diff --git a/2020-2021/src/TD2/interactions/App.java b/2020-2021/src/TD2/interactions/App.java
index 74ede4c..8603f41 100644
--- a/2020-2021/src/TD2/interactions/App.java
+++ b/2020-2021/src/TD2/interactions/App.java
@@ -8,10 +8,11 @@ public class App
 {
     public static void main( String[] args ) throws Exception
     {
-    	Controleur controleur;
+    	@SuppressWarnings("unused")
+		Controleur controleur;
 		try {
 			controleur = new Controleur();
-	    	controleur.start();
+	    	//controleur.start();
 		} catch (Exception e) {
 			e.printStackTrace();
 		}
diff --git a/2020-2021/src/TD4/cars/CarRental.java b/2020-2021/src/TD4/cars/CarRental.java
index 7341d19..5610c2f 100644
--- a/2020-2021/src/TD4/cars/CarRental.java
+++ b/2020-2021/src/TD4/cars/CarRental.java
@@ -2,6 +2,8 @@ package TD4.cars;
 
 import java.time.LocalDate;
 
+import TD4.core.PayingItem;
+
 /**
  * 
  * The Car Rental class represents the rental of a car, for a given duration and start date.
@@ -14,7 +16,7 @@ import java.time.LocalDate;
  * @author blay
  *
  */
-public class CarRental {
+public class CarRental implements PayingItem{
 
 	private Car car;
 	private double dayPrice;
diff --git a/2020-2021/src/TD4/cars/CarRentalService.java b/2020-2021/src/TD4/cars/CarRentalService.java
index 7ec358a..7b1672f 100644
--- a/2020-2021/src/TD4/cars/CarRentalService.java
+++ b/2020-2021/src/TD4/cars/CarRentalService.java
@@ -2,8 +2,10 @@ package TD4.cars;
 
 import java.time.LocalDate;
 import java.util.ArrayList;
+import java.util.Comparator;
 import java.util.List;
 
+import TD4.core.Service4PI;
 import TD4.util.DateTools;
 import TD4.util.NotPossibleCarRentalException;
 
@@ -17,22 +19,14 @@ import TD4.util.NotPossibleCarRentalException;
  *
  * 
  */
-public class CarRentalService {
+public class CarRentalService extends Service4PI<CarRental> {
 
 	//Set of cars for rent
 	private List<Car> cars;
-	
-	//All registered car rentals
-	private List<CarRental> carRentals = new ArrayList<>();
-
-
-
-	
-
 
 	//To create a car rental service,  you need to have cars.
 	public CarRentalService(List<Car> cars) {
-		super();
+		super(new ArrayList<>());
 		this.cars = cars;
 	}
 
@@ -55,7 +49,7 @@ public class CarRentalService {
 	
 
 	private boolean isAvailable(Car c, LocalDate[] dates) {
-		for (CarRental carRental : carRentals) {
+		for (CarRental carRental : super.payingItemList) {
 			if (c.equals(carRental.getCar()) &&
 				(carRental.includeADate(dates)) ) {
 				return false;	
@@ -81,11 +75,20 @@ public class CarRentalService {
 		LocalDate[] dates = DateTools.getDays(fromDate, numberOfDays);
 		if (isAvailable(c, dates)) {
 			carRental = new CarRental(c, fromDate, numberOfDays);
-			carRentals.add(carRental);
+			super.payingItemList.add(carRental);
 		}
 		return carRental;
 	}
 	
+	public List<CarRental> sortedByPrice() {
+		super.payingItemList.sort(Comparator.comparing(CarRental::getPrice));
+		return new ArrayList<>(super.payingItemList);
+	}
+	
+	public CarRental getCarRentalWithBestPrice() {
+		return this.sortedByPrice().get(0);
+	}
+	
 	
 
 }
diff --git a/2020-2021/src/TD4/flights/Flight.java b/2020-2021/src/TD4/flights/Flight.java
index 03276a2..e0ff92e 100644
--- a/2020-2021/src/TD4/flights/Flight.java
+++ b/2020-2021/src/TD4/flights/Flight.java
@@ -4,6 +4,8 @@ import java.time.LocalDate;
 import java.time.LocalTime;
 import java.util.Random;
 
+import TD4.core.PayingItem;
+
 /**
  * 
  * 
@@ -15,7 +17,7 @@ import java.util.Random;
  *
  *
  */
-public class Flight {
+public class Flight implements PayingItem {
 
 	private static final   int NOT_ASSIGNED = -1;
 	private static final   String DEFAULT_DESTINATION= "Paris";
diff --git a/2020-2021/src/TD4/flights/FlightService.java b/2020-2021/src/TD4/flights/FlightService.java
index 87df449..06e3df2 100644
--- a/2020-2021/src/TD4/flights/FlightService.java
+++ b/2020-2021/src/TD4/flights/FlightService.java
@@ -7,6 +7,8 @@ import java.util.List;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
+import TD4.core.Service4PI;
+
 /**
  * This class allows the management of a set of flights
  * 
@@ -15,12 +17,12 @@ import java.util.stream.Stream;
  * 
  */
 
-public class FlightService {
+public class FlightService extends Service4PI<Flight> {
 
-	private List<Flight> flights = new ArrayList<>();
+	//private List<Flight> flights = new ArrayList<>();
 
 	public FlightService(List<Flight> flights) {
-		this.flights = flights;
+		super(flights);
 	}
 
 	/**
@@ -28,7 +30,7 @@ public class FlightService {
 	 * @return the list of flights available on a given date {@code LocalDate}
 	 */
 	public List<Flight> getFlights(LocalDate aDate) {
-		Stream<Flight> parallelStream = flights.parallelStream();
+		Stream<Flight> parallelStream = super.payingItemList.parallelStream();
 		Stream<Flight> results = parallelStream.filter(f -> (f.getDepartDate().equals(aDate)));
 		return results.collect(Collectors.toCollection(ArrayList::new));
 	}
@@ -41,7 +43,7 @@ public class FlightService {
 	 *         a place to another place
 	 */
 	public List<Flight> getFlights(LocalDate d, String from, String to) {
-		Stream<Flight> parallelStream = flights.parallelStream();
+		Stream<Flight> parallelStream = super.payingItemList.parallelStream();
 		Stream<Flight> results = parallelStream.filter(f -> f.match(d, from, to));
 		return results.collect(Collectors.toCollection(ArrayList::new));
 	}
@@ -51,8 +53,8 @@ public class FlightService {
 	 *         not cloned.
 	 */
 	public List<Flight> sortedByPrice() {
-		flights.sort(Comparator.comparing(Flight::getPrice));
-		return new ArrayList<>(flights);
+		super.payingItemList.sort(Comparator.comparing(Flight::getPrice));
+		return new ArrayList<>(super.payingItemList);
 	}
 	
 	public Flight getFlyWithBestPrice() {