BAEL-2789 - The Dependency Inversion Principle in Java (#6613)

* Initial Commit

* Added dip dipmodular modules to pom.xml

* Delete pom.xml

* Add pom.xml

* Update pom.xml

* Update pom.xml

* Update pom.xml

* Update Application.java

* Update CustomerDaoUnitTest.java

* Update CustomerServiceUnitTest.java

* Update Application.java

* Update CustomerDaoUnitTest.java

* Update CustomerServiceUnitTest.java

* Update CustomerDaoUnitTest.java

* Update CustomerServiceUnitTest.java

patterns/dip/pom.xml

@@ -0,0 +1,39 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<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">
+    <modelVersion>4.0.0</modelVersion>
+    <groupId>com.baeldung.dip</groupId>
+    <artifactId>dip</artifactId>
+    <name>dip</name>
+    <version>1.0-SNAPSHOT</version>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>patterns</artifactId>
+        <version>1.0.0-SNAPSHOT</version>
+        <relativePath>..</relativePath>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <version>4.12</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>3.12.1</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+    
+    <properties>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+        <maven.compiler.source>11</maven.compiler.source>
+        <maven.compiler.target>11</maven.compiler.target>
+    </properties>
+
+</project>

patterns/dip/src/main/java/com/baeldung/dip/application/Application.java

@@ -0,0 +1,18 @@
+package com.baeldung.dip.application;
+
+import com.baeldung.dip.daoimplementations.SimpleCustomerDao;
+import com.baeldung.dip.entities.Customer;
+import com.baeldung.dip.services.CustomerService;
+import java.util.Map;
+import java.util.HashMap;
+
+public class Application {
+
+    public static void main(String[] args) {
+        Map<Integer, Customer> customers = new HashMap<>();
+        customers.put(1, new Customer("John"));
+        customers.put(2, new Customer("Susan"));
+        CustomerService customerService = new CustomerService(new SimpleCustomerDao(customers));
+        customerService.findAll().forEach(System.out::println);
+    }
+}

patterns/dip/src/main/java/com/baeldung/dip/daoimplementations/SimpleCustomerDao.java

@@ -0,0 +1,28 @@
+package com.baeldung.dip.daoimplementations;
+
+import com.baeldung.dip.entities.Customer;
+import com.baeldung.dip.daointerfaces.CustomerDao;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+
+public class SimpleCustomerDao implements CustomerDao {
+
+    private Map<Integer, Customer> customers = new HashMap<>();
+
+    public SimpleCustomerDao(Map<Integer, Customer> customers) {
+        this.customers = customers;
+    }
+
+    @Override
+    public Optional<Customer> findById(int id) {
+        return Optional.ofNullable(customers.get(id));
+    }
+
+    @Override
+    public List<Customer> findAll() {
+        return new ArrayList<>(customers.values());
+    }
+}

patterns/dip/src/main/java/com/baeldung/dip/daointerfaces/CustomerDao.java

@@ -0,0 +1,13 @@
+package com.baeldung.dip.daointerfaces;
+
+import com.baeldung.dip.entities.Customer;
+import java.util.List;
+import java.util.Optional;
+
+public interface CustomerDao {
+
+    Optional<Customer> findById(int id);
+
+    List<Customer> findAll();
+
+}

patterns/dip/src/main/java/com/baeldung/dip/entities/Customer.java

@@ -0,0 +1,19 @@
+package com.baeldung.dip.entities;
+
+public class Customer {
+
+    private final String name;
+
+    public Customer(String name) {
+        this.name = name;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    @Override
+    public String toString() {
+        return "Customer{" + "name=" + name + '}';
+    }
+}

patterns/dip/src/main/java/com/baeldung/dip/services/CustomerService.java

@@ -0,0 +1,23 @@
+package com.baeldung.dip.services;
+
+import com.baeldung.dip.daointerfaces.CustomerDao;
+import com.baeldung.dip.entities.Customer;
+import java.util.List;
+import java.util.Optional;
+
+public class CustomerService {
+
+    private final CustomerDao customerDao;
+
+    public CustomerService(CustomerDao customerDao) {
+        this.customerDao = customerDao;
+    }
+
+    public Optional<Customer> findById(int id) {
+        return customerDao.findById(id);
+    }
+
+    public List<Customer> findAll() {
+        return customerDao.findAll();
+    }
+}

patterns/dip/src/test/java/com/baeldung/dip/tests/CustomerDaoUnitTest.java

@@ -0,0 +1,46 @@
+package com.baeldung.dip.tests;
+
+import com.baeldung.dip.daoimplementations.SimpleCustomerDao;
+import com.baeldung.dip.daointerfaces.CustomerDao;
+import com.baeldung.dip.entities.Customer;
+import java.util.Map;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Optional;
+import static org.assertj.core.api.Assertions.assertThat;
+import org.junit.Before;
+import org.junit.Test;
+
+public class CustomerDaoUnitTest {
+
+    private CustomerDao customerDao;
+
+    @Before
+    public void setUpCustomerDaoInstance() {
+        Map<Integer, Customer> customers = new HashMap<>();
+        customers.put(1, new Customer("John"));
+        customers.put(2, new Customer("Susan"));
+        customerDao = new SimpleCustomerDao(customers);
+    }
+
+    @Test
+    public void givenCustomerDaoInstance_whenCalledFindById_thenCorrect() {
+        assertThat(customerDao.findById(1)).isInstanceOf(Optional.class);
+    }
+
+    @Test
+    public void givenCustomerDaoInstance_whenCalledFindAll_thenCorrect() {
+        assertThat(customerDao.findAll()).isInstanceOf(List.class);
+    }
+
+    @Test
+    public void givenCustomerDaoInstance_whenCalledFindByIdWithNullCustomer_thenCorrect() {
+        Map<Integer, Customer> customers = new HashMap<Integer, Customer>();
+        customers.put(1, null);
+        CustomerDao customerDaoObject = new SimpleCustomerDao(customers);
+
+        Customer customer = customerDaoObject.findById(1).orElseGet(() -> new Customer("Non-existing customer"));
+
+        assertThat(customer.getName()).isEqualTo("Non-existing customer");
+    }
+}

patterns/dip/src/test/java/com/baeldung/dip/tests/CustomerServiceUnitTest.java

@@ -0,0 +1,46 @@
+package com.baeldung.dip.tests;
+
+import com.baeldung.dip.daoimplementations.SimpleCustomerDao;
+import com.baeldung.dip.entities.Customer;
+import com.baeldung.dip.services.CustomerService;
+import java.util.Map;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Optional;
+import static org.assertj.core.api.Assertions.assertThat;
+import org.junit.Before;
+import org.junit.Test;
+
+public class CustomerServiceUnitTest {
+
+    private CustomerService customerService;
+
+    @Before
+    public void setUpCustomerServiceInstance() {
+        Map<Integer, Customer> customers = new HashMap<>();
+        customers.put(1, new Customer("John"));
+        customers.put(2, new Customer("Susan"));
+        customerService = new CustomerService(new SimpleCustomerDao(customers));
+    }
+
+    @Test
+    public void givenCustomerServiceInstance_whenCalledFindById_thenCorrect() {
+        assertThat(customerService.findById(1)).isInstanceOf(Optional.class);
+    }
+
+    @Test
+    public void givenCustomerServiceInstance_whenCalledFindAll_thenCorrect() {
+        assertThat(customerService.findAll()).isInstanceOf(List.class);
+    }
+
+    @Test
+    public void givenCustomerServiceInstance_whenCalledFindByIdWithNullCustomer_thenCorrect() {
+        Map<Integer, Customer> customers = new HashMap<>();
+        customers.put(1, null);
+        customerService = new CustomerService(new SimpleCustomerDao(customers));
+
+        Customer customer = customerService.findById(1).orElseGet(() -> new Customer("Non-existing customer"));
+
+        assertThat(customer.getName()).isEqualTo("Non-existing customer");
+    }
+}

patterns/pom.xml

@@ -19,6 +19,7 @@
         <module>design-patterns</module>
         <module>design-patterns-2</module>
         <module>solid</module>
+        <module>dip</module>     
     </modules>
 
     <dependencyManagement>