Added DAO implementation, modified readme, and added UML diagram

dao/etc/dao.ucls

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+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.1.8" icons="true" always-add-relationships="false" generalizations="true" realizations="true" 
+  associations="true" dependencies="false" nesting-relationships="true">  
+  <class id="1" language="java" name="com.iluwatar.Customer" project="dao" 
+    file="/dao/src/main/java/com/iluwatar/Customer.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="176" y="337"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="2" language="java" name="com.iluwatar.CustomerDaoImpl" project="dao" 
+    file="/dao/src/main/java/com/iluwatar/CustomerDaoImpl.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="540" y="334"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <interface id="3" language="java" name="com.iluwatar.CustomerDao" project="dao" 
+    file="/dao/src/main/java/com/iluwatar/CustomerDao.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="536" y="131"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </interface>  
+  <realization id="4">    
+    <end type="SOURCE" refId="2"/>    
+    <end type="TARGET" refId="3"/>  
+  </realization>  
+  <association id="5">    
+    <end type="SOURCE" refId="2" navigable="false">      
+      <attribute id="6" name="customers"/>      
+      <multiplicity id="7" minimum="0" maximum="2147483647"/>    
+    </end>    
+    <end type="TARGET" refId="1" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+    sort-features="false" accessors="true" visibility="true">    
+    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
+    <operations public="true" package="true" protected="true" private="true" static="true"/>  
+  </classifier-display>  
+  <association-display labels="true" multiplicity="true"/>
+</class-diagram>

dao/pom.xml

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+<?xml version="1.0"?>
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.0-SNAPSHOT</version>
+  </parent>
+  <artifactId>dao</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

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);
+	}
+}