import java.util.ArrayList;

public class Main {

	private static ArrayList<Settings> preference = new ArrayList<Settings>();
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		saveSettings();
	}

	private static void saveSettings() {
		// TODO Auto-generated method stub
		Settings mPreferences = new Settings();
		mPreferences.set(0, "Настройка 1");
		mPreferences.set(0, "Настройка 2");
		
		preference.set(0, mPreferences);
	}
	
}

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
	at java.util.ArrayList.rangeCheck(ArrayList.java:653)
	at java.util.ArrayList.set(ArrayList.java:444)
	at Main.saveSettings(Main.java:19)
	at Main.main(Main.java:10)

import java.io.Serializable;

public class Settings implements Serializable{

	public void set(int i, String string) {
		// TODO Auto-generated method stub
		
	}

}