code: public static ValueFilter createValueFilter(String operation, String value) {
          checkArgument(STRING_SUPPORTED_OPERATION.contains(operation), "String value only support operations in "
                  + STRING_SUPPORTED_OPERATION.toString());
          ValueFilter valueFilter = new ValueFilter(operation, SINGLE_QUOTATION + value + SINGLE_QUOTATION);
          return valueFilter;
      }
docs: Create value filter for String type.
  
  @param operation Operation for comparing which only support =, !=, &gt;, &lt;, &gt;= and &lt;=.
  @param value Value for comparing with.
  @return ValueFilter