Add data transfer hash pattern

data-transfer-hash/README.md

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+---
+layout: pattern
+title: Data Transfer Hash
+folder: data-transfer-hash
+permalink: /patterns/data-transfer-hash/
+categories: Architectural
+language: en
+tags:
+ - Structural
+ - Decoupling
+---
+
+## Intent
+
+Pass data with the help of a data transfer hash object 
+to reduce coupling between objects communicating with DTOs.
+
+## Explanation
+
+In plain words
+> The Data Transfer Hash Pattern aims to reduce coupling between objects communicating
+ between DTOs with the help of a data transfer hash object, which enables DTOs to send
+ data and receive data using well-known keys.
+
+> Data transfer hashes may be implemented as simple hash tables (or HashMaps).
+ A more robust implementation uses a container object to hold the hash, as well as identifying information, type-safe data retrieval methods, and well-known keys.
+
+**Programmatic Example**
+
+Let's first introduce our simple `DataTransferHashObject` class, which is the data transfer hash object
+and stores data for transport between layers as a set of values associated with well-known keys.
+
+```java
+public class DataTransferHashObject {
+ private final HashMap<String, Object> hash;
+
+ DataTransferHashObject() {
+  hash = new HashMap<>();
+ }
+
+ public void put(final String key, final Object value) {
+  hash.put(key, value);
+ }
+
+ public Object getValue(final String key) {
+  return this.hash.get(key);
+ }
+}
+```
+
+`Business` class acts as business object, which is one of the two layers that is involved in data transportation.
+
+```java
+public class Business {
+
+ public void createHash(final String k, final Object v, final DataTransferHashObject hash) {
+  hash.put(k, v);
+ }
+
+ public Object getData(final String k, final DataTransferHashObject hash) {
+  return hash.getValue(k);
+ }
+}
+```
+
+`Presentation` class acts as presentation tier, which is one of the two layers that is involved in data transportation.
+
+```java
+public class Presentation {
+
+ public void createHash(final String k, final Object v, final DataTransferHashObject hash) {
+  hash.put(k, v);
+ }
+
+ public Object getData(final String k, final DataTransferHashObject hash) {
+  return hash.getValue(k);
+ }
+}
+```
+
+Now two layers could transport data with low coupling through the transport data hash object.
+
+```java
+public class DataTransferHashApp {
+ public static void main(final String[] args) {
+  var hash = new DataTransferHashObject();
+  var business = new Business();
+  business.createHash("Alice", "88887777", hash);
+  LOGGER.info("Business object adds Alice's phone number 88887777.");
+  business.createHash("Bob", "67189923", hash);
+  LOGGER.info("Business object adds Bob's phone number 67189923");
+  business.createHash("Trump", "33521179", hash);
+  LOGGER.info("Business object adds Trump's phone number 33521179");
+  var presentation = new Presentation();
+  var alicePhoneNumber = presentation.getData("Alice", hash);
+  LOGGER.info("Presentation tier receives Alice's phone number: " + alicePhoneNumber);
+  var bobPhoneNumber = presentation.getData("Bob", hash);
+  LOGGER.info("Presentation tier receives Bob's phone number: " + bobPhoneNumber);
+  var trumpPhoneNumber = presentation.getData("Trump", hash);
+  LOGGER.info("Presentation tier receives Trump's phone number: " + trumpPhoneNumber);
+ }
+}
+```
+
+## Class diagram
+
+![alt text](./etc/data-transfer-hash.png "data-transfer-hash")
+
+## Applicability
+
+Use the Data Transfer Hash pattern when:
+
+* You want to reduce coupling between objects communicating with DTOs.
+
+## Credits
+
+* [J2EE Design Patterns](https://www.oreilly.com/library/view/j2ee-design-patterns/0596004273/re12.html)

data-transfer-hash/etc/data-transfer-hash.urm.puml

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+@startuml
+package com.iluwatar.datatransferhash {
+  class App {
+    - LOGGER : Logger {static}
+    + main(args : String[]) {static}
+  }
+  class Hash {
+    - hash : Hashtable<String, Object>
+    + put(void)
+    + getValue(key: String) : Object
+  }
+  class Business {
+      + createHash(key: String, value: Object, hash: Hash)
+      + getData(key: String, hash: Hash) : Object
+  }
+  class Presentation {
+      + createHash(key: String, value: Object, hash: Hash)
+      + getData(key: String, hash: Hash) : Object
+  }
+}
+Business .right.> Hash
+Presentation .left.> Hash
+@enduml

data-transfer-hash/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    The MIT License
+    Copyright © 2014-2021 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.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.iluwatar</groupId>
+        <artifactId>java-design-patterns</artifactId>
+        <version>1.26.0-SNAPSHOT</version>
+    </parent>
+    <artifactId>data-transfer-hash</artifactId>
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.maven.plugins</groupId>
+            <artifactId>maven-assembly-plugin</artifactId>
+            <version>3.0.0</version>
+        </dependency>
+    </dependencies>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-assembly-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <configuration>
+                            <archive>
+                                <manifest>
+                                    <mainClass>com.iluwatar.datatransferhash.DataTransferHashApp</mainClass>
+                                </manifest>
+                            </archive>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+</project>

data-transfer-hash/src/main/java/com/iluwater/datatransferhash/Business.java

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+package com.iluwater.datatransferhash;
+
+/**
+ * Business objects
+ * Create hashes for sending data, or use values in received hashes.
+ */
+public class Business {
+
+  /**
+   * Create hashes for sending data.
+   *
+   * @param k Hash key of data
+   * @param v Data Value
+   * @param hash The data transfer hash object, which stores data for transport between layers
+   */
+  public void createHash(final String k, final Object v, final DataTransferHashObject hash) {
+    hash.put(k, v);
+  }
+
+  /**
+   * Receive data according to hash key.
+   *
+   * @param k Hash key of data
+   * @param hash The data transfer hash object, which stores data for transport between layers
+   * @return Received Data from another layer
+   */
+  public Object getData(final String k, final DataTransferHashObject hash) {
+    return hash.getValue(k);
+  }
+}

data-transfer-hash/src/main/java/com/iluwater/datatransferhash/DataTransferHashApp.java

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+package com.iluwater.datatransferhash;
+
+import lombok.extern.slf4j.Slf4j;
+
+/**
+ * The Data Transfer Hash Pattern aims to reduce coupling between objects communicating
+ * between DTOs with the help of a data transfer hash object, which enables DTOs to send
+ * data and receive data using well-known keys.
+ *
+ * <p>In this implementation, we use a simple hash map to act as the data transfer hash object.
+ * For more robust implementation, a container object could be used to hold the hash,
+ * as well as identifying information, type-safe data retrieval methods, and well-known keys.
+ *
+ * <p>In the following example, {@link DataTransferHashApp} simulates a scenario
+ * where the presentation tier would retrieve one's phone number according to
+ * his or her name from business object.
+ */
+
+@Slf4j
+public class DataTransferHashApp {
+
+  /**
+  * This method simulates a scenario where the presentation tier would
+  * retrieve one's phone number according to his or her name from business object.
+
+  * @param args program argument.
+  */
+  public static void main(final String[] args) {
+    var hash = new DataTransferHashObject();
+    var business = new Business();
+    business.createHash("Alice", "88887777", hash);
+    LOGGER.info("Business object adds Alice's phone number 88887777.");
+    business.createHash("Bob", "67189923", hash);
+    LOGGER.info("Business object adds Bob's phone number 67189923");
+    business.createHash("Trump", "33521179", hash);
+    LOGGER.info("Business object adds Trump's phone number 33521179");
+    var presentation = new Presentation();
+    var alicePhoneNumber = presentation.getData("Alice", hash);
+    LOGGER.info("Presentation tier receives Alice's phone number: " + alicePhoneNumber);
+    var bobPhoneNumber = presentation.getData("Bob", hash);
+    LOGGER.info("Presentation tier receives Bob's phone number: " + bobPhoneNumber);
+    var trumpPhoneNumber = presentation.getData("Trump", hash);
+    LOGGER.info("Presentation tier receives Trump's phone number: " + trumpPhoneNumber);
+  }
+}

data-transfer-hash/src/main/java/com/iluwater/datatransferhash/DataTransferHashObject.java

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+package com.iluwater.datatransferhash;
+
+import java.util.HashMap;
+
+/**
+ * Data transfer hash
+ * Stores data for transport between layers as a set of values associated with well-known keys.
+ */
+public class DataTransferHashObject {
+  private final HashMap<String, Object> hash;
+
+  /**
+   * Constructor function for data transfer hash object.
+   * It initiates a HashMap for storing (key,value) pair.
+   */
+  DataTransferHashObject() {
+    hash = new HashMap<>();
+  }
+
+  /**
+   * Put (key,value) pair into Hashmap storage structure.
+   *
+   * @param key Hash key of data value.
+   * @param value The data value that needs to be stored.
+   */
+  public void put(final String key, final Object value) {
+    hash.put(key, value);
+  }
+
+  /**
+   * Get value from Hashmap storage structure according to given hash key.
+   *
+   * @param key Hash key of data value.
+   * @return Data value.
+   */
+  public Object getValue(final String key) {
+    return this.hash.get(key);
+  }
+}

data-transfer-hash/src/main/java/com/iluwater/datatransferhash/Presentation.java

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+package com.iluwater.datatransferhash;
+
+/**
+ * Presentation tier objects
+ * Create hashes for sending data, or use values in received hashes.
+ */
+
+public class Presentation {
+
+  /**
+   * Create hashes for sending data.
+   *
+   * @param k Hash key of data
+   * @param v Data Value
+   * @param hash The data transfer hash object, which stores data for transport between layers
+   */
+  public void createHash(final String k, final Object v, final DataTransferHashObject hash) {
+    hash.put(k, v);
+  }
+
+  /**
+   * Receive data according to hash key.
+   *
+   * @param k Hash key of data
+   * @param hash The data transfer hash object, which stores data for transport between layers
+   * @return Received Data from another layer
+   */
+  public Object getData(final String k, final DataTransferHashObject hash) {
+    return hash.getValue(k);
+  }
+}