fix solution

Author: SteffenLm

Exercise.java

@@ -21,6 +21,8 @@ public static void main(String[] args) {
     rental.addVehicle(truck1);
     rental.addVehicle(car2);
 
+    rental.transformAllTrucks();
+
     System.out.println(rental.toString());
   }
 }

Rental.java

@@ -32,7 +32,8 @@ public void transformAllTrucks() {
     for (Vehicle c : vehicles) {
       if (c instanceof Truck) {
         Truck t = (Truck) c;
-        t.transform();
+        if (!t.isTransformed())
+          t.transform();
       }
     }
   }