Choosing the right collection class has an impact on the performance of your application. In a previous post I gave a run down of Big O Notation. A hash map look up will have better performance generally than other types of collections when you can look up the data based on a key - O(1) since the underlying data structure is an array - and is useful when the order of the data is not important.
This article has a good run down of Java Maps including how they work and how to choose the right one for your application:
Java Map Collection Classes
In general the concept of optimizing for performance after the application has been built - by actually profiling the application and then making specific implementation decisions - is something I liked to hear. You need to understand the basics to implement a solid architecture but true optimization happens after you can measure the performance, taking all application variables into account from network to hardware to various software components and data distribution, and determining where your bottlenecks lie once you can actually measure the results of your particular application.
