Singleton Pattern In Android

In this blog, I am describing the most important concept of Android Development which is the Singleton Pattern. It is also a very important interview question that is asked to Android Developers.  

What is Singleton in Android?

It is basically a design pattern we use in Android Application Development. 

How it is achieved?

It is achieved by simply making the constructor of a class as private.

Purpose of this pattern?

This pattern is basically used to restrict a class of making new instances. We use this pattern when we want a class to not be able to create a new instance in any way.
Making a singleton is not difficult but making it perfect is a thing that is a little challenging.  
Below is the example of a perfect singleton.
  1. pubic class SingletonClass{  
  2.    private static SingletonClass sClassInstance;  
  3.    private SingletonClass(){  
  4.       if(sClassInstance != null){  
  5.          throw new RuntimeException("getInstance() method will be used to get single instance of this class");  
  6.       }  
  7.    }  
  8.    public synchronized static SingletonClass getInstance(){  
  9.       if(sClassInstance == null){  
  10.          sClassInstance = new SingletonClass();  
  11.       }  
  12.    return sClassInstance;  
  13.    }  
  14. }  
In this examle, a class is defined and named as SingletonClass, which is basically a singleton class. This class contains a private static instance of class, a private constructor, and a synchronized static method getInstance(). 

Why do we use synchronized?

It is necessary to use while working in a multithreaded environment. It prevents two threads creating NEW INSTANCE for each. There may be a time when two threads access the method at the same time and if we will not use synchronized Singleton pattern, the rule will break and a new instance will be created in the heap which is an extra load and nothing else. To avoid this, it is necessary to use synchronized with an instance method.