A Coding for Interviews [1] group member mentioned that reading through the Java collections library [2] was the most valuable step he took while preparing for his Google interviews.

In addition to getting a better understanding the standard data structures, hearing a candidate say "well the Java collections library uses this strategy..." is a strong positive signal.

[1]: http://codingforinterviews.com

[2]: He suggested reading the libraries here: http://www.docjar.com/html/api/java/util/HashMap.java.html