When and where to use the Singleton Pattern?

There are three use cases where we should go for singleton pattern.
1. When a class has no attribute ,so it has no state and we are passing value as method parameters, in that case there is no need to create multiple objects . By creating single object we can call that methods multiple times . See below for the example:
   
       class Circle{
         
          public area (int radius)
             {
                //display area
                 area=3.1412*r*r;
             }
        }


// here i am creating object for single time if i create multiple object also it refer to the same state only. So here no need to create multiple objects ,so declare a private constructor to make singleton class.
 
       Circle c=new Circle();
        c.area();

One more point also you can rise here that if we declare the methods as static then no need to create object.I can agree with this point but if we declare as static then there is a chance in the application we can create object but if we make the class as singleton then there is no chance that we can create more than one object in entire application.  

2. When the case is read-only state then at that case also there is no need to create multiple objects ,we can declare singleton.
         Ex:
             class Circle 
              {
                  private final pi=3.1412;
                 public area()
                 {
                       //display area
                  }
             } 

       In this case also declare class as singleton because here if we create multiple objects and call methods then it is also refer to the same object.


I hope this will helpful ,i have only given the idea ....so if anything wrong reply

Singleton pattern restricts the instantiation of a class and ensures that only one instance of the class exists in the java virtual machine. The singleton class must provide a global access point to get the instance of the class. 

 Singleton pattern is used for logging, drivers objects, caching and thread pool.(in log4j,Hibernate we are using the this pattern)

For more Details about Singleton Design Pattern then go through this blog http://adnjavainterview.blogspot.in/2014/06/singleton-design-pattern-in-java-with.html

I'd say avoid singleton pattern if you can. There are many ways to perform logging/security ect. You could use AOP techniques. The problem with the singleton patter is that its hard to test, it breaks the single responsibility pattern too.

They cause tightly coupled code
They're in control of their own lifecycle
You can mock or test them easily
You cannot extend them (open/closed principle)

I'm just saying really have a think about the design and whether a singleton is the right way to go.

logging, communication,Configuration classes reading from class [may be put in cache for performance hit] and Database access.

Hi,

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.

Hope that helps you..

You don't.

Singleton pattern is an anti-pattern. Please lookup why e.g. http://www.google.com/search?rlz=1C1MDNC_daDK434DK434&sugexp=chrome,mod=5&sourceid=chrome&ie=UTF-8&q=anti+pattern+singleton 

The best alternative I have found is to have a static class that registers managers by their interface e.g.
ISomeKindOfManager someKindOfManager = InstanceRegistry.Get<ISomeKindOfManager>();

Here is more information http://yoawsconsult.blogspot.dk/2009/06/feel-bliss-of-design-pattern.html.

Kind Regards,

Keld Ølykke

 

Hi

 We can make use of singleton in following scenarios.

1. When the class is stateless eg: Validator classes instead of using static methods for validation because we cant mock static methods so testing becomes difficult. instead if we have singleton we can mock the instance itself.
2. When  we need only one state of class at any given point of time. In this case we have to take care of synchronization.

Thanks
ChandraShekar

Hi, We used the singleton pattern in our utility layers which consist of Loggging, Caching, Service host repositories, Load Balancer... If the question is on how we arrived to the design. There was a performance lag on the utility layer eg, Logging, on diagnosing we observed that there are several instance getting created which are not required in my case. So we adopted Singleton pattern. Hope this helps

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.

Logger Instance

As you were asking about a real world example, here we go.

A few days ago, I had to implement holding a list of loaded assemblies and its references during program-execution. As this is a bigger project, we're talking about 150 dll-files to look at. In that case, I just didn't want to load the same data again and again whenever I need to get one of these assemblies. Therefore, I used a singleton-class, which creates this certain list on its first call and holds that values until the application gets closed.

Hi

hope you can understand here :)

http://www.allapplabs.com/java_design_patterns/singleton_pattern.htm 

The singleton pattern can be used for anything that you don't want to repeat. If the object in question is not expected to change, it is a good candidate for the singleton pattern. The singleton pattern is preferred over a static implementation in most cases. Static implementations can cause dependency headaches. Can you provide more specifics as to what exactly you're looking to do?

Hi,

You can use it in remoting when you want to create a shareable service....