ExpensesSelector.cls

public class ExpensesSelector {

	public static Map<Id, Expense> selectAll() {
		return dataService.selectAll();
	}

	private static ExpensesDataService dataService {
		get {
			if (null == dataService) {
				// use with sharing by default
				dataService = new WithSharing();
			}
			return dataService;
		}
		set;
	}

	public static void switchSharing(Boolean useSharing) {
		// this method is used to switch sharing model
		if (useSharing) {
			dataService = new WithSharing();
		} else {
			dataService = new WithoutSharing();
		}
	}

	private abstract class ExpensesDataService {
		public Map<Id, Expense> selectAll() {
			return new Map<Id, Expense>([SELECT Amount, TransactionDate FROM Expense]);
		}
	}

	private with sharing class WithSharing
			extends ExpensesDataService {

	}

	private without sharing class WithoutSharing
			extends ExpensesDataService {

	}
}