Shared Preferences on a Null Object Reference Android

I am new to android developing maybe its a silly question but please help me. I am getting this error while trying to save an int value.

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method ‘android.content.SharedPreferences android.content.Context.getSharedPreferences(java.lang.String, int)’ on a null object reference

And here is my code

SharedPreferences prefs = this.getSharedPreferences("com.example.app", Context.MODE_PRIVATE);
String value = "com.example.app.value";
int i = prefs.getInt(value, 0);

And for write

prefs.edit().putInt(number, i).apply();

I just want to set a SharedPreferences and want to read it at the first and write it inside the Activity. How can I solve it?

EDIT

public class MainActivity extends Activity {
SharedPreferences sharedpreferences;
public static final String MyPREFERENCES = "myprefs";
public static final  String value = "key";

int i = sharedpreferences.getInt(value, 0);

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    sharedpreferences = getSharedPreferences(MyPREFERENCES, Context.MODE_PRIVATE);
}
public void sendMessage(View view) {
    i += 1;    
    SharedPreferences.Editor editor = sharedpreferences.edit();
    editor.putInt(value, i);
    editor.apply();
}

I managed to save preferences with a different way but I couldn’t manage to read it in MainActivity extends Activity class.

log :

Caused by: java.lang.NullPointerException: Attempt to invoke interface method ‘int android.content.SharedPreferences.getInt(java.lang.String, int)’ on a null object reference

Answer

This is an example of SharedPreferences

To save name in SharedPreferences:

SharedPreferences sharedPreferences = PreferenceManager
                .getDefaultSharedPreferences(this);
        Editor editor = sharedPreferences.edit();
        editor.putString(key, name);
        editor.apply();

To get name from SharedPreferences:

 SharedPreferences sharedPreferences = PreferenceManager
                .getDefaultSharedPreferences(this);
        String name = sharedPreferences.getString(key, "default value");

For more details visit: http://developer.android.com/training/basics/data-storage/shared-preferences.html

Updated Code:

public class MainActivity extends Activity {
SharedPreferences sharedpreferences;
public static final String MyPREFERENCES = "myprefs";
public static final  String value = "key";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    sharedpreferences = getSharedPreferences(MyPREFERENCES, Context.MODE_PRIVATE);
    int i = sharedpreferences.getInt(value, 0);
    //use the value of i where needed.
}
public void saveMessage(View view) {
    i += 1;    
    SharedPreferences.Editor editor = sharedpreferences.edit();
    editor.putInt(value, i);
    editor.apply();
}