CODEWITHSUNDEEP
delphi
user4189838
user4189838

Reputation:

Java final variable in Delphi

I am curious about, what is the equivalent of final tag in delphi :

 public final int stackoverflow;

I use

class var

tag but class var equals static variable. But final is different from them.

Upvotes: 3

Views: 777

Answers (3)

Sir Rufo
Sir Rufo

Reputation: 19106

There is nothing comparable shipped with delphi.

But you can achieve this with a record

unit FinalType;

interface

uses
  System.SysUtils;

type
  Final<T> = record
  private
    FIsInitialized: string;
    FValue: T;
  public
    class operator implicit( const a: T ): Final<T>;
    class operator implicit( const a: Final<T> ): T;
  end;

implementation

{ Final<T> }

class operator Final<T>.implicit( const a: T ): Final<T>;
begin
  if Result.FIsInitialized <> ''
  then
    raise Exception.Create( 'Fehlermeldung' );

  Result.FIsInitialized := '*';
  Result.FValue := a;
end;

class operator Final<T>.implicit( const a: Final<T> ): T;
begin
  if a.FIsInitialized = ''
  then
    Result := default ( T )
  else
    Result := a.FValue;
end;

end.

program FinalCheck;

uses
  FinalType;

var
  MyFinalValue : Final<Integer>;

procedure OutputValue( AValue : Integer );
begin
  WriteLn( 'Value: ', AValue );
end;

begin
  MyFinalValue := 10; // everything is ok
  OutputValue( MyFinalValue );
  MyFinalValue := 12; // <- Exception
  OutputValue( MyFinalValue );
end.

UPDATE

There is one side effect you cannot catch:

program FinalCheck;

uses
  FinalType;

var
  MyFinalValue : Final<Integer>;

procedure SetFinalValue( AValue : Integer );
var
  LNewValue : Final<Integer>;
begin
  LNewValue := AValue;
  MyFinalValue := LNewValue;
end;

begin
  MyFinalValue := 10; // everything is ok
  SetFinalValue( 12 ); // no exception!!!
end.

In most cases I see this as a public ReadOnly field of a class. In Delphi you would use a public readonly property instead, but you have to take care, that the value is not changed from inside the class. The final flag ist just a guard, that you cannot change the value from inside.

This is a class with a final - but it is only final with the comment and if you respect this

TFoo = class
private
  // Please do not change this value once it is set
  FStackoverflow : Integer;
public
  property Stackoverflow : Integer read FStackoverflow;
end;

Or with the record

TFoo = class
public
  Stackoverflow : Final<Integer>;
end;

Upvotes: 1

David Heffernan
David Heffernan

Reputation: 613013

A Java final variable can be initialized once only. This initialization can be made by an initialization statement, or by an assignment statement.

As such there is not equivalent in Delphi. There is nothing that allows you to restrict a variable to being initialized no more than one time. The closest equivalent would be a constant, declared with const. This would restrict you to making the initialization in the const declaration.

Upvotes: 6

Hana Bzh
Hana Bzh

Reputation: 2260

You can use const keyword in Delphi

 Const MAX_LINES = 3;

Note that you could never change the value during the application running

Upvotes: 0

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