Singleton in Android

Question

I have followed this link and successfully made singleton class in Android  http   www devahead com blog 2011 06 extending-the-android-application-class-and-dealing-with-singleton   Problem is that i want a single object  like i have Activity A and Activity B  In Activity A I access the object from Singleton class  I use the object and made some changes to it   When I move to Activity B and access the object from Singleton Class it gave me the initialized object and does not keep the changes which i have made in Activity A  Is there any other way to save the changing  Please help me Experts  This is MainActivity  public class MainActivity extends Activity       protected MyApplication app              private OnClickListener btn2 new OnClickListener                  Override         public void onClick View arg0                Intent intent new Intent MainActivity this NextActivity class               startActivity intent                                       Override     protected void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           setContentView R layout activity main              Get the application instance         app    MyApplication getApplication                Call a custom application method         app customAppMethod                Call a custom method in MySingleton         Singleton getInstance   customSingletonMethod             Singleton getInstance               Read the value of a variable in MySingleton         String singletonVar   Singleton customVar           Log d  Test  singletonVar           singletonVar  World           Log d  Test  singletonVar            Button btn  Button findViewById R id button1           btn setOnClickListener btn2              This is NextActivity  public class NextActivity extends Activity             Override         protected void onCreate Bundle savedInstanceState                super onCreate savedInstanceState               setContentView R layout activity next                String singletonVar   Singleton customVar               Log d  Test  singletonVar                   Singleton Class  public class Singleton       private static Singleton instance       public static String customVar  Hello        public static void initInstance             if  instance    null                 Create the instance       instance   new Singleton                     public static Singleton getInstance                 Return the instance      return instance               private Singleton                  Constructor hidden because this is a singleton              public void customSingletonMethod                  Custom method             and MyApplication  public class MyApplication extends Application            Override     public void onCreate             super onCreate             Initialize the singletons so their instances         are bound to the application process       initSingletons                 protected void initSingletons                  Initialize the instance of MySingleton      Singleton initInstance                 public void customAppMethod                  Custom application method           When i run this code  i get Hello which i have initialized in Singleton then World which i gave it in MainActivity and again shows Hello in NextActivity in logcat  I want it to show world again in NextActivity  Please help me to correct this

User · Answer

As  Lazy stated in this answer  you can create a singleton from a template in Android Studio  It is worth noting that there is no need to check if the instance is null because the static ourInstance variable is initialized first  As a result  the singleton class implementation created by Android Studio is as simple as following code   public class MySingleton       private static MySingleton ourInstance   new MySingleton         public static MySingleton getInstance             return ourInstance             private MySingleton

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It is simple  as a java  Android also supporting singleton  -  Singleton is a part of Gang of Four design pattern and it is categorized under creational design patterns   -  Static member   This contains the instance of the singleton class   -  Private constructor   This will prevent anybody else to instantiate the Singleton class   -  Static public method   This provides the global point of access to the Singleton object and returns the instance to the client calling class    create private instance create private constructor use getInstance   of Singleton class  public class Logger  private static Logger   objLogger  private Logger               ToDo here    public static Logger getInstance         if  objLogger    null             objLogger   new Logger            return objLogger             while use singleton -   Logger getInstance

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Tip  To create singleton class In Android Studio  right click in your project and open menu   New - gt  Java Class - gt  Choose Singleton from dropdown menu

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The most clean and modern way to use singletons in Android is just to use the Dependency Injection framework called Dagger 2  Here you have an explanation of possible scopes you can use  Singleton is one of these scopes  Dependency Injection is not that easy but you shall invest a bit of your time to understand it  It also makes testing easier

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EDIT    The implementation of a Singleton in Android is not  safe   see here  and you should use a library dedicated to this kind of pattern like Dagger or other DI library to manage the lifecycle and the injection     Could you post an example from your code    Take a look at this gist   https   gist github com Akayh 5566992  it works but it was done very quickly    MyActivity   set the singleton for the first time   initialize mString attribute   Hello   in private constructor and show the value   Hello    Set new value to mString    Singleton   Launch activityB and show the mString value   Singleton  appears

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I put my version of Singleton below   public class SingletonDemo       private static SingletonDemo instance   null      private static Context context                  To initialize the class  It must be called before call the method getInstance           param ctx The Context used              public static void initialize Context ctx         context   ctx                        Check if the class has been initialized         return true  if the class has been initialized                false Otherwise             public static boolean hasBeenInitialized          return context    null                        The private constructor  Here you can use the context to initialize your variables             private SingletonDemo                Use context to initialize the variables                       The main method used to get the instance            public static synchronized SingletonDemo getInstance          if  context    null          throw new IllegalArgumentException  Impossible to get the instance  This class must be initialized before                 if  instance    null          instance   new SingletonDemo                 return instance              Override     protected Object clone   throws CloneNotSupportedException           throw new CloneNotSupportedException  Clone is not allowed               Note that the method initialize could be called in the main class Splash  and the method getInstance could be called from other classes  This will fix the problem when the caller class requires the singleton but it does not have the context   Finally the method hasBeenInitialized is uses to check if the class has been initialized  This will avoid that different instances have different contexts

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answer suggested by rakesh is great but still with some discription Singleton in Android is the same as Singleton in Java  The Singleton design pattern addresses all of these concerns  With the Singleton design pattern you can      1  Ensure that only one instance of a class is created       2  Provide a global point of access to the object      3  Allow multiple instances in the future without affecting a   singleton class s clients   A basic Singleton class example   public class MySingleton       private static MySingleton  instance       private MySingleton                     public static MySingleton getInstance                 if   instance    null                         instance   new MySingleton                      return  instance

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You are copying singleton s customVar into a singletonVar variable and changing that variable does not affect the original value in singleton      This does not update singleton variable    It just assigns value of your local variable Log d  Test  singletonVar   singletonVar  World   Log d  Test  singletonVar       This actually assigns value of variable in singleton Singleton customVar   singletonVar

[android] Singleton in Android

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