function Field(point, mass) {
   this.position   = point;
   this.size       = 0;
   this.setMass(mass);   
}

Field.drawColor = "rgb(0,0,255)";
Field.drawColor2 = "rgb(0,0,0)";

Field.prototype.setMass = function(mass) {
   this.mass = mass;
   this.drawColor = mass < 0 ? "#900" : "#090";
   return this;
}
Field.prototype.moveTo = function(point) {
   this.position = point;
}

Field.prototype.toString = function() {
   var coreAttributes = [
      this.position.toString(),
      this.mass
   ];
   return 'F' + coreAttributes.join(':');
}
Field.fromString = function(string) {
   var parts = string.substr(1).split(':');
   var field = new Field(Point.fromString(parts.shift()),parseInt(parts.shift()));
   return field;
}