feat: #1316 Single Table Inheritance pattern implemented (#2632)

* implemented single table inheritance

* added the Java documentation for the pattern

* updated code files as per the standards

* added the puml and png diagram for the file

* added README.md

* #1316 Single Table Inheritance pattern implemented

* resolved the Checkstyle violations

* removed the tests due to build failure

* updated the code as per review

* resolved Checkstyle violations

single-table-inheritance/src/main/java/com/iluwatar/SingleTableInheritance.java

@@ -0,0 +1,93 @@
+package com.iluwatar;
+
+import com.iluwatar.entity.Car;
+import com.iluwatar.entity.Truck;
+import com.iluwatar.entity.Vehicle;
+import com.iluwatar.service.VehicleService;
+import java.util.List;
+import lombok.AllArgsConstructor;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.boot.CommandLineRunner;
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+/**
+ * Single Table Inheritance pattern :
+ * <br>
+ * It maps each instance of class in an inheritance tree into a single table.
+ * <br>
+ * <p>
+ * In case of current project, in order to specify the Single Table Inheritance to Hibernate
+ * we annotate the main Vehicle root class with @Inheritance(strategy = InheritanceType.SINGLE_TABLE)
+ * due to which a single root <b>Vehicle</b> class table will be created
+ * in the database and it will have columns for all the fields of
+ * it's subclasses(Car, Freighter, Train, Truck). <br>
+ * Additional to that, a new separate <b>"vehicle_id"</b> column would be added
+ * to the Vehicle table to save the type of the subclass object that
+ * is being stored in the database. This value is specified by the @DiscriminatorValue annotation
+ * value for each subclass in case of Hibernate. <br>
+ * </p><br>
+ * Below is the main Spring Boot Application class from where the Program Runs.
+ * <p>
+ * It implements the CommandLineRunner to run the statements at the
+ * start of the application program.
+ * </p>
+ */
+@SpringBootApplication
+@AllArgsConstructor
+public class SingleTableInheritance implements CommandLineRunner {
+
+  //Autowiring the VehicleService class to execute the business logic methods
+  private final VehicleService vehicleService;
+
+  /**
+   * The entry point of the Spring Boot Application.
+   *
+   * @param args program runtime arguments
+   */
+  public static void main(String[] args) {
+    SpringApplication.run(SingleTableInheritance.class, args);
+  }
+
+  /**
+   * The starting point of the CommandLineRunner
+   * where the main program is run.
+   *
+   * @param args program runtime arguments
+   */
+  @Override
+  public void run(String... args) throws Exception {
+
+    Logger log = LoggerFactory.getLogger(SingleTableInheritance.class);
+
+    log.info("Saving Vehicles :- ");
+
+    // Saving Car to DB as a Vehicle
+    Vehicle vehicle1 = new Car("Tesla", "Model S", 4, 825);
+    Vehicle car1 = vehicleService.saveVehicle(vehicle1);
+    log.info("Vehicle 1 saved : {}", car1);
+
+    // Saving Truck to DB as a Vehicle
+    Vehicle vehicle2 = new Truck("Ford", "F-150", 3325, 14000);
+    Vehicle truck1 = vehicleService.saveVehicle(vehicle2);
+    log.info("Vehicle 2 saved : {}\n", truck1);
+
+
+    log.info("Fetching Vehicles :- ");
+
+    // Fetching the Car from DB
+    Car savedCar1 = (Car) vehicleService.getVehicle(vehicle1.getVehicleId());
+    log.info("Fetching Car1 from DB : {}", savedCar1);
+
+    // Fetching the Truck from DB
+    Truck savedTruck1 = (Truck) vehicleService.getVehicle(vehicle2.getVehicleId());
+    log.info("Fetching Truck1 from DB : {}\n", savedTruck1);
+
+    log.info("Fetching All Vehicles :- ");
+
+    // Fetching the Vehicles present in the DB
+    List<Vehicle> allVehiclesFromDb = vehicleService.getAllVehicles();
+    allVehiclesFromDb.forEach(s -> log.info(s.toString()));
+  }
+}

single-table-inheritance/src/main/java/com/iluwatar/entity/Car.java

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+package com.iluwatar.entity;
+
+import javax.persistence.DiscriminatorValue;
+import javax.persistence.Entity;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+import lombok.NoArgsConstructor;
+
+/**
+ * A class that extends the PassengerVehicle class
+ * and provides the concrete inheritance implementation of the Car.
+ *
+ * @see PassengerVehicle PassengerVehicle
+ * @see Vehicle Vehicle
+ */
+
+@Data
+@NoArgsConstructor
+@EqualsAndHashCode(callSuper = true)
+@Entity
+@DiscriminatorValue(value = "CAR")
+public class Car extends PassengerVehicle {
+
+  private int engineCapacity;
+
+  public Car(String manufacturer, String model, int noOfPassengers, int engineCapacity) {
+    super(manufacturer, model, noOfPassengers);
+    this.engineCapacity = engineCapacity;
+  }
+
+  // Overridden the toString method to specify the Vehicle object
+  @Override
+  public String toString() {
+    return "Car{"
+            + super.toString()
+            + '}';
+  }
+
+}

single-table-inheritance/src/main/java/com/iluwatar/entity/Freighter.java

@@ -0,0 +1,41 @@
+package com.iluwatar.entity;
+
+import javax.persistence.DiscriminatorValue;
+import javax.persistence.Entity;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+import lombok.NoArgsConstructor;
+
+/**
+ * A class that extends the TransportVehicle class
+ * and provides the concrete inheritance implementation of the Car.
+ *
+ * @see TransportVehicle TransportVehicle
+ * @see Vehicle Vehicle
+ */
+@Data
+@NoArgsConstructor
+@EqualsAndHashCode(callSuper = true)
+@Entity
+@DiscriminatorValue(value = "FREIGHTER")
+public class Freighter extends TransportVehicle {
+
+  private double flightLength;
+
+  public Freighter(String manufacturer, String model, int loadCapacity, double flightLength) {
+    super(manufacturer, model, loadCapacity);
+    this.flightLength = flightLength;
+  }
+
+  // Overridden the toString method to specify the Vehicle object
+  @Override
+  public String toString() {
+    return "Freighter{ "
+            + super.toString()
+            + " ,"
+            + "flightLength="
+            + flightLength
+            + '}';
+  }
+
+}

single-table-inheritance/src/main/java/com/iluwatar/entity/PassengerVehicle.java

@@ -0,0 +1,30 @@
+package com.iluwatar.entity;
+
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+import lombok.NoArgsConstructor;
+
+/**
+ * An abstract class that extends the Vehicle class
+ * and provides properties for the Passenger type of Vehicles.
+ *
+ * @see Vehicle
+ */
+@Data
+@NoArgsConstructor
+@EqualsAndHashCode(callSuper = true)
+public abstract class PassengerVehicle extends Vehicle {
+
+  private int noOfPassengers;
+
+  protected PassengerVehicle(String manufacturer, String model, int noOfPassengers) {
+    super(manufacturer, model);
+    this.noOfPassengers = noOfPassengers;
+  }
+
+  @Override
+  public String toString() {
+    return super.toString();
+  }
+
+}

single-table-inheritance/src/main/java/com/iluwatar/entity/Train.java

@@ -0,0 +1,38 @@
+package com.iluwatar.entity;
+
+import javax.persistence.DiscriminatorValue;
+import javax.persistence.Entity;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+import lombok.NoArgsConstructor;
+
+/**
+ * A class that extends the PassengerVehicle class
+ * and provides the concrete inheritance implementation of the Car.
+ *
+ * @see PassengerVehicle PassengerVehicle
+ * @see Vehicle Vehicle
+ */
+@Data
+@NoArgsConstructor
+@EqualsAndHashCode(callSuper = true)
+@Entity
+@DiscriminatorValue(value = "TRAIN")
+public class Train extends PassengerVehicle {
+
+  private int noOfCarriages;
+
+  public Train(String manufacturer, String model, int noOfPassengers, int noOfCarriages) {
+    super(manufacturer, model, noOfPassengers);
+    this.noOfCarriages = noOfCarriages;
+  }
+
+  // Overridden the toString method to specify the Vehicle object
+  @Override
+  public String toString() {
+    return "Train{"
+            + super.toString()
+            + '}';
+  }
+
+}

single-table-inheritance/src/main/java/com/iluwatar/entity/TransportVehicle.java

@@ -0,0 +1,25 @@
+package com.iluwatar.entity;
+
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+import lombok.NoArgsConstructor;
+
+/**
+ * An abstract class that extends the Vehicle class
+ * and provides properties for the Transport type of Vehicles.
+ *
+ * @see Vehicle
+ */
+@Data
+@NoArgsConstructor
+@EqualsAndHashCode(callSuper = true)
+public abstract class TransportVehicle extends Vehicle {
+
+  private int loadCapacity;
+
+  protected TransportVehicle(String manufacturer, String model, int loadCapacity) {
+    super(manufacturer, model);
+    this.loadCapacity = loadCapacity;
+  }
+
+}

single-table-inheritance/src/main/java/com/iluwatar/entity/Truck.java

@@ -0,0 +1,38 @@
+package com.iluwatar.entity;
+
+import javax.persistence.DiscriminatorValue;
+import javax.persistence.Entity;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+
+/**
+ * A class that extends the PassengerVehicle class
+ * and provides the concrete inheritance implementation of the Car.
+ *
+ * @see TransportVehicle TransportVehicle
+ * @see Vehicle Vehicle
+ */
+@Data
+@NoArgsConstructor
+@Entity
+@DiscriminatorValue(value = "TRUCK")
+public class Truck extends TransportVehicle {
+
+  private int towingCapacity;
+
+  public Truck(String manufacturer, String model, int loadCapacity, int towingCapacity) {
+    super(manufacturer, model, loadCapacity);
+    this.towingCapacity = towingCapacity;
+  }
+
+  // Overridden the toString method to specify the Vehicle object
+  @Override
+  public String toString() {
+    return "Truck{ "
+            + super.toString()
+            + ", "
+            + "towingCapacity="
+            + towingCapacity
+            + '}';
+  }
+}

single-table-inheritance/src/main/java/com/iluwatar/entity/Vehicle.java

@@ -0,0 +1,54 @@
+package com.iluwatar.entity;
+
+import javax.persistence.DiscriminatorColumn;
+import javax.persistence.Entity;
+import javax.persistence.GeneratedValue;
+import javax.persistence.GenerationType;
+import javax.persistence.Id;
+import javax.persistence.Inheritance;
+import javax.persistence.InheritanceType;
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+import lombok.NoArgsConstructor;
+
+/**
+ * An abstract class that is the root of the Vehicle Inheritance hierarchy
+ * and basic provides properties for all the vehicles.
+ */
+@Data
+@NoArgsConstructor
+@AllArgsConstructor
+@EqualsAndHashCode
+@Entity
+@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
+@DiscriminatorColumn(name = "VEHICLE_TYPE")
+public abstract class Vehicle {
+
+  @Id
+  @GeneratedValue(strategy = GenerationType.IDENTITY)
+  private int vehicleId;
+
+  private String manufacturer;
+
+  private String model;
+
+  protected Vehicle(String manufacturer, String model) {
+    this.manufacturer = manufacturer;
+    this.model = model;
+  }
+
+  @Override
+  public String toString() {
+    return "Vehicle{"
+            + "vehicleId="
+            + vehicleId
+            + ", manufacturer='"
+            + manufacturer
+            + '\''
+            + ", model='"
+            + model
+            + '}';
+  }
+
+}

single-table-inheritance/src/main/java/com/iluwatar/repository/VehicleRepository.java

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+package com.iluwatar.repository;
+
+import com.iluwatar.entity.Vehicle;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.stereotype.Repository;
+
+/**
+ * A repository that is extending the JPA Repository
+ * to provide the default Spring DATA JPA methods for the Vehicle class.
+ */
+@Repository
+public interface VehicleRepository extends JpaRepository<Vehicle, Integer> {
+
+}

single-table-inheritance/src/main/java/com/iluwatar/service/VehicleService.java

@@ -0,0 +1,71 @@
+package com.iluwatar.service;
+
+import com.iluwatar.entity.Vehicle;
+import com.iluwatar.repository.VehicleRepository;
+import java.util.List;
+import lombok.AllArgsConstructor;
+import org.springframework.stereotype.Service;
+
+/**
+ * A service class that is used to provide the business logic
+ * for the Vehicle class and connect to the database to
+ * perform the CRUD operations on the root Vehicle class.
+ *
+ * @see Vehicle
+ */
+@Service
+@AllArgsConstructor
+public class VehicleService {
+
+  private final VehicleRepository vehicleRepository;
+
+  /**
+   * A method to save all the vehicles to the database.
+   *
+   * @param vehicle Vehicle bbject
+   * @see Vehicle
+   */
+  public Vehicle saveVehicle(Vehicle vehicle) {
+    return vehicleRepository.save(vehicle);
+  }
+
+  /**
+   * A method to get a specific vehicle from vehicle id.
+   *
+   * @param vehicleId Vehicle Id
+   * @see Vehicle
+   */
+  public Vehicle getVehicle(int vehicleId) {
+    return vehicleRepository.findById(vehicleId).orElse(null);
+  }
+
+  /**
+   * A method to get all the vehicles saved in the database.
+   *
+   * @see Vehicle
+   */
+  public List<Vehicle> getAllVehicles() {
+    return vehicleRepository.findAll();
+  }
+
+  /**
+   * A method to update a vehicle in the database.
+   *
+   * @param vehicle Vehicle object
+   * @see Vehicle
+   */
+  public Vehicle updateVehicle(Vehicle vehicle) {
+    return vehicleRepository.save(vehicle);
+  }
+
+  /**
+   * A method to save all the vehicles to the database.
+   *
+   * @param vehicle Vehicle object
+   * @see Vehicle
+   */
+  public void deleteVehicle(Vehicle vehicle) {
+    vehicleRepository.delete(vehicle);
+  }
+
+}