From http://stackoverflow.com/questions/10975913/how-to-make-a-new-list-with-a-property-of-an-object-which-is-in-another-list

GuavaFunctionSample.java

private enum StudentToId implements Function<Student, Integer> {
        INSTANCE;

        @Override
        public Integer apply(Student input) {
            return input.getId();
        }
    }