How to transform List to Map with Google collections?

Use Maps.uniqueIndex(Iterable, Function) :

Returns an immutable map for which the
Map.values() are the given elements in
the given order, and each key is the
product of invoking a supplied
function on its corresponding value.(from javadoc)

Example:

Map<String,String> mappedRoles = Maps.uniqueIndex(yourList, new Function<String,String>() {
  public String apply(String from) {
    // do stuff here
    return result;
  }});