iluwatar#590 add explanation for DTO

Author: iluwatar

data-transfer-object/README.md

@@ -9,8 +9,89 @@ tags:
 ---
 
 ## Intent
-Pass data with multiple attributes in one shot from client to server,
-to avoid multiple calls to remote server. 
+Pass data with multiple attributes in one shot from client to server, to avoid multiple calls to remote server. 
+
+## Explanation
+
+Real world example
+
+> We need to fetch information about customers from remote database. Instead of querying the attributes one at a time, we use DTOs to transfer all the relevant attributes in a single shot.     
+
+In plain words
+
+> Using DTO relevant information can be fetched with a single backend query. 
+
+Wikipedia says
+
+> In the field of programming a data transfer object (DTO) is an object that carries data between processes. The 
+motivation for its use is that communication between processes is usually done resorting to remote interfaces 
+(e.g., web services), where each call is an expensive operation. Because the majority of the cost of each call is 
+related to the round-trip time between the client and the server, one way of reducing the number of calls is to use an 
+object (the DTO) that aggregates the data that would have been transferred by the several calls, but that is served by 
+one call only.
+
+**Programmatic Example**
+
+Let's first introduce our simple customer DTO class.
+
+```java
+public class CustomerDto {
+  private final String id;
+  private final String firstName;
+  private final String lastName;
+
+  public CustomerDto(String id, String firstName, String lastName) {
+    this.id = id;
+    this.firstName = firstName;
+    this.lastName = lastName;
+  }
+
+  public String getId() {
+    return id;
+  }
+
+  public String getFirstName() {
+    return firstName;
+  }
+
+  public String getLastName() {
+    return lastName;
+  }
+}
+```
+
+Customer resource class acts as the server for customer information.
+
+```java
+public class CustomerResource {
+  private List<CustomerDto> customers;
+
+  public CustomerResource(List<CustomerDto> customers) {
+    this.customers = customers;
+  }
+
+  public List<CustomerDto> getAllCustomers() {
+    return customers;
+  }
+
+  public void save(CustomerDto customer) {
+    customers.add(customer);
+  }
+
+  public void delete(String customerId) {
+    customers.removeIf(customer -> customer.getId().equals(customerId));
+  }
+}
+```
+
+Now fetching customer information is easy since we have the DTOs.
+
+```java
+    var allCustomers = customerResource.getAllCustomers();
+    allCustomers.forEach(customer -> LOGGER.info(customer.getFirstName()));
+    // Kelly
+    // Alfonso
+```
 
 ## Class diagram
 ![alt text](./etc/data-transfer-object.urm.png "data-transfer-object")

data-transfer-object/etc/data-transfer-object.ucls

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <class-diagram version="1.2.0" icons="true" always-add-relationships="false" generalizations="true" realizations="true" 
   associations="true" dependencies="false" nesting-relationships="true" router="FAN">  
-  <class id="1" language="java" name="com.iluwatar.datatransfer.CustomerClientApp" project="data-transfer-object" 
+  <class id="1" language="java" name="com.iluwatar.datatransfer.App" project="data-transfer-object"
     file="/data-transfer-object/src/main/java/com/iluwatar/datatransfer/CustomerClientApp.java" binary="false" 
     corner="BOTTOM_RIGHT">    
     <position height="-1" width="-1" x="145" y="93"/>    

data-transfer-object/pom.xml

@@ -48,7 +48,7 @@
                         <configuration>
                             <archive>
                                 <manifest>
-                                    <mainClass>com.iluwatar.datatransfer.CustomerClientApp</mainClass>
+                                    <mainClass>com.iluwatar.datatransfer.App</mainClass>
                                 </manifest>
                             </archive>
                         </configuration>

data-transfer-object/src/main/java/com/iluwatar/datatransfer/CustomerClientApp.java renamed to data-transfer-object/src/main/java/com/iluwatar/datatransfer/App.java

@@ -32,15 +32,15 @@
  * The Data Transfer Object pattern is a design pattern in which an data transfer object is used to
  * serve related information together to avoid multiple call for each piece of information.
  *
- * <p>In this example, ({@link CustomerClientApp}) as as customer details consumer i.e. client to
+ * <p>In this example, ({@link App}) as as customer details consumer i.e. client to
  * request for customer details to server.
  *
  * <p>CustomerResource ({@link CustomerResource}) act as server to serve customer information. And
  * The CustomerDto ({@link CustomerDto} is data transfer object to share customer information.
  */
-public class CustomerClientApp {
+public class App {
 
-  private static final Logger LOGGER = LoggerFactory.getLogger(CustomerClientApp.class);
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
 
   /**
    * Method as act client and request to server for details.

data-transfer-object/src/test/java/com/iluwatar/datatransfer/AppTest.java

@@ -0,0 +1,33 @@
+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.datatransfer;
+
+import org.junit.jupiter.api.Test;
+
+public class AppTest {
+    @Test
+    public void test() throws Exception {
+        App.main(new String[]{});
+    }
+}