implement solution

Author: SteffenLm

Rental.java

@@ -20,6 +20,15 @@ public void addAllVehicles(Vehicle... vehicles) {
     }
   }
 
+  public void transformAllTrucks() {
+    for (Vehicle c : vehicles) {
+      if (c instanceof Truck) {
+        Truck t = (Truck) c;
+        t.transform();
+      }
+    }
+  }
+
   public void print() {
     System.out.println(name);
     System.out.println("Unsere Fahrzeuge: ");