Activity.java

public class Activity extends AppCompatActivity {
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        SingletonSharePrefrences.getInstance().putString("KEY_NAME" , "MACK");
    }
}

App.java

import android.app.Application;

public class App extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        SingletonSharePrefrences.init(this, "PERSON_STORE");

    }
}

SingletonSharePrefrences.java

import android.annotation.SuppressLint;
import android.content.Context;
import android.content.SharedPreferences;

public class SingletonSharePrefrences {
    private static SingletonSharePrefrences instance;


    private SharedPreferences sharedPreferences;
    private SharedPreferences.Editor editor;

    public synchronized static void init(Context context, String name) {
        instance = new SingletonSharePrefrences(context, name);
    }


    public static SingletonSharePrefrences getInstance() {
        if (instance == null)
            throw new IllegalStateException("You must to call init() before use getInstance()");

        return instance;

    }


    @SuppressLint("CommitPrefEdits")
    private SingletonSharePrefrences(Context context, String name) {
        sharedPreferences = context.getSharedPreferences(name, Context.MODE_PRIVATE);
        editor = sharedPreferences.edit();
    }
  
    public void putString(String key, String value) {
        editor.putString(key, value).commit(); 
    }
}