to do this, it will create the proper object for you first. If it builds a new object for every injection, or only one object for all injections (Singleton), or e.g. one object per Unit of Work (if you use such a pattern), can be configured for each type.
As a sidenote: If you are unit testing your code, you can also use the DI framework to define completely other bindings, suitable for your test szenario. Easy way to inject some mocks!
I have little doubt about adapter class. I know what's the goal of adapter class. And when should be used. My doubt is about class construction. I've checked some tutorials and all of them say that I should pass "Adaptee" class as a dependency to my "Adapter".
e.g.
Class SampleAdapter implements MyInterface
{
private AdapteeClass mInstance;
public SampleAdapter(AdapteeClass instance)
{
mInstance=instance;
}
}
This example is copied from wikipedia. As you can see AdapteeClass is passed to my object as dependency. The question is why? If I'm changing interface of an object It's obvious I'm going to use "new" interface and I won't need "old" one. Why I need to create instance of "old" class outside my adapter. Someone may say that I should use dependency injection so I can pass whatever I want, but this is adapter - I need to change interface of concrete class. Personally I think code bellow is better.
Class SampleAdapter implements MyInterface
{
private AdapteeClass mInstance;
public SampleAdapter()
{
mInstance= new AdapteeClass();
}
}
What is your opinion?
I would say that you should always avoid the new operator in a class when it comes to complex objects (except when the class is a Builder or Factory) to reduce coupling and make your code better testable. Off course objects like a List or Dictionary or value objects can be constructed inside a class method (which is probably the purpose of the class method!)
Lets say for example that your AdapteeClass is a Remote Proxy. If you want to use Unit Testing, your unit tests will have to use the real proxy class because there is no way to replace it in your unit tests.
If you use the first approach, you can easily inject a mock or fake into the constructor when running your unit test so you can test all code paths.
Google has a guide on writing testable code which describes this in more detail but some important points are:
Warning Signs for not testable code
new keyword in a constructor or at field declaration
Static method calls in a constructor or at field declaration
Anything more than field assignment in constructors
Object not fully initialized after the constructor finishes (watch out for initialize methods)
Control flow (conditional or looping logic) in a constructor
Code does complex object graph construction inside a constructor rather than using a factory or builder
Adding or using an initialization block
AdapteeClass can have one or more non-trivial constructors. In this case you'll need to duplicate all of them in your SampleAdapter constructor to have the same flexibility. Passing already constructed object is simpler.
I think creating the Adaptee inside the Adapter is limiting. What if some day you want to adapt a pre-existing instance?
To be honest though, I'd do both if at all possible.
Class SampleAdapter implements MyInterface
{
private AdapteeClass mInstance;
public SampleAdapter()
: base (new AdapteeClass())
{
}
public SampleAdapter(AdapteeClass instance)
{
mInstance=instance;
}
}
Let's assume you have an external hard drive with a regular USB port and you are trying to hook it up with a Mac which only has type-c ports. Yes, you can buy a new drive which has a type-c port but what about the data in it?
It's the same for the adapter pattern. There're times you initialize AdapteeClass with tons of flavors. When you do the conversion, you want to keep all the context.
I often use a pattern where I have a static member function in a class which instantiates object of itself, uses it, and destroys it.
Is this a good pattern? I think so. Does the pattern have a name?
I guess it's sort of a combination of Singleton and Factory method patterns. "Singletory" maybe?
The pattern is called 'Factory method'.
I often use this pattern, if using a factory class is a bit overkill, and when creating an instance of the class is a bit cumbersome (some initialization that has to be done for instance on other objects), or, when you want to have an easy way of creating different types of instances of that class.
are you saying you are doing this
class MyClass {
static void util(){
obj = new MyClass();
obj.InstanceMem();
obj.destroy();
}
void InstanceMem(){}
}
i see this more of a utility method.
well if you think it solves a common reoccurring problem then it may be called as a pattern.
I want to implement singleton for httpconnection....
I have a servlet as a server and client side I use android. I have to use the connection many times. Currently, each time I am doing a new connection, but that is not the right way.. So I want to implement singleton for the httpconnection so I can use that instance in each android class.....
so help me.. what type of code i write..
Show some code from what you have, and I can help you more...
Off the cuff, I would say make the httpconnection object a member variable of the class, and any time you're making a new one, reference the member variable instead.
While binding a singleton with a live HTTPUrlConnection instance is good from a reusable practice, but you should also consider how often a call will be made to the server. If your Android application[s] make a lot of connections and need fast response, then there's no point creating a chokehold on such a resource. But if that's not the case, you can simply have an instance variable, and initialise it within the constructor of your particular class. Something like -
public SomeClass {
HTTTPUrlConnection conn = new HTTPUrlConnection(...);
SomeClass instance = new SomeClass();
private SomeClass() {}
public static getInstance() {
return instance;
}
}
Some might contend why initialise the instances inline and not within the getInstance method, but i believe this is best in terms of thread safety. Just Google on Singletons are not good from thread-safety perspective.
I am going over some OO basics and trying to understand why is there a use of Interface reference variables.
When I create an interface:
public interface IWorker
{
int HoneySum { get; }
void getHoney();
}
and have a class implement it:
public class Worker : Bee, IWorker
{
int honeySum = 15;
public int HoneySum { get { return honeySum; } }
public void getHoney()
{
Console.WriteLine("Worker Bee: I have this much honey: {0}", HoneySum);
}
}
why do people use:
IWorker worker = new Worker();
worker.getHoney();
instead of just using:
Worker worker3 = new Worker();
worker3.getHoney();
whats the point of a interface reference variable when you can just instatiate the class and use it's methods and fields that way?
If your code knows what class will be used, you are right, there is no point in having an interface type variable. Just like in your example. That code knows that the class that will be instantiated is Worker, because that code won't magically change and instantiate anything else than Worker. In that sense, your code is coupled with the definition and use of Worker.
But you might want to write some code that works without knowing the class type. Take for example the following method:
public void stopWorker(IWorker worker) {
worker.stop(); // Assuming IWorker has a stop() method
}
That method doesn't care about the specific class. It would handle anything that implements IWorker.
That is code you don't have to change if you want later to use a different IWorker implementation.
It's all about low coupling between your pieces of code. It's all about maintainability.
Basically it's considered good programming practice to use the interface as the type. This allows different implementations of the interface to be used without effecting the code. I.e. if the object being assigned was passed in then you can pass in anything that implements the interface without effecting the class. However if you use the concrete class then you can only passin objects of that type.
There is a programming principle I cannot remember the name of at this time that applies to this.
You want to keep it as generic as possible without tying to specific implementation.
Interfaces are used to achieve loose coupling between system components. You're not restricting your system to the specific concrete IWorker instance. Instead, you're allowing the consumer to specify which concrete implementation of IWorker they'd like to use. What you get out of it is loosely dependent components and better flexibility.
One major reason is to provide compatibility with existing code. If you have existing code that knows how to manipulate objects via some particular interface, you can instantly make your new code compatible with that existing code by implementing that interface.
This kind of capability becomes particularly important for long-term maintenance. You already have an existing framework, and you typically want to minimize changes to other code to fit your new code into the framework. At least in the ideal case, you do this by writing your new code to implement some number of existing interfaces. As soon as you do, the existing code that knows how to manipulate objects via those interfaces can automatically work with your new class just as well as it could with the ones for which it was originally designed.
Think about interfaces as protocols and not classes i.e. does this object implement this protocol as distinct from being a protocol? For example can my number object be serialisable? Its class is a number but it might implement an interface that describes generally how it can be serialised.
A given class of object may actually implement many interfaces.