what causes this error? When constructing an instance of class 'circular', the constructor must preserve the class of the returned object.

I'm an old Java programmer (actually I program in many languages). I am trying to create some simple classes but get this error (question line). I am enclosing the code that is causing the error when I invoke it.

classdef circular
    %This creates a curve based on a previous curve, often just a straight
    %line.  The parameters are:
    %
    % n: the number of points in the obj.curve - deduced from obj.curve
    % m: the number of twists to this obj.curve
    % t: the index into the unit circle
    % obj.curve: matrix of points in R3 - the obj.curve itself is the first 3
    % columns
    % N: the list of normal vectors to the obj.curve in columns 4 to 6
    % ON1: list of unit vectors orthogonal to N and ON2 in columns 7 to 9
    % ON2: list of unit vectors orthogonal to N and ON1 in columns 10 to 12
    % obj.ocurve: output: a matrix of points in R3 - same structure as obj.curve
    %   Detailed explanation goes here

      properties
          n = 0;
          m = 0;
          r = 0
          t = 1;
          curve = zeros(1,12);
          ocurve = zeros(1,12);
      end
      %
      methods
          function obj = circular(j, rad, initialcurve)
              obj.m = j;
              obj.curve = initialcurve;
              v = size(obj.curve);
              obj.n = v(1);
              inc = 2 * pi / obj.n;
              obj.t = 0:inc:2*pi - inc;
              obj.r = rad;
              obj.ocurve = zeros(obj.n,12);
              %
              for k = 1 : obj.n
                  % compute the obj.curve
                  ct = obj.r*cos(obj.m * obj.t(k));
                  st = obj.r*sin(obj.m * obj.t(k));
                  oc1 = obj.curve(k,7)*ct + obj.curve(k,10)*st;
                  oc2 = obj.curve(k,8)*ct + obj.curve(k,11)*st;
                  oc3 = obj.curve(k,9)*ct + obj.curve(k,12)*st;
                  e1 = obj.curve(k, 7:9);
                  e2 = obj.curve(k, 10:12);
                  p = obj.r*cos(obj.m * obj.t(k))*e1 + obj.r*sin(obj.m * obj.t(k))*e2 + obj.curve(k, 1:3);
                  obj.ocurve(k, 1:3) = p;
                  mg = sqrt(oc1^2 + oc2^2 + oc3^2);
                  %
                  % compute the normals
                  nc = obj.m * obj.r * (cos(obj.m * obj.t(k)) * e2 - sin(obj.m * obj.t(k))*e1);
                  pt = obj.curve(k, 4:6);
                  obj.ocurve(k, 4:6) = pt + nc;
                  % compute orthogonal vector 1: circle displacement at t(k)
                  obj.ocurve(k,7) = oc1 / mg;
                  obj.ocurve(k,8) = oc2 /mg;
                  obj.ocurve(k,9) = oc3/mg;
                  % compute orthogonal vector 2 as C X N / || C X N ||
                  ox = obj.ocurve(k,2) * obj.ocurve(k,6) - obj.ocurve(k,3) * obj.ocurve(k,5);
                  oy = obj.ocurve(k,3) * obj.ocurve(k,4) - obj.ocurve(k,1) * obj.ocurve(k,6);
                  oz = obj.ocurve(k,1) * obj.ocurve(k,5) - obj.ocurve(k,2) * obj.ocurve(k,4);
                  mg = sqrt(ox^2 + oy^2 + oz^2);   
                  obj.ocurve(k,10) = ox / mg;
                  obj.ocurve(k,11) = oy / mg;
                  obj.ocurve(k,12) = oz / mg;
              end
              obj = obj.ocurve;
          end
          %
          function a = mag(x,y,z)
              a = sqrt(x^2 + y^2 + z^2);
          end
      end
  end

[per isakson]

What is the intent of

obj = obj.ocurve;

In this line a double vector is assigned to obj, which violates "the constructor must preserve the class of the returned object"

[Ivan Handler up]

I figured out the problem. This is a rather clunky implementation of OOP, but I am starting to get the hang of it.