Difference between static class and singleton pattern

Question

What real  i e  practical  difference exists between a static class and a singleton pattern   Both can be invoked without instantiation  both provide only one  Instance  and neither of them is thread-safe  Is there any other difference

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One main advantage for Singleton   Polymorphism  Eg   create instance using a Class factory  Say based on some configuration   and we want this object to be really singleton

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From a client perspective  static behavior is known to the client but Singleton behavior can be completed hidden from a client  Client may never know that there only one single instance he s playing around with again and again

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static classes are not for anything that needs state  It is useful for putting a bunch of functions together i e Math  or Utils in projects   So the class name just gives us a clue where we can find the functions and nothing more   Singleton is my favorite pattern and I use it to manage something at a single point  It s more flexible than static classes and can maintain it s state  It can implement interfaces  inherit from other classes and allow inheritance   My rule for choosing between static and singleton   If there is a bunch of functions that should be kept together  then static is the choice  Anything else which needs single access to some resources  could be implemented as a singleton

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Lazy Loading Support of interfaces  so that separate implementation can be provided Ability to return derived type  as a combination of lazyloading and interface implementation

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As I understand the difference between a Static class and non-Static Singleton class  the static is simply a non-instantiated  type  in C   where the Singleton is a true  object   In other words  all the static members in a static class are assigned to the type but in the Singleton are housed under the object  But keep in mind  a static class still behaves like a reference type as its not a value type like a Struct   That means when you create a Singleton  because the class itself isnt static but its member is  the advantage is the static member inside the Singleton that refers to itself is connected to an actual  object  rather than a hollow  type  of itself  That sort of clarifies now the difference between a Static and a Non-Static Singleton beyond its other features and memory usage  which is confusing for me   Both use static members which are single copies of a member  but the Singleton wraps the referenced member around a true instantiated  object  who s address exists in addition to its static member  That object itself has properties wherein in can be passed around and referenced  adding value  The Static class is just a type so it doesn t exist except to point to its static members  That concept sort of cemented the purpose of the Singleton vs Static Class beyond the inheritance and other issues

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I m agree with this definition      The word  single  means single object across the application life   cycle  so the scope is at application level        The static does not have   any Object pointer  so the scope is at App Domain level        Moreover both should be implemented to be thread-safe    You can find interesting other differences about  Singleton Pattern Versus Static Class

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There is a huge difference between a single static class instance  that is  a single instance of a class  which happens to be a static or global variable  and a single static pointer to an instance of the class on the heap   When your application exits  the destructor of the static class instance will be called  That means if you used that static instance as a singleton  your singleton ceased working properly  If there is still code running that uses that singleton  for example in a different thread  that code is likely to crash

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What makes you say that either a singleton or a static method isn t thread-safe  Usually both should be implemented to be thread-safe   The big difference between a singleton and a bunch of static methods is that singletons can implement interfaces  or derive from useful base classes  although that s less common  in my experience   so you can pass around the singleton as if it were  just another  implementation

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To expand on Jon Skeet s Answer     The big difference between a singleton and a bunch of static methods is that singletons can implement interfaces  or derive from useful base classes  although that s less common IME   so you can pass around the singleton as if it were  just another  implementation    Singletons are easier to work with when unit testing a class  Wherever you pass singletons as a parameter  constructors  setters or methods  you can instead substitute a mocked or stubbed version of the singleton

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The difference in my head is implementing object oriented programming  Singleton Prototype  or functional programming Static    We are too focused on the number of objects created by singleton pattern when what we should focus on is that in the end we hold an object  Like others have already said  it can be extended  passed as a parameter but most importantly it is state-full   On the other hand static is used to implement functional programming  The static members belongs to a class  They are stateless   By the way did you know that you can create singleton static classes

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When I want class with full functionality  e g  there are many methods and variables  I use singleton   If I want class with only one or two methods in it  e g  MailService class  which has only 1 method SendMail   I use static class and method

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To illustrate Jon s point what s shown below cannot be done if Logger was a static class The class SomeClass expects an instance of ILogger implementation to be passed into its constructor    Singleton class is important for dependency injection to be possible   using System  using System Collections Generic  using System Linq  using System Text   namespace ConsoleApplication2       class Program               static void Main string   args                         var someClass   new SomeClass Logger GetLogger                            public class SomeClass                public SomeClass ILogger MyLogger                                  public class Logger   ILogger               private static Logger  logger          private Logger                public static Logger GetLogger                         if   logger  null                                 logger   new Logger                               return  logger                     public void Log                                     public interface ILogger                void Log

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I read the following and think it makes sense too   Taking Care of Business Remember  one of the most important OO rules is that an object is responsible for itself  This means that issues regarding the life cycle of a class should be handled in the class  not delegated to language constructs like static  and so on   from the book Objected-Oriented Thought Process 4th Ed

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Example with a static class  public class Any        private static Any instance   new Any         private Singleton             System out println  creating              with singleton patterns only exist one instance   public class Singleton        private static Singleton instance   new Singleton         private Singleton             System out println  creating            if  instance    null                throw new RuntimeException  Imposible create a new instance

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In an article I wrote I have described my point of view about why the singleton is much better than a static class    Static class is not actually canonical class     it   s a namespace with functions and variables Using static class is not a good practice because of breaking object-oriented programming principles Static class cannot be passed as a parameter for other Static class is not suitable for    lazy    initialization Initialization and using of static class is always hard tracked Implementing thread management is hard

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Main differences are    Singleton has an instance object while static class is a bunch of static methods Singleton can be extended e g  through an interface while static class can t be  Singleton can be inherited which supports open close principles in SOLID principles on the other hand static class can t be inherited and we need to make changes in itself  Singleton object can be passed to methods while static class as it does not have instance can t be passed as parameters

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static classes usually are used for libraries  singletons are used if I need only one instance of a particular class  From the point of view of memory there are some differences though  usually in the heap there are allocated only objects  the only methods allocated are methods that are current running  a static class has all methods static too and that will be in the heap from begin  so in general the static class consume more memory

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A singleton is nothing more than a write-once static variable on a class that always refers to the same instance of itself once it s initialized    So  you cannot  use a singleton instead of static variables  nor can you avoid keeping state in static variables by using singletons    The advantage of a singleton is only this  it does not get reinitialized even if some other code tries to reinitialize it a thousand times  This is great for something like a network handler where  y know  it would suck if an instance got replaced with another instance in the middle of waiting for a response    Unless you want an entire app WITH NO INSTANCES ANYWHERE   all static    then the singleton makes sense for these cases where we cannot rely on a lack of human error as the only guarantee something won t get overwritten   But beware  singleton is no guarantee against state living everywhere  Your network handler may within itself also rely on other singletons  etc  Now we have state living in a host of singletons    marvelous   And there is no compiler in existence that will ensure that the singleton is where all your state lives  or any other such ideas  at compile time  You can have a hundred static variables on a class that has a singleton  And the singleton can access the static variables  And the compiler will not care    So I would caution anyone from assuming that using a singleton guarantees anything at all about where state lives  Its only guarantee is simply that it is the only instance of its class  ever  And that is also its only advantage   Any other advantages that the other answers claim for singletons are things the compiler does not guarantee and that may vary from language to language  Dependency injection is a supplementary pattern that may rely on a singleton  though it may or may not be the best solution or the only solution in a given language  In languages that lack generics  or that place arbitrary restrictions on calling static accessors and functions  resorting to the singleton pattern may indeed be the best available solution to a given problem   Singleton is not necessary at all in a language like Swift  to get dependency injection  testable code  well-managed state  thread-safe accessors  polymorphism  etc  However it s still useful for the guarantee of a single instance ever   To recap  a singleton is nothing more than a static variable that serves as a safeguard against multiple instances of given class existing  and against the single instance being overwritten by a new one  That s it  period  full stop

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The true answer is by Jon Skeet  on another forum here      A singleton allows access to a single   created instance - that instance  or   rather  a reference to that instance    can be passed as a parameter to other   methods  and treated as a normal   object       A static class allows only static   methods

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Here s a good article  http   javarevisited blogspot com au 2013 03 difference-between-singleton-pattern-vs-static-class-java html  Static classes   a class having all static methods  better performance  static methods are bonded on compile time  can t override methods  but can use method hiding   What is method hiding in Java  Even the JavaDoc explanation is confusing   public class Animal       public static void foo             System out println  Animal             public class Cat extends Animal       public static void foo         hides Animal foo           System out println  Cat               Singleton   an object that can only be instantiated once  methods can be overridden  Why doesn  39 t Java allow overriding of static methods   easier to mock then static methods better at maintaining state   In summary  I would only use static classes for holding util methods  and using Singleton for everything else     Edits   static classes are lazy loaded as well  Thanks  jmoreno   When does static class initialization happen   method hiding for static classes  Thanks  MaxPeng

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In many cases  these two have no practical difference  especially if the singleton instance never changes or changes very slowly e g  holding configurations    I d say the biggest difference is a singleton is still a normal Java Bean as oppose to a specialized static-only Java class  And because of this  a singleton is accepted in many more situations  it is in fact the default Spring Framework s instantiation strategy  The consumer may or may not know it s a singleton being passed around  it just treat it like a normal Java bean  If requirement changes and a singleton needs to become a prototype instead  as we often see in Spring  it can be done totally seamlessly without a line of code change to the consumer   Someone else has mentioned earlier that a static class should be purely procedural e g  java lang Math  In my mind  such a class should never be passed around and they should never hold anything other than static final as attributes  For everything else  use a singleton since it s much more flexible and easier to maintain

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I ll try to rise above the WTMI and WTL DR responses    Singletons are an instance of an object    full stop    Your question is fundamentally asking about the difference between a class an an instance of that class   I think that s pretty clear and requires no elaboration    The singleton s class generally takes steps to ensure construction of one instance  that s smart but it s not required    Example  var connection   Connection Instance    Say this is the Connection class   public sealed class Connection        static readonly Connection  instance   new Connection         private Connection                     public static Connection Instance               get                      return  instance                       Note that you can throw an interface on that class and mock it for testing purposes  something you cannot easily do with a static class

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We have our DB framework that makes connections to Back end To Avoid Dirty reads across Multiple users we have used singleton pattern to ensure we have single instance available at any point of time   In c  a static class cannot implement an interface  When a single instance class needs to implement an interface for a business contracts or IoC purposes  this is where I use the Singleton pattern without a static class  Singleton provides a way to maintain state in stateless scenarios  Hope that helps you

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The Singleton pattern has several advantages over static classes  First  a singleton can extend classes and implement interfaces  while a static class cannot  it can extend classes  but it does not inherit their instance members   A singleton can be initialized lazily or asynchronously while a static class is generally initialized when it is first loaded  leading to potential class loader issues  However the most important advantage  though  is that singletons can be handled polymorphically without forcing their users to assume that there is only one instance

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Singleton is better approach from testing perspective  Unlike static classes   singleton could implement interfaces and you can use mock instance and inject them   In the example below I will illustrate this  Suppose you have a method isGoodPrice   which uses a method getPrice   and you implement getPrice   as a method in a singleton   singleton that   s provide getPrice functionality   public class SupportedVersionSingelton        private static ICalculator instance   null       private SupportedVersionSingelton                public static ICalculator getInstance            if instance    null               instance   new SupportedVersionSingelton                       return instance              Override     public int getPrice                calculate price logic here         return 0            Use of getPrice   public class Advisor        public boolean isGoodDeal             boolean isGoodDeal   false          ICalculator supportedVersion   SupportedVersionSingelton getInstance            int price   supportedVersion getPrice                logic to determine if price is a good deal          if price  lt  5               isGoodDeal   true                     return isGoodDeal            In case you would like to test the method isGoodPrice   with mocking the getPrice   method you could do it by  Make your singleton implement an interface and inject it        public interface ICalculator           int getPrice            Final Singleton implementation   public class SupportedVersionSingelton implements ICalculator        private static ICalculator instance   null       private SupportedVersionSingelton                public static ICalculator getInstance            if instance    null               instance   new SupportedVersionSingelton                       return instance              Override     public int getPrice             return 0                for testing purpose     public static void setInstance ICalculator mockObject           if instance    null    instance   mockObject          test class   public class TestCalculation        class SupportedVersionDouble implements ICalculator           Override         public int getPrice                  return 1                          Before     public void setUp   throws Exception           ICalculator supportedVersionDouble   new SupportedVersionDouble            SupportedVersionSingelton setInstance supportedVersionDouble                Test     public void test               Advisor advidor   new Advisor              boolean isGoodDeal   advidor isGoodDeal              Assert assertEquals isGoodDeal  true               In case we take the alternative of using static method for implementing getPrice     it was difficult to the mock getPrice    You could mock static with power mock  yet not all product could use it

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In singleton pattern you can create the singleton as an instance of a derived type  you can t do that with a static class   Quick Example   if  useD3D       IRenderer  instance   new D3DRenderer else     IRenderer  instance   new OpenGLRenderer

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Another advantage of a singleton is that it can easily be serialized  which may be necessary if you need to save its state to disc  or send it somewhere remotely

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Singleton s are instantiated  it s just there s only one instance ever instantiated  hence the single in Singleton   A static class can t be instantiated by anything other than itself

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Both can be invoked without instantiation  both provide only one  Instance  and neither of them is thread-safe  Is there any other difference    The question is wrong  both its statements  Please note  that static class here means nested static class And not a class with only static methods   I am assuming i e  static class meaning nested static class and not a class with only static members  so because if I see most popular Singleton implementation i e  DCL way  its nothing but static declaration of instance and static method to get the Singleton instance  Its one implementation  So in this case what s the difference between Singleton and a class with only static members  Though other implementations are possible like that using Enum   Let me correct the statements    Singleton class can have single instance application-wide  Nested static class can have multiple instance see the code below as a proof   Read the basics of nested class here  No class is inherently thread-safe  it has to be made thread-safe programmatically  It can be done for both nested static class and Singleton    Some more myth busters are below Most of answers to this question has given these statements so thought it would be good to programmatically prove it      Nested static class can implement interface like any other class  Nested static class can extend other non-final class  Nested static class can have instance variables  Nested static class can have parameterized constructor s     In the code below you can see that nested static class NestedStaticClass implements interface  extends another class  have instance variable and parameterized constructor      package com demo core       public class NestedStaticClassTest               public static void main String   args                        OuterClass NestedStaticClass obj1   new OuterClass NestedStaticClass                OuterClass NestedStaticClass obj2   new OuterClass NestedStaticClass                 if obj1    obj2                                System out println  Both nested static objects are equal                                  else                               System out println  NOT EQUAL                                     System out println OuterClass NestedStaticClass d                obj1 setD 5                System out println OuterClass NestedStaticClass d                System out println obj1 sum                          class OuterClass               int a  1          static int b   2           static class NestedStaticClass extends OneClass implements Sample                       int c   3              static int d   4               public NestedStaticClass                                              Parameterized constructor             public NestedStaticClass int z                                c   z                             public int sum                                 int sum   0                  sum   b   c   d   getE                    return sum                             public static int staticSum                                 int sum   0                  sum   b   d                  return sum                             public int getC                                 return c                            public void setC int c                                this c   c                            public static int getD                                 return d                            public static void setD int d                                NestedStaticClass d   d                                     interface Sample                   class OneClass               int e   10          static int f   11           public int getE                         return e                    public void setE int e                        this e   e                    public static int getF                         return f                    public static void setF int f                        OneClass f   f

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Static Class -   You cannot create the instance of static class  Loaded automatically by the  NET Framework common language runtime  CLR  when the program or namespace containing the class is loaded  Static Class cannot have constructor  We cannot pass the static class to method  We cannot inherit Static class to another Static class in C   A class having all static methods  Better performance  static methods are bonded on compile time    Singleton -   You can create one instance of the object and reuse it  Singleton instance is created for the first time when the user requested   Singleton class can have constructor  You can create the object of singleton class and pass it to method  Singleton class does not say any restriction of Inheritance  We can dispose the objects of a singleton class but not of static class  Methods can be overridden  Can be lazy loaded when need  static classes are always loaded   We can implement interface static class can not implement interface

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A static class is one that has only static methods  for which a better word would be  functions   The design style embodied in a static class is purely procedural   Singleton  on the other hand  is a pattern specific to OO design  It is an instance of an object  with all the possibilities inherent in that  such as polymorphism   with a creation procedure that ensures that there is only ever one instance of that particular role over its entire lifetime

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A static class in Java has only static methods  It is a container of functions  It is created based on procedural programming design   Singleton class is a pattern in Object Oriented Design  A Singleton class has only one instance of an object in JVM  This pattern is implemented in such a way that there is always only one instance of that class present in JVM

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Well a singleton is just a normal class that IS instantiated but just once and indirectly from the client code  Static class is not instantiated  As far as I know static methods  static class must have static methods  are faster than non-static      Edit  FxCop Performance rule description   Methods which do not access instance data or call instance methods can be marked as static  Shared in VB   After doing so  the compiler will emit non-virtual call sites to these members which will prevent a check at runtime for each call that insures the current object pointer is non-null  This can result in a measurable performance gain for performance-sensitive code  In some cases  the failure to access the current object instance represents a correctness issue   I don t actually know if this applies also to static methods in static classes

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Singleton class provides an object only one instance  during the application lifeCycle such as java lang Runtime While Static class only provide static methods such as java lang Math  Static methods in Java cannot be overridden  but methods defined in Singleton class can be overridden by extending it   Singleton Class is capable of  Inheritance and Polymorphism to extend a base class  implement an interface and capable of providing different implementations  whereas static not    For eg  java lang Runtime is a Singleton Class in Java  call to getRuntime   method returns the runtime object associated with the current Java application but ensures only one instance per JVM

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One notable difference is differed instantiation that comes with Singletons   With static classes  it gets created by the CLR and we have not control on it  with singletons  the object gets instantiated on the first instance it s tried to be accessed

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Singleton objects are stored in Heap  but static objects are stored in stack  We can clone  if the designer did not disallow it  the singleton object  but we can not clone the static class object   Singleton classes follow the OOP  object oriented principles   static classes do not  We can implement an interface with a Singleton class  but a class s static methods  or e g  a C  static class  cannot

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a  Serialization - Static members belong to the class and hence can t be serialized   b  Though we have made the constructor private  static member variables still will be carried to subclass   c  We can t do lazy initialization as everything will be loaded upon class loading only

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We can create the object of singleton class and pass it to method   Singleton class doesn t any restriction of inheritance   We can t dispose the objects of a static class but can singleton class

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I m not a great OO theorist  but from what I know  I think the only OO feature that static classes lack compared to Singletons is polymorphism  But if you don t need it  with a static class you can of course have inheritance   not sure about interface implementation   and data and function encapsulation   The comment of Morendil   The design style embodied in a static class is purely procedural  I may be wrong  but I disagree  In static methods you can access static members  which would be exactly the same as singleton methods accessing their single instance members     edit  I m actually thinking now that another difference is that a Static class is instantiated at program start  and lives throughout the whole life span of the program  while a singleton is explicitly instantiated at some point and can be destroyed also       or it may be instantiated at first use  depending on the language  I think

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Distinction from static class  JDK has examples of both singleton and static  on the one hand java lang Math is a final class with static methods  on the other hand java lang Runtime is a singleton class   Advantages of singleton   If your need to maintain state than singleton pattern is better choice than static class  because maintaining state in static class leads to bugs  especially in concurrent environment  that could lead to race conditions without adequate synchronization parallel modification by multiple threads  Singleton class can be lazy loaded if its a heavy object  but static class doesn t have such advantages and always eagerly loaded  With singleton  you can use inheritance and polymorphism to extend a base class  implement an interface and provide different implementations  Since static methods in Java cannot be overridden  they lead to inflexibility  On the other hand  you can override methods defined in singleton class by extending it    Disadvantages of static class   It is easier to write unit test for singleton than static class  because you can pass mock object whenever singleton is expected    Advantages of static class   Static class provides better performance than singleton  because static methods are bonded on compile time    There are several realization of singleton pattern each one with advantages and disadvantages    Eager loading singleton Double-checked locking singleton Initialization-on-demand holder idiom The enum based singleton   Detailed description each of them is too verbose so I just put a link to a good article - All you want to know about Singleton

[design-patterns] Difference between static class and singleton pattern?

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