Java enum method to Delphi

Question

public enum HTTPHeaderKey {
    CACHE_CONTROL("Cache-Control"), CONNECTION("Connection"), TRANSFER_ENCODING("Transfer-Encoding"), HOST("Host"), USER_AGENT("User-Agent"), CONTENT_LENGTH("Content-Length"), CONTENT_TYPE("Content-Type");
    private final String str;

    private HTTPHeaderKey(final String _str) {
        str = _str;
    }

    /** Over ridden toString returns the HTTP/1.1 compatible header */
    public String toString() {
        return str;
    }
};

I am trying to convert this enum to Delphi. I know that how to define enum variables but i have no idea, how can i insert a method in enum?

Or can someone suggest another way to convert this ?

Answer 1

You can get part way there with a record helper, which is available for value types from XE3 onwards. For instance:

{$APPTYPE CONSOLE}

uses
  System.SysUtils, System.TypInfo;

type
  TMyEnum = (enumValue1, enumValue2);

  TMyEnumHelper = record helper for TMyEnum
  public
    function ToString: string;
  end;

function TMyEnumHelper.ToString: string;
begin
  Result := GetEnumName(TypeInfo(TMyEnum), ord(Self));
end;

begin
  Writeln(enumValue1.ToString);
  Writeln(enumValue2.ToString);
end.

This program outputs the following:

enumValue1
enumValue2

Of course you may prefer to do it like this:

{$APPTYPE CONSOLE}

uses
  System.SysUtils, System.TypInfo;

type
  TMyEnum = (enumValue1, enumValue2);

  TMyEnumHelper = record helper for TMyEnum
  private
    const
      EnumNames: array [TMyEnum] of string = ('Friendly name 1', 'Friendly name 2');
  public
    function ToString: string;
  end;

function TMyEnumHelper.ToString: string;
begin
  Result := EnumNames[Self];
end;

begin
  Writeln(enumValue1.ToString);
  Writeln(enumValue2.ToString);
end.

The output here is:

Friendly name 1
Friendly name 2

This would presumably allow you to tackle the fact the Delphi enumerated types don't support the textual naming that is available in Java.

The other method, the constructor HTTPHeaderKey cannot be supported with an enumerated type. The reason being that it requires state, and the only state for a Delphi enumerated type is the enumerated type value itself. You cannot graft on an extra instance variable as is done in the Java code.

All things considered, I don't think an attempt at a literal translation with an enumerated type will work out. I suggest that you translate using either a record or a class, and build the equivalent functionality using the available Delphi language constructs.

Answer 2

Based on Davids answer, this modified version should be able to print the HTTP header names:

type
  THTTPHeaderKey = (hkCACHE_CONTROL, hkCONNECTION, hkTRANSFER_ENCODING);

  TTHTTPHeaderKeyHelper = record helper for THTTPHeaderKey
  private
    const
      EnumNames: array [THTTPHeaderKey] of string = ('Cache-Control', 'Connection', 'Transfer-Encoding');
  public
    function ToString: string;
  end;

  // ToString returns the HTTP/1.1 compatible header
  function TTHTTPHeaderKeyHelper.ToString: string;
  begin
    Result := EnumNames[Self];
  end;

  ...
  begin
    Writeln(hkCACHE_CONTROL.ToString);
    Writeln(hkTRANSFER_ENCODING.ToString);
  end.

should output

Cache-Control
Transfer-Encoding