fixtureDef.filter doesn't work

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What steps will reproduce the problem?
FixtureDef fixtureDef = new FixtureDef();
fixtureDef.density = 1;
fixtureDef.friction = 0;
fixtureDef.filter.groupIndex = -1;
fixtureDef.restitution = 1;
createDynamicBox(fixtureDef);

What is the expected output?
my game sprites don't collide with each other

What do you see instead?
they still collide

What version of Rokon are you using?
new trunk

On which version of Android are you experiencing this?
1.5

Please provide any additional information below.
It seems that fixtureDef.filter.groupIndex = -1, I have also used c++ box2d 
before, and I confrim that negative means never collide.
Then I have tried to set categoryBits and maskBits, it doesn't work either.

Original issue reported on code.google.com by [email protected] on 25 Aug 2010 at 9:53