adjust solutoin

Author: SteffenLm

Engine.java

@@ -4,7 +4,7 @@ public enum Engine {
   GAS("Gas"),
   ELECTRO("Elektro");
 
-  private String description;
+  private final String description;
 
   Engine(String description) {
     this.description = description;

Vehicle.java

@@ -1,9 +1,9 @@
 public class Vehicle {
 
-  private String make;
-  private String model;
+  private final String make;
+  private final String model;
   private double speedInKmh;
-  private Engine engine;
+  private final Engine engine;
   private static int numberOfVehicles;
 
   public Vehicle(String make, String model, Engine engine) {