Generic parsing class

[Miguel Muñoz]

Comrades,

I have a class I'd like to submit to the protocol buffer code base.
It's a class called GeneratedMessageParser, and it parses messages
from generic parameters. Here's why:

I have a glass called MessageTranceiver<M extends Message>. This class
needs to call the parseFrom() method of whatever message we specify
when we instantiate the class. But if you have this instance:

M message;

you can't say this:

message.parseFrom(buf);

because you don't have an instance of message, and you can't say

M.parseFrom(buf) either, because it's a static message. So I wrote
my parser class, which lets me say this:

GeneratedMessageParser<Requests.LoginRequest> parser
= new GeneratedMessageParser(Requests.LoginRequest.class);
...
message = parser.parseFrom(buf);

I've used protocol buffers on two projects, and this parser class has
been useful to both, so others may find it useful as well. It has
static methods that parallel all the standard parse methods, with
exactly the same signatures, including the declared exceptions. If
you'd like to see it, where do I send it?

[Kenton Varda]

Try this:

M prototype = M.getDefaultInstance();

// later on

M instance = prototype.newBuilderForType().mergeFrom(data).build();

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[Miguel Muñoz]

Kenton,

  You try it!

  You can't say M.getDefaultInstanceForType() for the same reason you can't say M.parseFrom(buffer). You can't call any of the methods from M, because it's type information has been erased at runtime. That's why I wrote my parser class.

-- Miguel

-------------------------------------------

Miguel Muñoz

SwingG...@yahoo.com

323/225-7285

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  -- Galileo

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[Kenton Varda]

Sorry, I should have been a bit clearer.

You've defined a new interface called GeneratedMessageParser, and when someone instantiates a generic class which needs to parse things, they need to pass in an instance of your interface to do the parsing.

I'm saying that they can just pass in the message type's default instance instead.  The default instance is explicitly intended to serve the purpose that your GeneratedMessageParser serves (among other things).

class Frobber<M extends Message> {

  M prototype;

  public Frobber(M prototype) {

    this.prototype = prototype;

  }

  public M parse(ByteString data) {

    return prototype.newBuilderForType().mergeFrom(data).build();

  }

}

// some code that uses this

Frobber<MyType> frobber = new Frobber<MyType>(MyType.getDefaultInstance());

MyType result = frobber.parse(someData);