Why I can't create extension node?

[fankobebryant]

I can't create extension node with my code:
Waypoint point=new Waypoint(34.287683, 121.2898392);
point.setElevation(1000);
HashMap<String,Object> extension=new HashMap<String,Object>();
extension.put("speed", 120);
point.setExtensionData(extension);
xml result:

1000

I don't know why the result is above.
Could you tell me what should I do？