@Override
      public Collection<OM> apply(M message) {
        return new ArrayList<OM>() {
          {
            OM r = mapFn.apply(message);
            if (r != null) {
              this.add(r);
            }
          }
        };
      }
The spec for an operator that transforms each input message to a single output message.

@param <M> type of input message
@param <OM> type of output messages
samza-core/src/main/java/org/apache/samza/operators/spec/MapOperatorSpec.java