code: @Nullable
    @Override
    public T get(String s) {
      checkNotNull(s);
      checkArgument(!s.isEmpty());
      for (T t : supplier.get()) {
        if (t.id().equals(s)) {
          return t;
        }
      }
      return null;
    }
docs: Searches for the given id in the supplier.
  
  @param s the id.
  @return the found resource or null.
  @throws NullPointerException if the given id is null.
  @throws IllegalArgumentException if the given id is empty.