How to transform List<String> to Map<String,String> 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;
  }});

As of 7/26/2012, Guava master contains two new ways to do this. They should be in release 14.0.

Maps.asMap(Set<K>, Function<? super K, V>) (and two overloads for SortedSet and NavigableSet) allows you to view a Set plus a Function as a Map where the value for each key in the set is the result of applying the function to that key. The result is a view, so it doesn't copy the input set and the Map result will change as the set does and vice versa.

Maps.toMap(Iterable<K>, Function<? super K, V>) takes an Iterable and eagerly converts it to an ImmutableMap where the distinct elements of the iterable are the keys and the values are the results of applying the function to each key.

EDIT: It's entirely possible that Sean's right and I misunderstood the question.

If the original list is meant to be keys, then it sounds like you might be able to just use a computing map, via MapMaker.makeComputingMap, and ignore the input list to start with. EDIT: As noted in comments, this is now deprecated and deleted in Guava 15.0. Have a look at CacheBuilder instead.

On the other hand, that also doesn't give you a map which will return null if you ask it for a value corresponding to a key which wasn't in the list to start with. It also won't give you In other words, this may well not be appropriate, but it's worth consideration, depending on what you're trying to do with it. :)

I'll leave this answer here unless you comment that neither approach here is useful to you, in which case I'll delete it.

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Original answer

Using Guava you can do this pretty easily with Maps.uniqueIndex:

Map<String, String> map = Maps.uniqueIndex(list, keyProjection);

(I mentioned Guava specifically as opposed to Google collections, as I haven't checked whether the older Google collections repository includes Maps.uniqueIndex.)