code: protected IFilter getFilter4Field(final Field _field)
      {
          final IFilter ret;
          switch (_field.getFilter().getType()) {
              case STATUS:
              case PICKLIST:
                  ret = new ListFilter(_field.getId());
                  break;
              case FREETEXT:
              case FORM:
                  ret = new MapFilter(_field.getId());
                  break;
              case CLASSIFICATION:
                  ret = new ClassificationFilter(_field.getId());
                  break;
              case NONE:
              default:
                  ret = new IFilter()
                  {
                      /** The Constant serialVersionUID. */
                      private static final long serialVersionUID = 1L;
  
                      @Override
                      public long getFieldId()
                      {
                          return _field.getId();
                      }
                  };
                  break;
          }
          return ret;
      }
docs: Gets the filter for field.
  
  @param _field the field
  @return the filter for field