code: public QueryBuilder withinCenter(final double x, final double y, final double radius) {
          addOperand(QueryOperators.WITHIN,
                     new BasicDBObject(QueryOperators.CENTER, asList(asList(x, y), radius)));
          return this;
      }
docs: Equivalent of the $within operand, used for geospatial operation
  
  @param x      x coordinate
  @param y      y coordinate
  @param radius radius
  @return {@code this}