Incorporated review changes - 1) Created sql file for central schema 2)

Changed getById return type to Optional

dao/src/main/java/com/iluwatar/dao/App.java

@@ -73,15 +73,14 @@ public class App {
   private static void deleteSchema(DataSource dataSource) throws SQLException {
     try (Connection connection = dataSource.getConnection();
         Statement statement = connection.createStatement()) {
-      statement.execute("DROP TABLE CUSTOMERS");
+      statement.execute(CustomerSchemaSql.DELETE_SCHEMA_SQL);
     }
   }
 
   private static void createSchema(DataSource dataSource) throws SQLException {
     try (Connection connection = dataSource.getConnection();
         Statement statement = connection.createStatement()) {
-      statement.execute("CREATE TABLE CUSTOMERS (ID NUMBER, FNAME VARCHAR(100), "
-          + "LNAME VARCHAR(100))");
+      statement.execute(CustomerSchemaSql.CREATE_SCHEMA_SQL);
     }
   }
 

dao/src/main/java/com/iluwatar/dao/CustomerDao.java

@@ -23,6 +23,7 @@
 
 package com.iluwatar.dao;
 
+import java.util.Optional;
 import java.util.stream.Stream;
 
 /**
@@ -51,10 +52,11 @@ public interface CustomerDao {
   
   /**
    * @param id unique identifier of the customer.
-   * @return customer with unique identifier <code>id</code> if found, null otherwise.
+   * @return an optional with customer if a customer with unique identifier <code>id</code>
+   *     exists, empty optional otherwise.
    * @throws Exception if any error occurs.
    */
-  Customer getById(int id) throws Exception;
+  Optional<Customer> getById(int id) throws Exception;
 
   /**
    * @param customer the customer to be added.

dao/src/main/java/com/iluwatar/dao/CustomerSchemaSql.java

@@ -0,0 +1,9 @@
+package com.iluwatar.dao;
+
+public interface CustomerSchemaSql {
+
+  String CREATE_SCHEMA_SQL = "CREATE TABLE CUSTOMERS (ID NUMBER, FNAME VARCHAR(100), " 
+      + "LNAME VARCHAR(100))";
+  
+  String DELETE_SCHEMA_SQL = "DROP TABLE CUSTOMERS";
+}

dao/src/main/java/com/iluwatar/dao/DbCustomerDao.java

@@ -27,6 +27,7 @@ import java.sql.Connection;
 import java.sql.PreparedStatement;
 import java.sql.ResultSet;
 import java.sql.SQLException;
+import java.util.Optional;
 import java.util.Spliterator;
 import java.util.Spliterators;
 import java.util.function.Consumer;
@@ -109,7 +110,7 @@ public class DbCustomerDao implements CustomerDao {
    * {@inheritDoc}
    */
   @Override
-  public Customer getById(int id) throws Exception {
+  public Optional<Customer> getById(int id) throws Exception {
     try (Connection connection = getConnection();
         PreparedStatement statement = 
             connection.prepareStatement("SELECT * FROM CUSTOMERS WHERE ID = ?")) {
@@ -117,9 +118,9 @@ public class DbCustomerDao implements CustomerDao {
       statement.setInt(1, id);
       ResultSet resultSet = statement.executeQuery();
       if (resultSet.next()) {
-        return createCustomer(resultSet);
+        return Optional.of(createCustomer(resultSet));
       } else {
-        return null;
+        return Optional.empty();
       }
     } catch (SQLException ex) {
       throw new Exception(ex.getMessage(), ex);
@@ -131,7 +132,7 @@ public class DbCustomerDao implements CustomerDao {
    */
   @Override
   public boolean add(Customer customer) throws Exception {
-    if (getById(customer.getId()) != null) {
+    if (getById(customer.getId()).isPresent()) {
       return false;
     }
 

dao/src/main/java/com/iluwatar/dao/InMemoryCustomerDao.java

@@ -25,6 +25,7 @@ package com.iluwatar.dao;
 
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Optional;
 import java.util.stream.Stream;
 
 /**
@@ -46,13 +47,13 @@ public class InMemoryCustomerDao implements CustomerDao {
   }
 
   @Override
-  public Customer getById(final int id) {
-    return idToCustomer.get(id);
+  public Optional<Customer> getById(final int id) {
+    return Optional.ofNullable(idToCustomer.get(id));
   }
 
   @Override
   public boolean add(final Customer customer) {
-    if (getById(customer.getId()) != null) {
+    if (getById(customer.getId()).isPresent()) {
       return false;
     }