Complexity of method Graph::getNode(String id)

[<simone.davico AT usi.ch>]

Hello everyone,

I was wondering what is the complexity of the method Graph::getNode(String 
id).
What is used to retrieve the node in the graph? Is it implemented as linear
search? Binary Search? Or maybe it uses a Map and allows constant time
retrieval?

This is very critical for the project I'm working on, as I could find myself 
in
the situation of handling a huge quantity of nodes and I have to know if calls
to this method are computationally expensive.

Thank you for your help!

Simone