Status: New
Owner: liuj...@google.com
Labels: Type-Defect Priority-Medium
New issue 460 by dav.co...@gmail.com: Add isDefault() to Java Message
interface as a fast means to verify whether it is a default instance
http://code.google.com/p/protobuf/issues/detail?id=460
What steps will reproduce the problem?
1. The only way currently to check whether a message is the default is to
use (1) message == message.getDefaultInstanceForType(), or worse, via (2)
Message#equals(). The (1) works okay for a concrete GeneratedMessage
because it is a singleton (implementation detail), however, in
DynamicMessage (1) won't work (not a singleton), and equals() is relatively
expensive.
What is the expected output? What do you see instead?
message == message.getDefaultInstanceForType() doesn't work for
DynamicMessage, and very implementation dependent. On the other hand, the
equals() is relatively expensive as a means for a simple isDefault() check.
The proposed "boolean isDefault()" method on Message (possibly deeper, in
the MessageLite and/or MessageBuilder) interface strictly defines this
capability, which is VERY easy to implement in concrete classes, and also
in DynamicMessage and the likes (the "empty" message should return
isDefault()==true).
Alternatively, "boolean isDefault()" could be declared as an abstract
method of AbstractMessage. This would avoid messing with the base
interfaces, won't break any external code at the expense of implicit
understanding that this method should be available via appropriate casting,
for example, ((AbstractMessage) message).isDefault() for instances of
AbstractMessage, and ((T) message).isDefault() for any non-subclass of
AbstractMessage.
Obviously, the former alternative is more elegant and logically proper, but
will break all non-subclasses of AbstractMessage. To mitigate this a bit, I
propose:
1. Implement a fail-safe AbstractMessage#isDefault():
@Override
public boolean isDefault() {
return this.equals(getDefaultInstanceForType());
}
2. Implement GeneratedMessage#isDefault():
@Override
public boolean isDefault() {
return this == getDefaultInstanceForType();
}
2. Implement DynamicMessage#isDefault():
@Override
public boolean isDefault() {
return (fields == FieldSet.<FieldDescriptor>emptySet())
&& (unknownFields == UnknownFieldSet.getDefaultInstance());
}
3. All GeneratedMessage and DynamicMessage will work out of the box
efficiently, and only direct AbstractMessage subclasses will have slow
performance of isDefault(), unless they override it.
What version of the product are you using? On what operating system?
protobuf 1.4.1, 1.5RC1 with Java 1.5 - 1.7 on Windows and Linux
Please provide any additional information below.
In Java 1.5RC1 the Message interface already enhanced with
getParserForType(), so this is the good time to add the proposed
isDefault() method and make small changes described above.
Thanks in advance,
Daid
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