docs: Added README.md explanation in Data mapper (iluwatar#2307)

* Added wikipedia says for explanation section

* Added explanation on README.md

Author: XianWu99

data-mapper/README.md

@@ -7,7 +7,147 @@ tags:
 ---
 
 ## Intent
-A layer of mappers that moves data between objects and a database while keeping them independent of each other and the mapper itself
+>Data Mapper is the software layer that separates the in-memory objects from the database.
+>Its responsibility is to transfer data between the objects and database and isolate them from each other. 
+>If we obtain a Data Mapper, it is not necessary for the in-memory object to know if the database exists or not.
+>The user could directly manipulate the objects via Java command without having knowledge of SQL or database.
+
+## Explanation
+
+Real world example
+>When a user accesses a specific web page through a browser, he only needs to do several operations to the browser.
+> The browser and the server will take the responsibility of saving data separately.
+> You don't need to know the existence of the server or how to operate the server.
+
+In plain words
+>A layer of mappers that moves data between objects and a database while keeping them independent of each other.
+
+Wikipedia says
+>A Data Mapper is a Data Access Layer that performs bidirectional transfer of data between a persistent data store 
+> (often a relational database) and an in-memory data representation (the domain layer). The goal of the pattern is to 
+> keep the in-memory representation and the persistent data store independent of each other and the data mapper itself. 
+> This is useful when one needs to model and enforce strict business processes on the data in the domain layer that do 
+> not map neatly to the persistent data store. 
+
+**Programmatic Example**
+
+>We have the student class to defining Students' attributes includes studentId, name and grade.
+>We have an interface of StudentDataMapper to lists out the possible behaviour for all possible student mappers.
+>And StudentDataMapperImpl class for the implementation of actions on Students Data.
+
+```java
+
+public final class Student implements Serializable {
+
+    private static final long serialVersionUID = 1L;
+
+    @EqualsAndHashCode.Include
+    private int studentId;
+    private String name;
+    private char grade;
+
+
+    public interface StudentDataMapper {
+
+        Optional<Student> find(int studentId);
+
+        void insert(Student student) throws DataMapperException;
+
+        void update(Student student) throws DataMapperException;
+
+        void delete(Student student) throws DataMapperException;
+    }
+
+    public final class StudentDataMapperImpl implements StudentDataMapper {
+        @Override
+        public Optional<Student> find(int studentId) {
+            return this.getStudents().stream().filter(x -> x.getStudentId() == studentId).findFirst();
+        }
+
+        @Override
+        public void update(Student studentToBeUpdated) throws DataMapperException {
+            String name = studentToBeUpdated.getName();
+            Integer index = Optional.of(studentToBeUpdated)
+                    .map(Student::getStudentId)
+                    .flatMap(this::find)
+                    .map(students::indexOf)
+                    .orElseThrow(() -> new DataMapperException("Student [" + name + "] is not found"));
+            students.set(index, studentToBeUpdated);
+        }
+
+        @Override
+        public void insert(Student studentToBeInserted) throws DataMapperException {
+            Optional<Student> student = find(studentToBeInserted.getStudentId());
+            if (student.isPresent()) {
+                String name = studentToBeInserted.getName();
+                throw new DataMapperException("Student already [" + name + "] exists");
+            }
+
+            students.add(studentToBeInserted);
+        }
+
+        @Override
+        public void delete(Student studentToBeDeleted) throws DataMapperException {
+            if (!students.remove(studentToBeDeleted)) {
+                String name = studentToBeDeleted.getName();
+                throw new DataMapperException("Student [" + name + "] is not found");
+            }
+        }
+
+        public List<Student> getStudents() {
+            return this.students;
+        }
+    }
+}
+
+```
+
+>The below example demonstrates basic CRUD operations: Create, Read, Update, and Delete between the in-memory objects
+> and the database.
+
+```java
+@Slf4j
+public final class App {
+
+  private static final String STUDENT_STRING = "App.main(), student : ";
+
+  public static void main(final String... args) {
+
+    final var mapper = new StudentDataMapperImpl();
+
+    var student = new Student(1, "Adam", 'A');
+
+    mapper.insert(student);
+
+    LOGGER.debug(STUDENT_STRING + student + ", is inserted");
+
+    final var studentToBeFound = mapper.find(student.getStudentId());
+
+    LOGGER.debug(STUDENT_STRING + studentToBeFound + ", is searched");
+
+    student = new Student(student.getStudentId(), "AdamUpdated", 'A');
+
+    mapper.update(student);
+
+    LOGGER.debug(STUDENT_STRING + student + ", is updated");
+    LOGGER.debug(STUDENT_STRING + student + ", is going to be deleted");
+
+    mapper.delete(student);
+  }
+
+  private App() {
+  }
+}
+```
+
+Program output:
+
+15:05:00.264 [main] DEBUG com.iluwatar.datamapper.App - App.main(), student : Student(studentId=1, name=Adam, grade=A), is inserted
+15:05:00.267 [main] DEBUG com.iluwatar.datamapper.App - App.main(), student : Optional[Student(studentId=1, name=Adam, grade=A)], is searched
+15:05:00.268 [main] DEBUG com.iluwatar.datamapper.App - App.main(), student : Student(studentId=1, name=AdamUpdated, grade=A), is updated
+15:05:00.268 [main] DEBUG com.iluwatar.datamapper.App - App.main(), student : Student(studentId=1, name=AdamUpdated, grade=A), is going to be deleted
+
+
 
 This layer consists of one or more mappers (or data access objects) that perform data transfer. The scope of mapper implementations varies.
 A generic mapper will handle many different domain entity types, a dedicated mapper will handle one or a few.