How to create a variable only once when app is created in Android?

I am creating an android application. In my application, I need a variable that initializes only once when the application is first created. It should not be initialized even when the app gets updated. I added, "entered". So whenever I open the app I should have ArrayList which contains entered String.

ArrayList<String> strings=new ArrayList<String>();
strings.add("entered");

As @blackapps mentioned You can use Shared Preferences

In MainActivity.java:

@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
SharedPreferences sharedPref = getPreferences(Context.MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPref.edit();
String yourVariable = sharedPref.getString(getString(R.string.nameOfValue), getString(R.string.defaultValue));
if (yourVariable.equals(getString(R.string.defaultValue))) //app was run for 1st time
{
editor.putString(getString(R.string.nameOfValue), "String which You want to save"); //value that You want to put
editor.apply();
}
else
{
// app wasn't run for 1st time
}
}

In string.xml

<string name="nameOfValue">NAME_OF_VALUE</string>
<string name="defaultValue">DEFAULT_VALUE</string>

I saved String but You can save any type in Shared Preferences.
<hr>

Edit:
To save ArrayList You can use GSON

First add dependency to grade and sync project:

implementstion 'com.google.code.gson:gson:2.8.6'

And then use this code:

public class MainActivity extends AppCompatActivity
{
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
SharedPreferences sharedPref = getPreferences(Context.MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPref.edit();
boolean firstRun = sharedPref.getBoolean(getString(R.string.firstRun), true);
ArrayList<String> arrayList;
Gson gson = new Gson();
if (firstRun) //app was run for 1st time
{
System.out.println("First run");
// initialize array as You want
arrayList = new ArrayList<>();
arrayList.add("Some string");
String arrayJSON = gson.toJson(arrayList);
// add values to SharedPreferences
editor.putString(getString(R.string.nameOfValue), arrayJSON);
editor.putBoolean(getString(R.string.firstRun), false);
// apply changes
editor.apply();
}
else // app wasn't run for 1st time
{
System.out.println("Not first run");
String arrayJSON = sharedPref.getString(getString(R.string.nameOfValue), getString(R.string.defaultValue));
arrayList = gson.fromJson(arrayJSON, new TypeToken<ArrayList<String>>()
{
}.getType());
}
for (String s : arrayList)
{
System.out.println(s);
}
}
}