Can't use SharedPreferences in method that doesn't get called in onCreate()?

In my app, I'm trying to save an ArrayList of Strings. I have an Activity Favourites, which has an add() method. This method开发者_开发技巧 is called in another Activity to add something to the ArrayList. I want that each time something is added, it first gets all the values that I've stored in SharedPreferences, checks if the value isn't already there and if it's not, adds it to the ArrayList. Finally I want it to store all the values again in SharedPreferences. When the Activity Favourites is called, it shows a list with all the favourites (Gets each value in database and all that).

So I've made a method, but as onCreate() is never called when an item is added to the ArrayList, I can't seem to instantiate my SharedPreferences. Is there anyone who can give me some hints to get this to work?

Note: I don't want to work with SQLite database, because it would be for only one class in my app, and it would contain very little values.

Also, the code I used:

public void add(String v, String k){
    prefs = PreferenceManager.getDefaultSharedPreferences(this);
    int countvaca = prefs.getInt("vacacount", 0);
    int countkantoor = prefs.getInt("kantoorcount", 0);

    for(int i = 0; i <countvaca; i++){
        vaca.add(prefs.getString("Vaca"+i, null));
    }
    for(int j = 0; j <countkantoor; j++){
        kantoor.add(prefs.getString("Kantoor"+j, null));
    }

    boolean blnFound = vaca.contains(v);
    System.out.println("Does arrayList contain vacature ? " + blnFound);
    if(!blnFound){
        vaca.add(v);
        kantoor.add(k);
    }

    Editor edit = prefs.edit();
    countvaca = vaca.size();
    countkantoor = kantoor.size();
    edit.putInt("Vacaturelijstgrootte", countvaca);
    edit.putInt("Kantoorlijstgrootte", countkantoor);
    for(int a = 0; a<countvaca; a++){
         edit.putString("Vaca"+a, vaca.get(a));
    }
    for(int b = 0; b< countkantoor; b++){
        edit.putString("Kantoor"+b, kantoor.get(b));
    }
    edit.commit();


}

If onCreate never gets called then you activity is never started, so the "this" in

prefs = PreferenceManager.getDefaultSharedPreferences(this);

doesn't refer to a properly initialized activity.

Either you should start your activity that contains your "add" method (which automatically calls the onCreate method), or you can pass a reference to the currently active Activity to your add method, i.e. something like this:

public void add(String v, String k, Activity a){
     prefs = PreferenceManager.getDefaultSharedPreferences(a);

However, it is not considered good programming style to directly call methods from one activity to another. Best is to create a separate class that contains the code that the two activities have in common.