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From Doug Cutting <cutt...@apache.org>
Subject Re: schema defaults not reflected in generated objects (1.3.2)
Date Mon, 07 Jun 2010 22:30:07 GMT
On 06/07/2010 03:11 PM, Bill de hOra wrote:
> This means writers can't leverage schema defaults, so writers should do
> something like this?
>
> Message message = new Message();
> // no defaults set
> String quux = message
> .getSchema()
> .getField("foo")
> .defaultValue()
> .getTextValue();
> message.foo=new Utf8(quux);
>
> [ignoring that the writer needs to know the schema type]. I suspect
> people will just write in garbage (like empty strings).

No, we don't expect folks to do that.  If a writer never sets a field 
then they might be better off dropping that field from their schema.  If 
the writer only rarely sets it, then a schema which is a union with null 
might be better, making the field optional.  But if the field is usually 
set but it's awkward for the programmer to know whether its set, then 
automatically filling in a default might be a useful feature and the 
default from the schema is probably a good value to use.

Like Philip, I too am +1 for enhancing the SpecificCompiler to set 
default values from the schema in generated code.  The only downside I 
see is perhaps a slight performance loss: if the default value is always 
overwritten then the allocation and setting of it will still be executed 
for each instance.

> Why is it incorrect to not provide defaults when defaults are part of
> the schema author's intention? Or put another way, why is reader/writer
> asymmetry a goal under a given schema?

It's not incorrect nor is asymmetry a goal.  (I don't think Scott meant 
either of those to be the case.)  Java instance default values were not 
the primary purpose of default values in a schema, but they're a fine, 
complementary use of them.

Doug

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