code: public void mapField(String sourceField, String targetField, Map<String, String> mapping) {
          TransformationStep step = new TransformationStep();
          step.setTargetField(targetField);
          step.setSourceFields(sourceField);
          step.setOperationParams(mapping);
          step.setOperationName("map");
          steps.add(step);
      }
docs: Adds a map transformation step to the transformation description. The value of the source field is mapped based
  on the mapping to another value which is forwarded to the target field. The values in the models need to be
  working with the values of the mapping (e.g. assigning 'hello' to Integer field will not work).