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/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()
+            + '}';
+  }
+
+}