java11-vavr-exception-handling

Simple project to show how to handle exceptions using vavr.

preface

Please refer my other github projects:

• https://github.com/mtumilowicz/java11-vavr-option
• https://github.com/mtumilowicz/java11-vavr-try
• https://github.com/mtumilowicz/java11-vavr-either

project description

1. Suppose we have outer repository (to backup database) that we cannot modify. What is more - find method throws EntityNotFoundException:

   public class LegacyBackupUserRepository {
       User find(int id) {
           return MockBackupDatabase.users.find(User.withId(id))
                   .getOrElseThrow(EntityNotFoundException::new);
       }
   }
   

2. in our application we have also a repository that:
   • has find method that will lookup into our database
   • has backupLookup method that will call find method from repository mentioned above (LegacyBackupUserRepository)
   • has findOrBackup method that will try to lookup in our database and:
     • if user is found returns it
     • if user is not found - try to lookup in backup database
3. we want to model our UserRepository in a right way, so:
   • Option is perfect for modelling existence:

     Option<User> find(int id) {
         return MockDatabase.users.find(User.withId(id));
     }
     

   • Try is perfect to cope with third-party libraries that throws exceptions

     Try<User> backupLookup(int id) {
         return Try.of(() -> legacyBackupUserRepository.find(id));
     }
     

   • Either is perfect as a tailored error container

     Either<List<DatabaseUserFindReport>, User> findOrBackup(int id) {
         Option<User> normalUser = find(id);
         if (normalUser.isDefined()) {
             return Either.right(normalUser.get());
         }
         
         Try<User> backupUser = backupLookup(id);
         if (backupUser.isSuccess()) {
             return Either.right(backupUser.get());
         }
         
         return Either.left(List.of(
                 new DatabaseUserFindReport(id, DatabaseType.NORMAL),
                 new DatabaseUserFindReport(id, DatabaseType.BACKUP)
         ));
     
     }
     

     summary: we return user if found otherwise list databases that we looked-up into.

tests

We provide tests for mentioned above functionality.