Added DAO implementation, modified readme, and added UML diagram

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

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+package com.iluwatar;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * 
+ * With the DAO pattern, we can use various method calls to retrieve/add/delete/update data without directly
+ * interacting with the data directly. The below example demonstrates basic operations: select, add, update, and delete.
+ */
+public class App {
+
+	public static void main(String[] args) {
+
+		CustomerDaoImpl customerDao = new CustomerDaoImpl(generateSampleCustomers());
+
+		System.out.println("customerDao.getAllCustomers(): " + customerDao.getAllCustomers());
+		System.out.println("customerDao.getCusterById(2): " + customerDao.getCusterById(2));
+
+		Customer customer = new Customer(4, "Dan", "Danson");
+		customerDao.addCustomer(customer);
+
+		System.out.println("customerDao.getAllCustomers(): " + customerDao.getAllCustomers());
+
+		customer.setFirstName("Daniel");
+		customer.setLastName("Danielson");
+		customerDao.updateCustomer(customer);
+
+		System.out.println("customerDao.getAllCustomers(): " + customerDao.getAllCustomers());
+
+		customerDao.deleteCustomer(customer);
+
+		System.out.println("customerDao.getAllCustomers(): " + customerDao.getAllCustomers());
+	}
+
+	public static List<Customer> generateSampleCustomers() {
+		Customer customer1 = new Customer(1, "Adam", "Adamson");
+		Customer customer2 = new Customer(2, "Bob", "Bobson");
+		Customer customer3 = new Customer(3, "Carl", "Carlson");
+
+		List<Customer> customers = new ArrayList<Customer>();
+		customers.add(customer1);
+		customers.add(customer2);
+		customers.add(customer3);
+		return customers;
+	}
+}

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

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+package com.iluwatar;
+
+public class Customer {
+    private int id;
+    private String firstName;
+    private String lastName;
+
+    public Customer(int id, String firstName, String lastName) {
+        this.id = id;
+        this.firstName = firstName;
+        this.lastName = lastName;
+    }
+
+    public int getId() {
+        return id;
+    }
+
+    public void setId(int id) {
+        this.id = id;
+    }
+
+    public String getFirstName() {
+        return firstName;
+    }
+
+    public void setFirstName(String firstName) {
+        this.firstName = firstName;
+    }
+
+    public String getLastName() {
+        return lastName;
+    }
+
+    public void setLastName(String lastName) {
+        this.lastName = lastName;
+    }
+
+    @Override
+    public String toString() {
+        return "Customer{" +
+                "id=" + id +
+                ", firstName='" + firstName + '\'' +
+                ", lastName='" + lastName + '\'' +
+                '}';
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+
+        Customer customer = (Customer) o;
+
+        if (id != customer.id) return false;
+
+        return true;
+    }
+
+    @Override
+    public int hashCode() {
+        int result = id;
+        return result;
+    }
+}

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

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+package com.iluwatar;
+
+import java.util.List;
+
+public interface CustomerDao {
+    public List<Customer> getAllCustomers();
+    public Customer getCusterById(int id);
+    public void addCustomer(Customer customer);
+    public void updateCustomer(Customer customer);
+    public void deleteCustomer(Customer customer);
+}

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

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+package com.iluwatar;
+
+import java.util.List;
+
+/**
+ * The data access object (DAO) is an object that provides an abstract interface to some type of database or other persistence mechanism.
+ * By mapping application calls to the persistence layer, DAO provide some specific data operations without exposing details of the database.
+ * This isolation supports the Single responsibility principle. It separates what data accesses the application needs, in terms of
+ * domain-specific objects and data types (the public interface of the DAO), from how these needs can be satisfied with a specific DBMS,
+ * database schema, etc.
+ */
+public class CustomerDaoImpl implements CustomerDao {
+
+    // Represents the DB structure for our example so we don't have to managed it ourselves
+    // Note: Normally this would be in the form of an actual database and not part of the Dao Impl.
+    private List<Customer> customers;
+
+    public CustomerDaoImpl(List<Customer> customers) {
+        this.customers = customers;
+    }
+
+    @Override
+    public List<Customer> getAllCustomers() {
+        return customers;
+    }
+
+    @Override
+    public Customer getCusterById(int id) {
+        for (int i = 0; i < customers.size(); i++) {
+            if (customers.get(i).getId() == id) {
+                return customers.get(i);
+            }
+        }
+        // No customer found
+        return null;
+    }
+
+    @Override
+    public void addCustomer(Customer customer) {
+        customers.add(customer);
+    }
+
+
+    @Override
+    public void updateCustomer(Customer customer) {
+        if (customers.contains(customer)) {
+            customers.set(customers.indexOf(customer), customer);
+        }
+    }
+
+    @Override
+    public void deleteCustomer(Customer customer) {
+        customers.remove(customer);
+    }
+}

dao/src/test/java/com/iluwatar/AppTest.java

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+package com.iluwatar;
+
+import org.junit.Test;
+
+public class AppTest {
+
+	@Test
+	public void test() {
+		String[] args = {};
+		App.main(args);
+	}
+}