Updated unit .equals() and .hashCode() methods

Formatted code using this formatter:
https://github.com/google/styleguide/blob/gh-pages/eclipse-java-google-style.xml
Removed argument final declaration on interface
Updated addCustomer logic for cases where the added customer already
exists

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

@@ -43,23 +43,18 @@ public class Customer {
 
   @Override
   public String toString() {
-        return "Customer{" +
-                "id=" + getId() +
-                ", firstName='" + getFirstName() + '\'' +
-                ", lastName='" + getLastName() + '\'' +
-                '}';
+    return "Customer{" + "id=" + getId() + ", firstName='" + getFirstName() + '\'' + ", lastName='"
+        + getLastName() + '\'' + '}';
   }
 
-    /**
-     * Checks if two objects are the same.
-     * 
-     * @return true if the two objects are Customer objects and have the same id value, false otherwise.
-     */
   @Override
   public boolean equals(final Object o) {
     boolean isEqual = false;
+    if (this == o) {
+      isEqual = true;
+    } else if (o != null && (getClass() == o.getClass())) {
       final Customer customer = (Customer) o;
-        if (getId() == customer.getId()) {
+      if (getId() == customer.getId())
         isEqual = true;
     }
     return isEqual;
@@ -68,8 +63,6 @@ public class Customer {
   @Override
   public int hashCode() {
     int result = getId();
-        id += getFirstName().hashCode();
-        id += getLastName().hashCode();
     return result;
   }
 }

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

@@ -10,8 +10,12 @@ import java.util.List;
 public interface CustomerDao {
 
   List<Customer> getAllCustomers();
-    Customer getCustomerById(final int id);
-    void addCustomer(final Customer customer);
-    void updateCustomer(final Customer customer);
-    void deleteCustomer(final Customer customer);
+
+  Customer getCustomerById(int id);
+
+  void addCustomer(Customer customer);
+
+  void updateCustomer(Customer customer);
+
+  void deleteCustomer(Customer customer);
 }

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

@@ -4,11 +4,12 @@ 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.
+ * 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 {
@@ -40,8 +41,10 @@ public class CustomerDaoImpl implements CustomerDao {
 
   @Override
   public void addCustomer(final Customer customer) {
+    if (getCustomerById(customer.getId()) == null) {
       customers.add(customer);
     }
+  }
 
 
   @Override

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

@@ -50,7 +50,7 @@ public class CustomerDaoImplTest {
 
   @Test
   public void updateNonExistingCustomer() {
-		final int nonExistingId = 999;
+    final int nonExistingId = getNonExistingCustomerId();
     final String newFirstname = "Douglas";
     final String newLastname = "MacArthur";
     final Customer customer = new Customer(nonExistingId, newFirstname, newLastname);
@@ -68,6 +68,15 @@ public class CustomerDaoImplTest {
     assertEquals(2, impl.getAllCustomers().size());
   }
 
+  @Test
+  public void addAlreadyAddedCustomer() {
+    final Customer newCustomer = new Customer(3, "George", "Patton");
+    impl.addCustomer(newCustomer);
+    assertEquals(2, impl.getAllCustomers().size());
+    impl.addCustomer(newCustomer);
+    assertEquals(2, impl.getAllCustomers().size());
+  }
+
   @Test
   public void getExistinCustomerById() {
     assertEquals(CUSTOMER, impl.getCustomerById(CUSTOMER.getId()));
@@ -78,4 +87,13 @@ public class CustomerDaoImplTest {
     final int nonExistingId = 999;
     assertNull(impl.getCustomerById(nonExistingId));
   }
+
+  /**
+   * An arbitrary number which does not correspond to an active Customer id.
+   * 
+   * @return an int of a customer id which doesn't exist
+   */
+  private int getNonExistingCustomerId() {
+    return 999;
+  }
 }

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

@@ -1,8 +1,7 @@
 package com.iluwatar.dao;
 
 import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.assertNotEquals;
 
 import org.junit.Before;
 import org.junit.Test;
@@ -20,64 +19,52 @@ public class CustomerTest {
   }
 
   @Test
-	public void getIndex() {
-		assertEquals(ID, customer.getId());
-	}
-	
-	@Test
-	public void getFirstname() {
-		assertEquals(FIRSTNAME, customer.getFirstName());
-	}
-	
-	@Test
-	public void getLastname() {
-		assertEquals(LASTNAME, customer.getLastName());
-	}
-	
-	@Test
-	public void setIndex() {
+  public void getAndSetId() {
     final int newId = 2;
     customer.setId(newId);
     assertEquals(newId, customer.getId());
   }
 
   @Test
-	public void setFirstname() {
+  public void getAndSetFirstname() {
     final String newFirstname = "Bill";
     customer.setFirstName(newFirstname);
     assertEquals(newFirstname, customer.getFirstName());
   }
 
   @Test
-	public void setLastname() {
+  public void getAndSetLastname() {
     final String newLastname = "Clinton";
     customer.setLastName(newLastname);
     assertEquals(newLastname, customer.getLastName());
   }
 
   @Test
-	public void equalsWithDifferentId() {
+  public void notEqualWithDifferentId() {
     final int newId = 2;
     final Customer otherCustomer = new Customer(newId, FIRSTNAME, LASTNAME);
-		assertFalse(customer.equals(otherCustomer));
+    assertNotEquals(customer, otherCustomer);
+    assertNotEquals(customer.hashCode(), otherCustomer.hashCode());
+  }
+
+  @Test
+  public void equalsWithSameObjectValues() {
+    final Customer otherCustomer = new Customer(ID, FIRSTNAME, LASTNAME);
+    assertEquals(customer, otherCustomer);
+    assertEquals(customer.hashCode(), otherCustomer.hashCode());
   }
 
   @Test
   public void equalsWithSameObjects() {
-		final Customer otherCustomer = new Customer(ID, FIRSTNAME, LASTNAME);
-		assertTrue(customer.equals(otherCustomer));
-	}
-	
-	@Test
-	public void testHashCode() {
-		assertTrue(customer.hashCode() > 0);
+    assertEquals(customer, customer);
+    assertEquals(customer.hashCode(), customer.hashCode());
   }
 
   @Test
   public void testToString() {
     final StringBuffer buffer = new StringBuffer();
     buffer.append("Customer{id=");
-        buffer.append(""+customer.getId());
+    buffer.append("" + customer.getId());
     buffer.append(", firstName='");
     buffer.append(customer.getFirstName());
     buffer.append("\', lastName='");