@jfeng523
You don't provide much information here; about all I can do is make guesses about what you're trying to do. Knowing which API call you're making and what you're trying to accomplish would help us better help you.
My first suggestion would be to use a different API call, you don't say which one you're using, but most calls only return one type and you're not going to be able to get the other without changing calls. In the documentation, the bottom of most API calls end with "Returns a xxx" where xxx is an object type.
The second thing I would do is make sure that you have permissions to view the information. This becomes my first guess if you're making a call that says "Returns a User" but you're not getting a User object.
Also, realize those examples in the documentations are examples and you may not get all of the information or you may get more than what's shown depending on parameters that you pass and permissions you have.