@Override
      public Object apply(Object m, Object om) {
        return joinFn.apply(om, m);
      }
Creates a new {@link OperatorImpl} instance for the provided {@link OperatorSpec}.

@param prevOperatorSpec the original {@link OperatorSpec} that produces output for {@code operatorSpec} from {@link OperatorSpecGraph}
@param operatorSpec  the original {@link OperatorSpec} from {@link OperatorSpecGraph}
@param context  the {@link Context} required to instantiate operators
@return  the {@link OperatorImpl} implementation instance
samza-core/src/main/java/org/apache/samza/operators/impl/OperatorImplGraph.java