When Should I Use Singleton?

Which is better singleton or static class?

While a static class allows only static methods and and you cannot pass static class as parameter.

A Singleton can implement interfaces, inherit from other classes and allow inheritance.

While a static class cannot inherit their instance members.

So Singleton is more flexible than static classes and can maintain state..

Why is Singleton bad?

By using singletons in your project, you start to create technical debt. Singletons tend to spread like a virus because it’s so easy to access them. It’s difficult to keep track of where they’re used and getting rid of a singleton can be a refactoring nightmare in large or complex projects.

Why should we not use Singleton pattern?

Singleton pattern should only be used to guarantee that one and only one instance of a given class exists during run time. People think Singleton is evil because they are using it for globals. It is because of this confusion that Singleton is looked down upon.

Why is Singleton bad for testing?

While they provide a quick and easy solution, singletons are considered bad because they make unit testing and debugging difficult. … This property allows you to substitute alternate implementations for collaborators during testing to achieve specific testing goals (think mock objects).

Why Singleton is not thread safe?

Singleton is mostly considered an anti-Pattern, because it brings inherent complexity to the system in terms of testing. Only DI frameworks (Spring, Dagger, etc) should be allowed to create Singletons for you rather than you writing the singletons.

Is Singleton class thread safe?

Thread Safe Singleton: A thread safe singleton in created so that singleton property is maintained even in multithreaded environment. To make a singleton class thread-safe, getInstance() method is made synchronized so that multiple threads can’t access it simultaneously. Pros: … It is also thread safe.

Can you inherit from a singleton?

You cannot inherit or instantiate a static class. Unlike static classes, Singleton classes can be inherited, can have base class, can be serialized and can implement interfaces. … Static classes can only have static members are these classes cannot be inherited — these classes are sealed implicitly.

Why is singleton class used?

The purpose of the singleton class is to control object creation, limiting the number of objects to only one. The singleton allows only one entry point to create the new instance of the class. … Singletons are often useful where we have to control the resources, such as database connections or sockets.

Can we override Singleton class?

The only way to override a singleton is to have a singleton that expects to be overridden. The simplest way to do this is to provide Singleton that implements an interface (or is otherwise fully abstract itself) that internally instantiates an injected singleton upon the first use of getInstance() .

Why is singleton class sealed?

The sealed keyword means that the class cannot be inherited from. … Marking the class sealed prevents someone from trivially working around your carefully-constructed singleton class because it keeps someone from inheriting from the class.

Is Singleton class immutable?

An immutable object is initialized by its constructor only, while a singleton is instantiated by a static method. A set of functions (or static methods) which manipulate some shared mutable state constitute a singleton. … This implies that every mutable member of a singleton collection is itself a singleton.

Can singleton class extend another class?

Usually you wouldn’t extend a Singleton class. In fact, usually a Singleton class would be designed in a way to prevent it being extended, in case extending it was used as a way to create multiple instances. Matthew Brown wrote: Usually you wouldn’t extend a Singleton class.

How can Singleton be prevented?

There are many ways to prevent Singleton pattern from Reflection API, but one of the best solutions is to throw a run-time exception in the constructor if the instance already exists. In this, we can not able to create a second instance.

What is lazy Singleton?

In software engineering, the initialization-on-demand holder (design pattern) idiom is a lazy-loaded singleton. … A singleton implementation may use lazy initialization, where the instance is created when the static method is first invoked.

When would you use a singleton?

A singleton should be used when managing access to a resource which is shared by the entire application, and it would be destructive to potentially have multiple instances of the same class. Making sure that access to shared resources thread safe is one very good example of where this kind of pattern can be vital.

What is the advantage of Singleton design pattern?

Instance control: Singleton prevents other objects from instantiating their own copies of the Singleton object, ensuring that all objects access the single instance. Flexibility: Since the class controls the instantiation process, the class has the flexibility to change the instantiation process.

Why can’t we use static class instead of Singleton?

You never can change how it behaves after the class is decorated with the static keyword. The Singleton class does not require you to use the static keyword everywhere. Static class objects cannot be passed as parameters to other methods whereas we can pass instances of a singleton as a parameter to another method.

What are the disadvantages of Singleton pattern?

One of the main disadvantages of singletons is that they make unit testing very hard. They introduce global state to the application. The problem is that you cannot completely isolate classes dependent on singletons. When you are trying to test such a class, you inevitably test the Singleton as well.