Efficient way to find if map contains any of the keys from a list/iterable

Surely something like:

for (String candidate : candidates) {
     String result = fields.get(key);
     if (result != null) {
         return result;
     }
}

The above only performs one map lookup per candidate key. It avoids the separate test for presence plus extraction, since extracting a non-existant key will simply give you a null. Note (thanks Slanec) that a null value for a valid key is indistinguishable from a non-existant key for this solution.

I don't quite understand why you're performing the case conversion, btw.

for(String candidate : candidates) {
 if(fields.containsKey(candidate)) {
  return fields.get(candidate)
 }
}

the best way if null values are possibly in map, and if only the first detected key is required.