преобразование строки json в объект класса pojo в java

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Я создал строку JSON как THS

{"vehicles":[{"vehicle":[{"department":[{"Item":[{"itemId":31,"itemName":"c7"},{"itemId":32,"itemName":"c2"}],"depId":21,"departmentName":"d1"}],"vId":11,"VName":"aaa"},{"department":[{"Item":[{"itemId":33,"itemName":"c3"},{"itemId":34,"itemName":"c4"}],"depId":22,"departmentName":"d2"},{"Item":[{"itemId":36,"itemName":"c1"}],"depId":24,"departmentName":"d3"}],"vId":12,"VName":"bbbb"},{"department":[{"Item":[{"itemId":30,"itemName":"c6"},{"itemId":35,"itemName":"c5"}],"depId":23,"departmentName":"d4"}],"vId":13,"VName":"cccc"},{"department":[{"Item":[{"itemid":37,"itemName":"c8","status":0}],"depId":25,"departmentName":"d5"}],"vId":14,"VName":"ddd"}]}]}

Я попытался преобразовать то же самое с помощью приведенного ниже кода

Gson gsonObj = new Gson();
Vehicles vehicles = gsonObj.fromJson(jsonData, Vehicles.class);

но переменная vehicles показывает нулевое значение. Как я могу преобразовать строку json в объект класса pojo?
мой класс pojo выглядит следующим образом

public class Vehicles {
private List<Vehicle> vehicle;
public List<Vehicle> getVehicle() {
    return vehicle;
}
public void setVehicle(List<Vehicle> vehicle) {
    this.vehicle= vehicle;
}
    }

и

public class Vehicle{
   private Integer vId;
  private String VName;
   private List<Department> department;
  //getters and setters;
   }

и

public class Department{
    private Integer depId;
private String departmentName;
private List<Item> item;
   //getters and setters
   }

и

public class Item{
 private Integer itemId;
 private String itemName;
 //getters and setters
  }

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