MrtStation: rename parameter.
Node: make name field final.
Node: rename getEdges method to getAllEdges and change its return type to Collection<Edge>.
MrtStation: make station field final.
Edge: rename node field to neighbouringNode.
Graph: prevent an existing node to be replaced by a new node with the same name for now to avoid data inconsistency.
Node, MrtStation: override toString method.
SingaporeMrt: temporary fix for testCase method
Various improvements.