feature: #1299 Add Identity Map Pattern (#2094)

* #1299 IMPLEMENT IDENTITY MAP PATTERN. * #1299 Add docstrings, README, testCases and class diagram. * #1299 Update a comment string in DB implementation. * #1299 Fix code smells. * #1299 Fix code smells and add comments to App.java * #1299 Update constant name. * #1299 Remove java version dependency and update README.md. * #1299 Add lombok to PersonFinder.java. * #1299 Add dependency to maven-assembly-plugin. * #1299 Use java streams in PersonDbSimulatorImplementation.java. * #1299 Add print statements while returning the person object. * #1299 Update README.md. * #1299 Add puml file. * Update README.md
2026-05-14 08:58:26 +00:00 · 2022-11-10 06:55:44 +11:00
parent 3831a82b69
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+---
+title: Identity Map
+categories: Data access
+language: en
+tags:
+- Performance
+---
+
+## Intent
+
+Ensures that each object gets loaded only once by keeping every loaded object in a map. 
+Looks up objects using the map when referring to them.
+
+## Explanation
+
+Real world example
+
+> We are writing a program which the user may use to find the records of a given person in a database.
+
+In plain words
+
+> Construct an Identity map which stores the records of recently searched for items in the database. When we look
+> for the same record next time load it from the map do not go to the database.
+
+Wikipedia says
+
+> In the design of DBMS, the identity map pattern is a database access design pattern used to improve performance by providing
+a context-specific, in-memory cache to prevent duplicate retrieval of the same object data from the database
+
+**Programmatic Example**
+
+* For the purpose of this demonstration assume we have already created a database instance **db**.
+* Let's first look at the implementation of a person entity, and it's fields:
+
+```java
+@EqualsAndHashCode(onlyExplicitlyIncluded = true)
+@Getter
+@Setter
+@AllArgsConstructor
+public final class Person implements Serializable {
+
+  private static final long serialVersionUID = 1L;
+
+  @EqualsAndHashCode.Include
+  private int personNationalId;
+  private String name;
+  private long phoneNum;
+
+  @Override
+  public String toString() {
+
+    return "Person ID is : " + personNationalId + " ; Person Name is : " + name + " ; Phone Number is :" + phoneNum;
+
+  }
+
+}
+
+```
+
+* The following is the implementation of the personFinder which is the entity that the user will utilize in order
+to search for a record in our database. It has the relevant DB attached to it. It also maintains an IdentityMap
+to store the recently read records.
+
+```java
+@Slf4j
+@Getter
+@Setter
+public class PersonFinder {
+  private static final long serialVersionUID = 1L;
+  //  Access to the Identity Map
+  private IdentityMap identityMap = new IdentityMap();
+  private PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();
+  /**
+   * get person corresponding to input ID.
+   *
+   * @param key : personNationalId to look for.
+   */
+  public Person getPerson(int key) {
+    // Try to find person in the identity map
+    Person person = this.identityMap.getPerson(key);
+    if (person != null) {
+      LOGGER.info("Person found in the Map");
+      return person;
+    } else {
+      // Try to find person in the database
+      person = this.db.find(key);
+      if (person != null) {
+        this.identityMap.addPerson(person);
+        LOGGER.info("Person found in DB.");
+        return person;
+      }
+      LOGGER.info("Person with this ID does not exist.");
+      return null;
+    }
+  }
+}
+
+```
+
+* The identity map field in the above class is simply an abstraction of a hashMap with **personNationalId**
+as the keys and the corresponding person object as the value. Here is its implementation:
+
+```java
+@Slf4j
+@Getter
+public class IdentityMap {
+  private Map<Integer, Person> personMap = new HashMap<>();
+  /**
+   * Add person to the map.
+   */
+  public void addPerson(Person person) {
+    if (!personMap.containsKey(person.getPersonNationalId())) {
+      personMap.put(person.getPersonNationalId(), person);
+    } else { // Ensure that addPerson does not update a record. This situation will never arise in our implementation. Added only for testing purposes.
+      LOGGER.info("Key already in Map");
+    }
+  }
+
+  /**
+   * Get Person with given id.
+   *
+   * @param id : personNationalId as requested by user.
+   */
+  public Person getPerson(int id) {
+    Person person = personMap.get(id);
+    if (person == null) {
+      LOGGER.info("ID not in Map.");
+    }
+    return person;
+  }
+
+  /**
+   * Get the size of the map.
+   */
+  public int size() {
+    if (personMap == null) {
+      return 0;
+    }
+    return personMap.size();
+  }
+
+}
+
+```
+
+* Now we should construct a dummy person for demonstration purposes and put that person in our database.
+
+```java
+  Person person1 = new Person(1, "John", 27304159);
+  db.insert(person1);
+```
+
+* Now let's create a person finder object and look for person with personNationalId = 1(assume that the personFinder
+object already has the db and an IdentityMap attached to it.):
+  
+```java
+  PersonFinder finder = new PersonFinder();
+  finder.getPerson(1);
+```
+
+* At this stage this record will be loaded from the database and the output would be:
+
+```java
+  ID not in Map.
+  Person ID is:1;Person Name is:John;Phone Number is:27304159
+  Person found in DB.
+```
+
+* However, the next we search for this record again we will find it in the map hence we will not need to go
+to the database.
+
+```java
+  Person ID is:1;Person Name is:John;Phone Number is:27304159
+  Person found in Map.
+```
+
+* If the corresponding record is not in the DB at all then an Exception is thrown. Here is its implementation.
+
+```java
+public class IdNotFoundException extends RuntimeException {
+  public IdNotFoundException(final String message) {
+    super(message);
+  }
+}
+```
+## Class diagram
+
+![alt text](./etc/IdentityMap.png "Identity Map Pattern")
+
+## Applicability
+
+* The idea behind the Identity Map pattern is that every time we read a record from the database,
+  we first check the Identity Map to see if the record has already been retrieved.
+  This allows us to simply return a new reference to the in-memory record rather than creating a new object,
+  maintaining referential integrity.
+* A secondary benefit to the Identity Map is that, since it acts as a cache,
+  it reduces the number of database calls needed to retrieve objects, which yields a performance enhancement.
+
+## Credits
+
+* [Identity Map](https://www.sourcecodeexamples.net/2018/04/identity-map-pattern.html)
@@ -0,0 +1,67 @@
+@startuml
+package com.iluwatar.identitymap {
+  class App {
+    - LOGGER : Logger {static}
+    + App()
+    + main(args : String[]) {static}
+  }
+  class IdentityMap {
+    - LOGGER : Logger {static}
+    - personMap : Map<Integer, Person>
+    + IdentityMap()
+    + addPerson(person : Person)
+    + getPerson(id : int) : Person
+    + getPersonMap() : Map<Integer, Person>
+    + size() : int
+  }
+  class Person {
+    - name : String
+    - personNationalId : int
+    - phoneNum : long
+    - serialVersionUID : long {static}
+    + Person(personNationalId : int, name : String, phoneNum : long)
+    + equals(o : Object) : boolean
+    + getName() : String
+    + getPersonNationalId() : int
+    + getPhoneNum() : long
+    + hashCode() : int
+    + setName(name : String)
+    + setPersonNationalId(personNationalId : int)
+    + setPhoneNum(phoneNum : long)
+    + toString() : String
+  }
+  interface PersonDbSimulator {
+    + delete(int) {abstract}
+    + find(int) : Person {abstract}
+    + insert(Person) {abstract}
+    + update(Person) {abstract}
+  }
+  class PersonDbSimulatorImplementation {
+    ~ ID_STR : String {static}
+    - LOGGER : Logger {static}
+    ~ NOT_IN_DATA_BASE : String {static}
+    - personList : List<Person>
+    + PersonDbSimulatorImplementation()
+    + delete(id : int)
+    + find(personNationalID : int) : Person
+    + insert(person : Person)
+    + size() : int
+    + update(person : Person)
+  }
+  class PersonFinder {
+    - LOGGER : Logger {static}
+    - db : PersonDbSimulatorImplementation
+    - identityMap : IdentityMap
+    + PersonFinder()
+    + getDB() : PersonDbSimulatorImplementation
+    + getIdentityMap() : IdentityMap
+    + getPerson(key : int) : Person
+    + setDb(db : PersonDbSimulatorImplementation)
+    + setIdentityMap(identityMap : IdentityMap)
+  }
+}
+PersonFinder -->  "-db" PersonDbSimulatorImplementation
+PersonFinder -->  "-identityMap" IdentityMap
+PersonDbSimulatorImplementation -->  "-personList" Person
+PersonDbSimulatorImplementation ..|> PersonDbSimulator 
+@enduml
@@ -0,0 +1,69 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+
+    The MIT License
+    Copyright © 2014-2022 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">
+    <parent>
+        <artifactId>java-design-patterns</artifactId>
+        <groupId>com.iluwatar</groupId>
+        <version>1.26.0-SNAPSHOT</version>
+    </parent>
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>identity-map</artifactId>
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-api</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-assembly-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <configuration>
+                            <archive>
+                                <manifest>
+                                    <mainClass>com.iluwatar.identitymap.App</mainClass>
+                                </manifest>
+                            </archive>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+</project>
@@ -0,0 +1,73 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.identitymap;
+
+import lombok.extern.slf4j.Slf4j;
+
+/**
+ * The basic idea behind the Identity Map is to have a series of maps containing objects that have been pulled from the database.
+ * The below example demonstrates the identity map pattern by creating a sample DB.
+ * Since only 1 DB has been created we only have 1 map corresponding to it for the purpose of this demo.
+ * When you load an object from the database, you first check the map.
+ * If there’s an object in it that corresponds to the one you’re loading, you return it. If not, you go to the database,
+ * putting the objects on the map for future reference as you load them.
+ */
+@Slf4j
+public class App {
+  /**
+   * Program entry point.
+   *
+   * @param args command line args.
+   */
+  public static void main(String[] args) {
+
+    // Dummy Persons
+    Person person1 = new Person(1, "John", 27304159);
+    Person person2 = new Person(2, "Thomas", 42273631);
+    Person person3 = new Person(3, "Arthur", 27489171);
+    Person person4 = new Person(4, "Finn", 20499078);
+    Person person5 = new Person(5, "Michael", 40599078);
+
+    // Init database
+    PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();
+    db.insert(person1);
+    db.insert(person2);
+    db.insert(person3);
+    db.insert(person4);
+    db.insert(person5);
+
+    // Init a personFinder
+    PersonFinder finder = new PersonFinder();
+    finder.setDb(db);
+
+    // Find persons in DataBase not the map.
+    LOGGER.info(finder.getPerson(2).toString());
+    LOGGER.info(finder.getPerson(4).toString());
+    LOGGER.info(finder.getPerson(5).toString());
+    // Find the person in the map.
+    LOGGER.info(finder.getPerson(2).toString());
+
+  }
+}
@@ -0,0 +1,31 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.identitymap;
+
+public class IdNotFoundException extends RuntimeException {
+  public IdNotFoundException(final String message) {
+    super(message);
+  }
+}
@@ -0,0 +1,77 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.identitymap;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import lombok.Getter;
+import lombok.extern.slf4j.Slf4j;
+
+/**
+ * This class stores the map into which we will be caching records after loading them from a DataBase.
+ * Stores the records as a Hash Map with the personNationalIDs as keys.
+ */
+@Slf4j
+@Getter
+public class IdentityMap {
+  private Map<Integer, Person> personMap = new HashMap<>();
+  /**
+   * Add person to the map.
+   */
+  public void addPerson(Person person) {
+    if (!personMap.containsKey(person.getPersonNationalId())) {
+      personMap.put(person.getPersonNationalId(), person);
+    } else { // Ensure that addPerson does not update a record. This situation will never arise in our implementation. Added only for testing purposes.
+      LOGGER.info("Key already in Map");
+    }
+  }
+
+  /**
+   * Get Person with given id.
+   *
+   * @param id : personNationalId as requested by user.
+   */
+  public Person getPerson(int id) {
+    Person person = personMap.get(id);
+    if (person == null) {
+      LOGGER.info("ID not in Map.");
+      return null;
+    }
+    LOGGER.info(person.toString());
+    return person;
+  }
+
+  /**
+   * Get the size of the map.
+   */
+  public int size() {
+    if (personMap == null) {
+      return 0;
+    }
+    return personMap.size();
+  }
+
+}
@@ -0,0 +1,57 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.identitymap;
+
+import java.io.Serializable;
+
+import lombok.AllArgsConstructor;
+import lombok.EqualsAndHashCode;
+import lombok.Getter;
+import lombok.Setter;
+
+/**
+ * Person definition.
+ */
+@EqualsAndHashCode(onlyExplicitlyIncluded = true)
+@Getter
+@Setter
+@AllArgsConstructor
+public final class Person implements Serializable {
+
+  private static final long serialVersionUID = 1L;
+
+  @EqualsAndHashCode.Include
+  private int personNationalId;
+  private String name;
+  private long phoneNum;
+
+  @Override
+  public String toString() {
+
+    return "Person ID is : " + personNationalId + " ; Person Name is : " + name + " ; Phone Number is :" + phoneNum;
+
+  }
+
+}
@@ -0,0 +1,36 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.identitymap;
+
+public interface PersonDbSimulator {
+  Person find(int personNationalID);
+
+  void insert(Person person);
+
+  void update(Person person);
+
+  void delete(int personNationalID);
+
+}
@@ -0,0 +1,108 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.identitymap;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Optional;
+
+import lombok.extern.slf4j.Slf4j;
+
+/**
+ * This is a sample database implementation. The database is in the form of an arraylist which stores records of
+ * different persons. The personNationalId acts as the primary key for a record.
+ * Operations :
+ * -> find (look for object with a particular ID)
+ * -> insert (insert record for a new person into the database)
+ * -> update (update the record of a person). To do this, create a new person instance with the same ID as the record you
+ * want to update. Then call this method with that person as an argument.
+ * -> delete (delete the record for a particular ID)
+ */
+@Slf4j
+public class PersonDbSimulatorImplementation implements PersonDbSimulator {
+
+  //This simulates a table in the database. To extend logic to multiple tables just add more lists to the implementation.
+  private List<Person> personList = new ArrayList<>();
+  static final String NOT_IN_DATA_BASE = " not in DataBase";
+  static final String ID_STR = "ID : ";
+
+  @Override
+  public Person find(int personNationalID) throws IdNotFoundException {
+    Optional<Person> elem = personList.stream().filter(p -> p.getPersonNationalId() == personNationalID).findFirst();
+    if (elem.isEmpty()) {
+      throw new IdNotFoundException(ID_STR + personNationalID + NOT_IN_DATA_BASE);
+    }
+    LOGGER.info(elem.get().toString());
+    return elem.get();
+  }
+
+  @Override
+  public void insert(Person person) {
+    Optional<Person> elem = personList.stream().filter(p -> p.getPersonNationalId() == person.getPersonNationalId()).findFirst();
+    if (elem.isPresent()) {
+      LOGGER.info("Record already exists.");
+      return;
+    }
+    personList.add(person);
+  }
+
+  @Override
+  public void update(Person person) throws IdNotFoundException {
+    Optional<Person> elem = personList.stream().filter(p -> p.getPersonNationalId() == person.getPersonNationalId()).findFirst();
+    if (elem.isPresent()) {
+      elem.get().setName(person.getName());
+      elem.get().setPhoneNum(person.getPhoneNum());
+      LOGGER.info("Record updated successfully");
+      return;
+    }
+    throw new IdNotFoundException(ID_STR + person.getPersonNationalId() + NOT_IN_DATA_BASE);
+  }
+
+  /**
+   * Delete the record corresponding to given ID from the DB.
+   *
+   * @param id : personNationalId for person whose record is to be deleted.
+   */
+  public void delete(int id) throws IdNotFoundException {
+    Optional<Person> elem = personList.stream().filter(p -> p.getPersonNationalId() == id).findFirst();
+    if (elem.isPresent()) {
+      personList.remove(elem.get());
+      LOGGER.info("Record deleted successfully.");
+      return;
+    }
+    throw new IdNotFoundException(ID_STR + id + NOT_IN_DATA_BASE);
+  }
+
+  /**
+   * Return the size of the database.
+   */
+  public int size() {
+    if (personList == null) {
+      return 0;
+    }
+    return personList.size();
+  }
+
+}
@@ -0,0 +1,69 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.identitymap;
+
+import lombok.Getter;
+import lombok.Setter;
+import lombok.extern.slf4j.Slf4j;
+
+/**
+ * Any object of this class stores a DataBase and an Identity Map. When we try to look for a key we first check if
+ * it has been cached in the Identity Map and return it if it is indeed in the map.
+ * If that is not the case then go to the DataBase, get the record, store it in the
+ * Identity Map and then return the record. Now if we look for the record again we will find it in the table itself which
+ * will make lookup faster.
+ */
+@Slf4j
+@Getter
+@Setter
+public class PersonFinder {
+  private static final long serialVersionUID = 1L;
+  //  Access to the Identity Map
+  private IdentityMap identityMap = new IdentityMap();
+  private PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();
+  /**
+   * get person corresponding to input ID.
+   *
+   * @param key : personNationalId to look for.
+   */
+  public Person getPerson(int key) {
+    // Try to find person in the identity map
+    Person person = this.identityMap.getPerson(key);
+    if (person != null) {
+      LOGGER.info("Person found in the Map");
+      return person;
+    } else {
+      // Try to find person in the database
+      person = this.db.find(key);
+      if (person != null) {
+        this.identityMap.addPerson(person);
+        LOGGER.info("Person found in DB.");
+        return person;
+      }
+      LOGGER.info("Person with this ID does not exist.");
+      return null;
+    }
+  }
+}
@@ -0,0 +1,35 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.identitymap;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+  class AppTest {
+  @Test
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
+  }
+}
@@ -0,0 +1,75 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.identitymap;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+class IdentityMapTest {
+  @Test
+  void addToMap(){
+    // new instance of an identity map(not connected to any DB here)
+    IdentityMap idMap = new IdentityMap();
+    // Dummy person instances
+    Person person1 = new Person(11, "Michael", 27304159);
+    Person person2 = new Person(22, "John", 42273631);
+    Person person3 = new Person(33, "Arthur", 27489171);
+    Person person4 = new Person(44, "Finn", 20499078);
+    // id already in map
+    Person person5 = new Person(11, "Michael", 40599078);
+    // All records go into identity map
+    idMap.addPerson(person1);
+    idMap.addPerson(person2);
+    idMap.addPerson(person3);
+    idMap.addPerson(person4);
+    idMap.addPerson(person5);
+    // Test no duplicate in our Map.
+    Assertions.assertEquals(4,idMap.size(),"Size of the map is incorrect");
+    // Test record not updated by add method.
+    Assertions.assertEquals(27304159,idMap.getPerson(11).getPhoneNum(),"Incorrect return value for phone number");
+  }
+  @Test
+  void testGetFromMap() {
+    // new instance of an identity map(not connected to any DB here)
+    IdentityMap idMap = new IdentityMap();
+    // Dummy person instances
+    Person person1 = new Person(11, "Michael", 27304159);
+    Person person2 = new Person(22, "John", 42273631);
+    Person person3 = new Person(33, "Arthur", 27489171);
+    Person person4 = new Person(44, "Finn", 20499078);
+    Person person5 = new Person(55, "Michael", 40599078);
+    // All records go into identity map
+    idMap.addPerson(person1);
+    idMap.addPerson(person2);
+    idMap.addPerson(person3);
+    idMap.addPerson(person4);
+    idMap.addPerson(person5);
+    // Test for dummy persons in the map
+    Assertions.assertEquals(person1,idMap.getPerson(11),"Incorrect person record returned");
+    Assertions.assertEquals(person4,idMap.getPerson(44),"Incorrect person record returned");
+    // Test for person with given id not in map
+    Assertions.assertNull(idMap.getPerson(1), "Incorrect person record returned");
+  }
+}
@@ -0,0 +1,122 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.identitymap;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+class PersonDbSimulatorImplementationTest {
+  @Test
+  void testInsert(){
+    // DataBase initialization.
+    PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();
+    Assertions.assertEquals(0,db.size(),"Size of null database should be 0");
+    // Dummy persons.
+    Person person1 = new Person(1, "Thomas", 27304159);
+    Person person2 = new Person(2, "John", 42273631);
+    Person person3 = new Person(3, "Arthur", 27489171);
+    db.insert(person1);
+    db.insert(person2);
+    db.insert(person3);
+    // Test size after insertion.
+    Assertions.assertEquals(3,db.size(),"Incorrect size for database.");
+    Person person4 = new Person(4, "Finn", 20499078);
+    Person person5 = new Person(5, "Michael", 40599078);
+    db.insert(person4);
+    db.insert(person5);
+    // Test size after more insertions.
+    Assertions.assertEquals(5,db.size(),"Incorrect size for database.");
+    Person person5duplicate = new Person(5,"Kevin",89589122);
+    db.insert(person5duplicate);
+    // Test size after attempt to insert record with duplicate key.
+    Assertions.assertEquals(5,db.size(),"Incorrect size for data base");
+  }
+  @Test
+  void findNotInDb(){
+    PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();
+    Person person1 = new Person(1, "Thomas", 27304159);
+    Person person2 = new Person(2, "John", 42273631);
+    db.insert(person1);
+    db.insert(person2);
+    // Test if IdNotFoundException is thrown where expected.
+    Assertions.assertThrows(IdNotFoundException.class,()->db.find(3));
+  }
+  @Test
+  void findInDb(){
+    PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();
+    Person person1 = new Person(1, "Thomas", 27304159);
+    Person person2 = new Person(2, "John", 42273631);
+    db.insert(person1);
+    db.insert(person2);
+    Assertions.assertEquals(person2,db.find(2),"Record that was found was incorrect.");
+  }
+  @Test
+  void updateNotInDb(){
+    PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();
+    Person person1 = new Person(1, "Thomas", 27304159);
+    Person person2 = new Person(2, "John", 42273631);
+    db.insert(person1);
+    db.insert(person2);
+    Person person3 = new Person(3,"Micheal",25671234);
+    // Test if IdNotFoundException is thrown when person with ID 3 is not in DB.
+    Assertions.assertThrows(IdNotFoundException.class,()->db.update(person3));
+  }
+  @Test
+  void updateInDb(){
+    PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();
+    Person person1 = new Person(1, "Thomas", 27304159);
+    Person person2 = new Person(2, "John", 42273631);
+    db.insert(person1);
+    db.insert(person2);
+    Person person = new Person(2,"Thomas",42273690);
+    db.update(person);
+    Assertions.assertEquals(person,db.find(2),"Incorrect update.");
+  }
+  @Test
+  void deleteNotInDb(){
+    PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();
+    Person person1 = new Person(1, "Thomas", 27304159);
+    Person person2 = new Person(2, "John", 42273631);
+    db.insert(person1);
+    db.insert(person2);
+    // Test if IdNotFoundException is thrown when person with this ID not in DB.
+    Assertions.assertThrows(IdNotFoundException.class,()->db.delete(3));
+  }
+  @Test
+  void deleteInDb(){
+    PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();
+    Person person1 = new Person(1, "Thomas", 27304159);
+    Person person2 = new Person(2, "John", 42273631);
+    db.insert(person1);
+    db.insert(person2);
+    // delete the record.
+    db.delete(1);
+    // test size of database after deletion.
+    Assertions.assertEquals(1,db.size(),"Size after deletion is incorrect.");
+    // try to find deleted record in db.
+    Assertions.assertThrows(IdNotFoundException.class,()->db.find(1));
+  }
+
+}
@@ -0,0 +1,112 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.identitymap;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+class PersonFinderTest {
+  @Test
+  void personFoundInDB(){
+    // personFinderInstance
+    PersonFinder personFinder = new PersonFinder();
+    // init database for our personFinder
+    PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();
+    // Dummy persons
+    Person person1 = new Person(1, "John", 27304159);
+    Person person2 = new Person(2, "Thomas", 42273631);
+    Person person3 = new Person(3, "Arthur", 27489171);
+    Person person4 = new Person(4, "Finn", 20499078);
+    Person person5 = new Person(5, "Michael", 40599078);
+    // Add data to the database.
+    db.insert(person1);
+    db.insert(person2);
+    db.insert(person3);
+    db.insert(person4);
+    db.insert(person5);
+    personFinder.setDb(db);
+
+    Assertions.assertEquals(person1,personFinder.getPerson(1),"Find person returns incorrect record.");
+    Assertions.assertEquals(person3,personFinder.getPerson(3),"Find person returns incorrect record.");
+    Assertions.assertEquals(person2,personFinder.getPerson(2),"Find person returns incorrect record.");
+    Assertions.assertEquals(person5,personFinder.getPerson(5),"Find person returns incorrect record.");
+    Assertions.assertEquals(person4,personFinder.getPerson(4),"Find person returns incorrect record.");
+  }
+  @Test
+  void personFoundInIdMap(){
+    // personFinderInstance
+    PersonFinder personFinder = new PersonFinder();
+    // init database for our personFinder
+    PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();
+    // Dummy persons
+    Person person1 = new Person(1, "John", 27304159);
+    Person person2 = new Person(2, "Thomas", 42273631);
+    Person person3 = new Person(3, "Arthur", 27489171);
+    Person person4 = new Person(4, "Finn", 20499078);
+    Person person5 = new Person(5, "Michael", 40599078);
+    // Add data to the database.
+    db.insert(person1);
+    db.insert(person2);
+    db.insert(person3);
+    db.insert(person4);
+    db.insert(person5);
+    personFinder.setDb(db);
+    // Assure key is not in the ID map.
+    Assertions.assertFalse(personFinder.getIdentityMap().getPersonMap().containsKey(3));
+    // Assure key is in the database.
+    Assertions.assertEquals(person3,personFinder.getPerson(3),"Finder returns incorrect record.");
+    // Assure that the record for this key is cached in the Map now.
+    Assertions.assertTrue(personFinder.getIdentityMap().getPersonMap().containsKey(3));
+    // Find the record again. This time it will be found in the map.
+    Assertions.assertEquals(person3,personFinder.getPerson(3),"Finder returns incorrect record.");
+  }
+  @Test
+  void personNotFoundInDB(){
+    PersonFinder personFinder = new PersonFinder();
+    // init database for our personFinder
+    PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();
+    personFinder.setDb(db);
+    Assertions.assertThrows(IdNotFoundException.class,()->personFinder.getPerson(1));
+    // Dummy persons
+    Person person1 = new Person(1, "John", 27304159);
+    Person person2 = new Person(2, "Thomas", 42273631);
+    Person person3 = new Person(3, "Arthur", 27489171);
+    Person person4 = new Person(4, "Finn", 20499078);
+    Person person5 = new Person(5, "Michael", 40599078);
+    db.insert(person1);
+    db.insert(person2);
+    db.insert(person3);
+    db.insert(person4);
+    db.insert(person5);
+    personFinder.setDb(db);
+    // Assure that the database has been updated.
+    Assertions.assertEquals(person4,personFinder.getPerson(4),"Find returns incorrect record");
+    // Assure key is in DB now.
+    Assertions.assertDoesNotThrow(()->personFinder.getPerson(1));
+    // Assure key not in DB.
+    Assertions.assertThrows(IdNotFoundException.class,()->personFinder.getPerson(6));
+
+  }
+}
@@ -0,0 +1,43 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 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.identitymap;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+class PersonTest {
+  @Test
+  void testEquality(){
+    // dummy persons.
+    Person person1 = new Person(1,"Harry",989950022);
+    Person person2 = new Person(2,"Kane",989920011);
+    Assertions.assertNotEquals(person1,person2,"Incorrect equality condition");
+    // person with duplicate nationalID.
+    Person person3 = new Person(2,"John",789012211);
+    // If nationalID is equal then persons are equal(even if name or phoneNum are different).
+    // This situation will never arise in this implementation. Only for testing.
+    Assertions.assertEquals(person2,person3,"Incorrect inequality condition");
+  }
+}
@@ -215,6 +215,7 @@
    <module>composite-view</module>
    <module>metadata-mapping</module>
    <module>service-to-worker</module>
+    <module>identity-map</module>
  </modules>
  <repositories>
    <repository>