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.

Answer 1

Tip: To create singleton class In Android Studio, right click in your project and open menu:

New -> Java Class -> Choose Singleton from dropdown menu 

Answer 2

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.

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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...