Guava Library & Examples com.google.common.collect.Multimaps allow multiple values to be stored for every key
Guava Library & Examples com.google.common.collect.BiMap one-to-one bidirectional relationship between key and value of the Map
Guava Library & Examples com.google.common.collect.Constraints Similar to preconditions in a way that they can restri ct what values are added to a collection
Guava Pros & Cons Guava targets Java 5 - Greatly benefits from the Java 5 features: generics Guava is very well designed / documented The code is useful patterns to make the API more readable, secure, thread-safe.. Functional programming support
Guava Pros & Cons Its not a sufficient replacement for Apache Commons, in particular commons-codec If youre in an environment requiring Java 1.3 or 1.4, youre out of luck.