Why we used Singleton Class?

 Purpose of Singleton:

The main purpose of the singleton class is to object creation, limiting the number of objects to only one. Since there is only one singleton instance fields of a singleton will occur only once per class, just like static.

Prons:

1. Singleton prevents other objects from instantiating their own copies of the singleton object, ensuring that all objects access the single instance.

2. Since the class controls the instantiation process, the class has the flexibility to change the instantiation process.

3. The advantage of singleton over global variable is that you are absolutely sure of the number of instances when you use Singleton.

Cons:

1. Their state is automatically shared across the entire app.

2. Bugs can often start occurring when that state changes unexpectedly.

3. The relationships between singleton and the code that depends on them is usually not very well     defined.

4. Managing their life-cycle can be tricky.

How to implement Singleton class.

Step 1. Create a class.

public class MyApplication  {

}

Step 2. Create an instance of the class which declare as a static.

public class MyApplication   {
    
    private static MyApplication appInstance;
}

 Step 3.  Make the initializer. final Singleton class

public class MyApplication   {
    
    private static MyApplication appInstance;
   
    public static synchronized MyApplication getAppInstance(){
        return appInstance;
    }

}

 Step 4.  final Singleton class in android.

public class MyApplication extends Application {
    
    private static MyApplication appInstance;
    

    @Override    public void onCreate() {
        super.onCreate();
        appInstance=this;

    }

    public static synchronized MyApplication getAppInstance(){
        return appInstance;
    }
 
}

How to used Singleton class in Activity in android.

 In this example we have save the login details in sharedpreferences  storage.

 

import android.app.Application;
import android.content.SharedPreferences;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.toolbox.Volley;

public class MyApplication extends Application {

    private final static String PREFS_NAME = "SaveValue";
    private static MyApplication appInstance;
   

    @Override    public void onCreate() {
        super.onCreate();
        appInstance=this;

    }

    public static synchronized MyApplication getAppInstance(){
        return appInstance;
    }

    public void saveString(String key, String data) {
        SharedPreferences settings = MyApplication.appInstance.getSharedPreferences(PREFS_NAME, 0);
        SharedPreferences.Editor editor = settings.edit();
        editor.putString(key, data);
        editor.commit();
    }

    public String getString(String key) {
        SharedPreferences settings = MyApplication.appInstance.getSharedPreferences(PREFS_NAME, 0);
        return settings.getString(key,"");
    }

}

Activity

public class MainActivity extends AppCompatActivity {

    @Override    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        MyApplication..getAppInstance()

        // Save login Details.
        MyApplication.getAppInstance().

                   saveString("Username", loginRequest.getUserName());

        MyApplication.getAppInstance().

                   saveString("Password", loginRequest.getPassword());

        // Get login Details

        MyApplication.getAppInstance().getString("Username");
        MyApplication..getAppInstance().getString("Password");
}

}