code: public ScanSpec withValueMap(Map<String, Object> valueMap) {
          if (valueMap == null)
              this.valueMap = null;
          else
              this.valueMap = Collections.unmodifiableMap(new LinkedHashMap<String, Object>(valueMap));
          return this;
      }
docs: Applicable only when an expression has been specified. Used to
  specify the actual values for the attribute-value placeholders.
  
  @see ScanRequest#withExpressionAttributeValues(Map)