OptionalOrElse.java

import java.util.Optional;

public class OptionalOrElse {

   public static void main(String[] args) {
       // Correct usage of orElseGet. createInventoryForUser is not invoked unless.
       System.out.println("Using the APIs in a correct way");
       findInventoryByUserId(1).orElseGet(() -> createInventoryForUser(1));
       findInventoryByUserId(2).orElseGet(() -> createInventoryForUser(2));

       System.out.println();
      
       // Incorrect usage of orElse. createInventoryForUser is always invoked, even though the user already has an inventory!
       // This mistake is easy to do because the code reads out so nicely.
       System.out.println("Using the APIs in an incorrect way");
       findInventoryByUserId(1).orElse(createInventoryForUser(1));
       findInventoryByUserId(2).orElse(createInventoryForUser(2));
   }

   private static String createInventoryForUser(int userId) {
       System.out.println("Created inventory for userId: " + userId);
       return "A new inventory for user: " + userId;
   }

   /**
    * Returns inventory for users. Only user with id 1 has an inventory;
    */
   private static Optional<String> findInventoryByUserId(int userId) {
       if (userId == 1) {
           System.out.println("User with id: " + userId + " has an inventory, returning it");
           return Optional.of("An inventory");
       } else {
           System.out.println("Could not find inventory for user with id: " + userId);
           return Optional.empty();
       }
   }
}