What is a singleton class and when do you use it?

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Vishal Deep

11+ year of IT Experience in Manual & Automation(Java + Selenium+Rest Assured) Testing

Singleton class have a private constructor. while we can't create an object from outside of class, we can create an object only once.
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One reason that tends to come up over and over again on the internets is that of a "logging" class (which you mentioned). In this case, a Singleton can be used instead of a single instance of a class because a logging class usually needs to be used over and over again ad nauseam by every class in a project....
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One reason that tends to come up over and over again on the internets is that of a "logging" class (which you mentioned). In this case, a Singleton can be used instead of a single instance of a class because a logging class usually needs to be used over and over again ad nauseam by every class in a project. If every class uses this logging class, dependency injection becomes cumbersome. Logging is a specific example of an "acceptable" Singleton because it doesn't affect the execution of your code. Disable logging, code execution remains the same. Enable it, same same. Misko puts it in the following way in Root Cause of Singletons, "The information here flows one way: From your application into the logger. Even though loggers are global state, since no information flows from loggers into your application, loggers are acceptable." read less
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Tutor

Singleton class has only one instance and is generally meant for shortened finish of a program
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Big Data Expert

A class is called singleton when it limits the number of object creation for that class to be one. We can't have more than a single object of that class. Singleton class are employed extensively in concepts like networking and database connectivity.
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Trainer

singleton is a design pattern that restricts the instantiation of a class to one object.
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B.E. (IT)

One reason that tends to come up over and over again on the internets is that of a "logging" class (which you mentioned). In this case, a Singleton can be used instead of a single instance of a class because a logging class usually needs to be used over and over again ad nauseam by every class in a project....
read more
One reason that tends to come up over and over again on the internets is that of a "logging" class (which you mentioned). In this case, a Singleton can be used instead of a single instance of a class because a logging class usually needs to be used over and over again ad nauseam by every class in a project. If every class uses this logging class, dependency injection becomes cumbersome. Logging is a specific example of an "acceptable" Singleton because it doesn't affect the execution of your code. Disable logging, code execution remains the same. Enable it, same same. Misko puts it in the following way in Root Cause of Singletons, "The information here flows one way: From your application into the logger. Even though loggers are global state, since no information flows from loggers into your application, loggers are acceptable." Singleton candidate must satisfy three requirements: controls concurrent access to a shared resource. access to the resource will be requested from multiple, disparate parts of the system. there can be only one object. If your proposed Singleton has only one or two of these requirements, a redesign is almost always the correct option. For example, a printer spooler is unlikely to be called from more than one place (the Print menu), so you can use mutexes to solve the concurrent access problem. A simple logger is the most obvious example of a possibly-valid Singleton, but this can change with more complex logging schemes. read less
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Java trainer , Data structure and algorithms

Singleton design pattern can be obtained by using following steps: 1.Private Constructor 2.Static Factory Method 3.Static Reference Object Creation As we are using a Static Reference Object Creation ,it is a "static" member and a static member can execute only one time and will create only one object. Creating...
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Singleton design pattern can be obtained by using following steps: 1.Private Constructor 2.Static Factory Method 3.Static Reference Object Creation As we are using a Static Reference Object Creation ,it is a "static" member and a static member can execute only one time and will create only one object. Creating only one Object for the entire class saves the memory space and which helps to not to degrade the performance, which is most important. Since we are able to create exactly one instance for one class ,it is called as Singleton class read less
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Talent with Professional Experience

Singleton class is a class which has one instance
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Trainer

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
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Trainer

A singleton class is one which has only one object of itself own which is a static object and outside the class we cant create another object as the constructor is private.
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