i'm a beginner in Dart programming and i'm a bit confused when it comes to making an instance of a class,
suppose we have a class named Student
what is the difference between these two:
Student student;
and
Student student = new Student();
In Student student; you are just declaring a field that you can later store a Student object in. You aren't creating an actual object here.
At Student student = new Student(); you are creating an Student object which is stored in student. You now have an instance of the Student class that you can use methods on by calling for example student.study().
In dart (as of Dart 2), unlike Java, you can also omit the "new" keyword.
Which means you can write like this instead: Student student = Student();
Example of using both your provided rows would look like this;
Student student;
student = Student();
Which however can just be writted like this: Student student = Student();
Related
I'd like to add a derived list to a DTO/model object when mapping from an entity. For example, I have an entity class named Company that has a list of Employees. I'd like for the Company Model object to contain the list of Employees as well as a list of Employees who are Managers. (This is not my actual class names, but this type of thing is what I'm trying to do). So, I want the Company DTO to have a list of Employees and a list of Managers where the Managers is a subset of Employees that is derived by filtering the list of Employees.
I tried using a mapping expression and a default method. However, I need to use the EmployeeMapper and it seems that I don't have access to the mapper.
Here's what I tried...
#Mapper(componentModel = "spring", uses = {EmployeeMapper.class})
public interface CompanyMapper extends IEntityMapper<CompanyModel, Entity> {
#Mapping(expression = "java(deriveManagers(model))", target = "managers")
EntityModel toModel(Company company);
default List<EmployeeModel> deriveManagers(Company company) {
List<EmployeeModel> managers = new ArrayList<EmployeeModel>();
company.getEmployees().forEach(nextEmployee -> {
if (nextEmployee.getJobTitle().equals(JobTitle.MANAGER)) {
managers.add(this.EmployeeMapper.toModel(nextEmployee);
}
});
return managers;
}
}
I decided to do this processing in the DTO/Model object rather than in the Mapper. The list can be derived as a subset of the EmployeeModel objects in the CompanyModel object. Therefore, I think that this is the right place to derive this list. It is not really a mapping from Entity to Model object as much as it is a derived attribute in the Model object.
Person[] p = new Person[5];
p[0] = new Student();
p[1] = new Student();
...
Arrays.sort(p);
Here Person is the parent class and Student is the child class. There is an overridden compareTo() method both in Person class and Student class. I want to sort Person array based on the method in Person class not in Student class. But JAVA's dynamic binding calling the method in Student class as Student is the actual type. How to solve it?
I have a GridView which needs to page and sort data which comes from a collection of Customer objects.
Unfortunately my customer information is stored separately...the customer information is stored as a Customer ID in my database, and the Customer Name in a separate DLL.
I retrieve the ID from the database using Entity Framework, and the name from the external DLL through a partial class.
I am getting the ID from my database as follows:
public class DAL
{
public IEnumberable<Customer> GetCustomers()
{
Entities entities = new Entities();
var customers = (from c in entities.Customers
select c);
//CustomerID is a field in the Customer table
return customers;
}
}
I have then created a partial class, which retrieves the data from the DLL:
public partial class Customer
{
private string name;
public string Name
{
if (name==null)
{
DLLManager manager = new DLLManager();
name= manager.GetName(CustomerID);
}
return name;
}
}
In my business layer I can then call something like:
public class BLL
{
public List<Customer> GetCustomers()
{
DAL customersDAL = new DAL();
var customers = customersDAL.GetCustomers();
return customers.ToList();
}
}
...and this gives me a collection of Customers with ID and Name.
My problem is that I wish to page and sort by Customer Name, which as we have seen, is populated from a DLL. This means I cannot page and sort in the database, which is my preferred solution. I am therefore assuming I am going to have to call of the database records into memory, and perform paging and sorting at this level.
My question is - what is the best way to page and sort an in-memory collection. Can I do this with my List in the BLL above? I assume the List would then need to be stored in Session.
I am interested in people's thoughts on the best way to page and sort a field that does not come from the database in an Entity Framework scenario.
Very grateful for any help!
Mart
p.s. This question is a development of this post here:
GridView sorting and paging Entity Framework with calculated field
The only difference here is that I am now using a partial class, and hopefully this post is a little clearer.
Yes, you can page and sort within you list in the BLL. As long as its fast enough I wouldn't care to much about caching something in the session. An other way would be to extend your database with the data from you DLL.
I posted this question slightly differently on a different forum, and got the following solution.
Basically I return the data as an IQueryable from the DAL which has already been forced to execute using ToList(). This means that I am running my sorting and paging against an object which consists of data from the DB and DLL. This also allows Scott's dynamic sorting to take place.
The BLL then performs OrderBy(), Skip() and Take() on the returned IQueryable and then returns this as a List to my GridView.
It works fine, but I am slightly bemused that we are perfoming IQueryable to List to IQueryable to List again.
1) Get the results from the database as an IQueryable:
public class DAL
{
public IQueryable<Customer> GetCustomers()
{
Entities entities = new Entities();
var customers = (from c in entities.Customers
select c);
//CustomerID is a field in the Customer table
return customers.ToList().AsQueryable();
}
}
2) Pull the results into my business layer:
public class BLL
{
public List<Customer> GetCustomers(intint startRowIndex, int maximumRows, string sortParameter)
{
DAL customersDAL = new DAL();
return customersDAL.GetCustomers().OrderBy(sortParameter).Skip(startRowIndex).Take(maximumRows).ToList();
}
}
Here is the link to the other thread.
http://forums.asp.net/p/1976270/5655727.aspx?Paging+and+sorting+Entity+Framework+on+a+field+from+Partial+Class
Hope this helps others!
I am trying to create a classic asp/vbscript class that will allow me to easily manage a small number of employees (30-40) along with some metrics associated with those employees, about 14 metrics each. I've done some tutorials online and can't quite get how I should proceed. What I have so far is below. It's not much, basically I think I can only add the employees to a dictionary in the class, but I don't know where to go from here.
class iagent
private di_agents
private ar_metrics
private pri_agent_counter
Public function add_agent(uuid)
di_agents.Add uuid, pri_agent_counter
pri_agent_counter=pri_agent_counter+1
end function
private sub Class_initialize
pri_agent_counter=1
dim ar_metrics(14, 5)
set di_agents = CreateObject("Scripting.Dictionary")
end sub
end class
The class you have is just a wrapper around a dictionary. Are you talking about creating a class that represents an employee?
Class Employee
Public Name
Public Age
Public Phone
'other properties
End Class
Then you can instantiate Employee like this and set your properties
Set e = New Employee
e.Name = "Some name"
You could then store your instances of Employee in a dictionary, perhaps paired with an ID:
Set d = CreateObject("Scripting.Dictionary")
Call d.Add(uuid, e)
However, you're better off using a database for this and using ASP/VBS to extract records... Unless this is just an exercise
I have the following scenario: A Doctor can have multiple Companions. A Companion can also have multiple Doctors. Here are my classses (minus the context):
Public Class Doctor
Public Property DoctorId As Integer
Public Property Regeration As Integer
Public Property Name As String
Public Property Actor As String
Public Property Companions As List(Of Companion)
End Class
Public Class Companion
Public Property CompanionId As Integer
Public Property Name As String
Public Property Actor As String
Public Property Doctors As List(Of Doctor)
End Class
Public Class DoctorViewModel
Public Property DoctorId As Integer
Public Property Actor As String
Public Property Companions As List(Of CompanionViewModel)
End Class
Public Class CompanionViewModel
Public Property Actor As String
End Class
I'm trying to fetch a singular Doctor with a list of companions who have travelled with him. This I can do quite simply, but I'm trying to shape the query to get only a few columns and not the entire entity. He is my bungled query - the X's are what I can't figure out.
Dim query = From d In context.Doctors
From c In context.Companions
Select New DoctorViewModel With {
.Actor = d.Actor,
.DoctorId = d.DoctorId,
.Companions = XXXXXXX}
EDIT: If I query:
(From d In Tardis.Doctors
Where d.Actor = "Tom Baker"
Select New DoctorViewModel With {.Actor = d.Actor, .Companions = d.Companions.Select(Function(c) New CompanionViewModel With {.Actor = c.Actor})}).SingleOrDefault
I get:
"Unable to cast the type 'System.Collections.Generic.IEnumerable1' to
type 'System.Collections.Generic.List1'. LINQ to Entities only
supports casting Entity Data Model primitive types."
Would it be considered nasty to ditch the ViewModel classes in the query and just get the stuff as an anonymous type, then pass this to a constuctor in a ViewModel (my models have a whack of functions that are needed) and fill the class like this?
It probably would be not only okay but much more readable (aka maintainable) as well. Especially since the query wouldn't return an anonymous type, it would return an IQueryable
Solved! I've spent days on this! Trick was to change the List inside the Doctor ViewModel to IEnumerable(Of CompanionViewModel)