TObjectList E2003 Undeclared identifier TObjectList<> - delphi

Question

I've been introduced to TObjectList, and I would like to make use of it, except that I can't seem to get even the sample code from Embarcadero's web site to work for me. Here is my code:
unit Test03Unit1;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;
type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
type
{ Declare a new object type. }
TNewObject = class
private
FName: String;
public
constructor Create(const AName: String);
destructor Destroy(); override;
end;
{ TNewObject }
var
Form1: TForm1;
implementation
{$R *.dfm}
constructor TNewObject.Create(const AName: String);
begin
FName := AName;
end;
destructor TNewObject.Destroy;
begin
{ Show a message whenever an object is destroyed. }
MessageDlg('Object "' + FName + '" was destroyed!', mtInformation, [mbOK], 0);
inherited;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
List: TObjectList<TNewObject>;
Obj: TNewObject;
begin
{ Create a new List. }
{ The OwnsObjects property is set by default to true -- the list will free the owned objects automatically. }
List := TObjectList<TNewObject>.Create();
{ Add some items to the List. }
List.Add(TNewObject.Create('One'));
List.Add(TNewObject.Create('Two'));
{ Add a new item, but keep the reference. }
Obj := TNewObject.Create('Three');
List.Add(Obj);
{
Remove an instance of the TNewObject class. Destructor
is called for the owned objects, because you have set the OwnsObjects
to true.
}
List.Delete(0);
List.Extract(Obj);
{ Destroy the List completely--more message boxes will be shown. }
List.Free;
end;
end.
When trying to compile this, I get the error [DCC Error] Test03Unit1.pas(51): E2003 Undeclared identifier: 'TObjectList<>'. Line 51 is the line which says:
List: TObjectList<TNewObject>;
I have never seen < > used in the Pascal language before, so this is completely new territory for me. Doing a google search for "Delphi and < >" doesn't seem to tell me what I need to know about this. From other examples I can find on the internet it seems to be the correct way to use it.
Using Delphi XE2.
What am I doing wrong?

Answer 1

You must add System.Generics.Collections to your uses clause. That is the unit which declares TObjectList<T>.
I've added documentation links to the answer. When you cannot find a class, look for it in the documentation. That will tell you which unit you need to use.
As well as TObjectList<T> there is TList<T>. It makes sense to use TObjectList<T> when you want the list to own its members. Otherwise, there is no benefit in using TObjectList<T> and you may as well use TList<T>. As well as the built-in Delphi generic containers, there are many excellent third-party containers that are often superior. For instance, the Delphi Spring Framework has a fine collection of containers.