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Difference between static class and singleton pattern?
Why would one ever require one and only one instance? Same purpose can be achieved using classes with static member variables and static methods.
As far as I can find out, there might be two possible answers to it -
When your class needs to have state and you want only one object of it. From the design point of view, class with static methods & variables are considered to be the Utility classes and shouldn't be keeping any state.
If your class needs to take part in polymorphism and you want only one object of the class(es) which are in the inheritance tree.
It would be really helpful if someone can provide an example from real life scenario or from any Java API where Singleton objects need to participate in Polymorphism / Inheritance?
Collections.emptySet() is a typical example of a singleton that can't be implemented as a static class since, obviously, its goal is to be an instance of the java.util.Set interface. It's not costly to create, but it would be stupid to create a new instance each time an empty set is needed, since the unique instance can be reused.
Classes that perform logging or common access to data bases frequently follow the Singleton pattern. Basically anything that should have instance methods and that is costly to construct.
Scope and behavior are different concerns and should NOT be mixed. You may want your object to be available per use, per thread, per web request, per session or global (Singleton). The reasons for making these adjustments are likely due to resource management and ultimately performance. The behavior inside your class shouldn't have to change if you change its scope.
Singleton is pattern for taking a regular object and controlling its scope with just a little bit of bolt-on code. Ideally though, you really shouldn't really deal with scope at all inside your object and delegate that to a factory or container.
My answer is quite short but it's enough to use exactly common singleton instead of it's static implementation. The answer is:
Popular paradigm (yes it is!)
Threads (synchronization etc.)
Interface implementation (your static class has some restrictions)
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Difference between static class and singleton pattern?
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what is the better approach?
Let us assume a scenario in which there is a utility class that is used by other classes. What is better to use in this case, a singleton class which can be instantiated exactly once or should i make all the fieds static?
In object-oriented programming generally you should avoid singletons and utility classes if possible.
However, if really needed I'd go with utility class without any fields - just static methods. By definition utilities should be rather set of stateless functions. Such are well testable in comparison to untestable singleton (which is done with static field). If you need to keep the state then go towards true objects.
As stated in the comment, you can have a safe singleton done by dependency injection, without static state.
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With the time ...lots of utility method are introduced in java project for more complex and simple task.
When using static methods we introduce tight coupling in our code and it make our code more difficult to test, especially if the utility methods are quite complex.
I am just thinking that it now difficult to manage and test these utilities. please guide me in avoiding these utilities methods and how can i organize existing project to remove all STATIC utilities.
Can you help me avoiding static method ?
There is nothing wrong with having lots of static methods.
Static methods are (or should be, read on) stateless, which makes them the easiest methods to test - there's no setup, just call them.
You don't need mocking, because there is no state to deal with.
Regarding being stateless, technically static methods can be stateful if they use static variables to store state. If this is the case, from a good-design perspective they should be converted to instance methods using instance variables to store state, employing the singleton pattern if required.
To contradict the other answers currently available: Static methods are bad!
They do introduce strong coupling. Yes there are cases where it is acceptable. Yes you can make a seam for inside a static method, by making the strategy used inside exchangeable. But as a rule of thumb static are still bad.
To answer the question, how to get rid of static methods. Simple: put them on a proper Object. All statics are gone. Have we improved our code? not much yet. If we replace
callToStaticMethod()
with
new X().callToNoLongerStaticMethod()
we replaced a static call with a constructor call which is essentially just another static method. But now your X is just another dependency, so you can inject it:
class A{
private final X x;
A(X aX){
x = aX;
}
}
Note: there is no need to use Spring or any other framework for this. If you feel like it provide a constructor which uses the default implementation. If you are a purist, introduce an interface for X.
Testing A without relying on the implementation of X becomes trivial and obvious. Same for replacing X in any way.
Static utility methods are not so bad. You can hide a package-private strategy behind the static call. This can be easily tested (and replaced) given that the test case belongs to the same package. Moreover, it makes the code very readable. Of course, the clients of the static utility method can still only use one implementation in their tests. So here is some inflexibility.
Bohemian is right when talking about state. If your static utilities have state you are doing something wrong.
About your question: If you want to avoid static methods you can use the spring framework and define different implementations of utilities that you use and test in different contexts. In this case, however, access to these objects is not so convenient as you must first obtain a reference to the context that knows your utility object.
Nothing wrong with a set of static utility methods that belong together in a class. See for example java.util.Collections. If every method in that class that operates on a List would be specified in the List interface itself, they would have to be implemented by all subclasses. As long as they can be implemented by the public List methods, there is no problem.
Of course, as soon as you start adding methods to the interface (or in case of a class, making methods public) only to be able to put functionality in static methods instead of the class itself, then you're on the wrong path.
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When I read about nesting an interface inside of a class, the intention appears to be to encapsulate the abstract behavior of the interface through composition. However, to me it makes more sense to create the interface outside the class, then use a getter/setter and return an instance of the interface type. There must be a benefit that I'm not seeing. Is this simply a matter of "choice".
If the interface strongly related with some class, it might be reasonable to nest it within the class. For example: SurfaceHolder.Callback which allows a client to receive information about changes to the surface in Android. The Callback interface is nested within the SurfaceHolder, and it is easier to access and find it within that context.
However, for generic interfaces such as Runnable which is implemented by a class whose instances to be executed by a thread, it is completely outside of a class (in the java.lang package for this example). This make more sense because, this interface could be used by any class, not necessarily within a specific context).
It is not a matter of choice if we talk about highly complex, scalable and re-usable systems. There are so named S.O.L.I.D. design principles (dependency injection and inversion of control if we talk specifically) which suppose the use of interfaces. The reasons to use them are:
1) the decoupling of the models inside your code. If you create the objects inside a class when your classes cannot be used in other projects as they are tightly coupled
2) possibility of creating mocks for right unit tests. You can test concrete layers, in your case by testing the highest level you invoke all the methods of the lower levels.
3) Interfaces can be combined and used throughout the whole system in many places so a lot of other classes can use the mailer server, for example
I have a helper class that creates some objects, like a builder. The helper class does not have a state. It is on a multi-threaded environment; specifically, a web server. Is this class a good candidate for being a singleton?
What would be the difference between implementing this class as a singleton and just using static methods?
What would the effect of thousands of users accessing this object/these methods be?
I could make the class a regular class, but instantiating it every time it is needed would be a waste of memory.
Infact instead of singleton you can make the methods static.
Singleton doesn't have to be only 1, you can create a pool of instances and delegate work depending on the requirement, where as you don't have such control with static methods.
discussion on Singleton vs Static methods is here
As the name suggests, singletons are used to have only one instance of the object present at the time. So singleton does have a state, but you're accessing to that one state wherever you're calling your singleton.
So if you don't need any state saved in your class/method I'd suggest to use static approach.
No need to use singleton here (since you do not need a state), you can use static methods.
Singleton in principle offers more control by allowing a state. There won't be much difference in your case, but static methods will be easier to implement and use.
What would the effect of thousands of users accessing this object/these methods be?
Again, not much difference in both cases, but in Singleton you can have a state, and if you do not implement carefully, your code will be non-thread-safe. Every user calling the static method gets its own "instance" of the method (I think this is what you ask), so no risk of running into thread-safety problems there.
As has been stated before, given that your class doesn't have object state, static methods would work just fine.
However, consider the following - Depending on the overall design of your system, you may want to be able to specify a different implementation of the methods. This is usually done with either subclassing (...), or interface implementation (now the preferred method) - look up the strategy pattern. In either case, being able to provide alternte implementations would require you to not use static methods, but to have an (empty) object to call methods on.
I have never found good answers to these simple questions about helper/utility classes:
Why would I create a singleton (stateless) instead of using static methods?
Why would an object instance be needed if an object has no state?
Often, singletons are used to introduce some kind of global state to an application. (More often than really necessary, to be honest, but that's a topic for another time.)
However, there are a few corner cases where even a stateless singleton can be useful:
You expect to extend it with state in the foreseeable future.
You need an object instance for some particular technical reason. Example: Synchonization objects for the C# lock or the Java synchronized statement.
You need inheritance, i.e., you want to be able to easily replace your singleton with another one using the same interface but a different implementation.Example: The Toolkit.getDefaultToolkit() method in Java will return a singleton whose exact type is system dependent.
You want reference equality for a sentinel value.Example: DBNull.Value in C#.
I could see a case for a stateless singleton being used instead of a static methods class, namely for Dependency Injection.
If you have a helper class of utility functions that you're using directly, it creates a hidden dependency; you have no control over who can use it, or where. Injecting that same helper class via a stateless singleton instance lets you control where and how it's being used, and replace it / mock it / etc. when you need to.
Making it a singleton instance simply ensures that you're not allocating any more objects of the type than necessary (since you only ever need one).
Actually i've found another answer not mentionned here: static methods are harder to test.
It seems most test frameworks work great for mocking instance methods but many of them no not handle in a decent way the mock of static methods.
In most programming languages classes elude a lot of the type system. While a class, with its static methods and variables is an object, it very often cannot implement an interface or extend other classes. For that reason, it cannot be used in a polymorphic manner, since it cannot be the subtype of another type. For example, if you have an interface IFooable, that is required by several method signatures of other classes, the class object StaticFoo cannot be used in place of IFooable, whereas FooSingleton.getInstance() can (assuming, FooSingleton implements IFooable).
Please note, that, as I commented on Heinzi's answer, a singleton is a pattern to control instantiation. It replaces new Class() with Class.getInstance(), which gives the author of Class more control over instances, which he can use to prevent the creation of unneccessary instances. The singleton is just a very special case of the factory pattern and should be treated as such. Common use makes it rather the special case of global registries, which often ends up bad, because global registries should not be used just willy-nilly.
If you plan to provide global helper functions, then static methods will work just fine. The class will not act as class, but rather just as a namespace. I suggest, you preserve high cohesion, or you might end up with weirdest coupling issues.
greetz
back2dos
There is a trade-off between using which one. Singletons may or may not have state and they refer to objects. If they are not keeping state and only used for global access, then static is better as these methods will be faster. But if you want to utilize objects and OOP concepts (Inheritance polymorphism), then singleton is better.
Consider an example: java.lang.Runtime is a singleton class in java. This class allows different implementations for each JVM. The implementation is single per JVM. If this class would have been static, we cannot pass different implementations based on JVM.
I found this link really helpful: http://javarevisited.blogspot.com/2013/03/difference-between-singleton-pattern-vs-static-class-java.html?
Hope it helps!!
Singleton is not stateless, it holds the global state.
Some reasons which I can think of using Singleton are:
To avoid memory leaks
To provide the same state for all modules in an application e.g database connection
For me "Want Object State use Singleton, Want Function use static method"
It depends on what you want. Whenever you want the object state (e.g. Polymorphism like Null state instead of null, or default state), singleton is the appropriate choice for you whereas the static method use when you need function (Receive inputs then return an output).
I recommend for the singleton case, it should be always the same state after it is instantiated. It should neither be clonable, nor receive any value to set into (except static configuration from the file e.g. properties file in java).
P.S. The performance between these 2 are different in milliseconds, so focus on Architecture first.
According to GoF’s book Design Patterns, chapter ‘Singleton’, class operations have the following drawbacks compared to singletons (bold emphasis mine):
More flexible than class operations. Another way to package singleton’s functionality is to use class operations (that is, static member functions in C++ or class methods in Smalltalk). But both of these language techniques make it hard to change a design to allow more than one instance of a class. Moreover, static member functions in C++ are never virtual, so subclasses can’t override them polymorphically.