Class object ownership objective c - objective-c

Question

Being noob in objective-c,I just came across a problem in object ownership in a class that cannot explain the reason.
Suppose that I have a class called Point that describes a point with x,y.
A class called rectangle has an object of type Point that represents its left bottom edge. With following implementation
#implementation Rectangle {
Point *origin;
}
-(Point *) origin {
return origin;
}
-(void) setOrigin: (Point *) pt {
origin = pt;
}
#end
Now I simply instantiate the Rectangle, assign its origin a pointer to object of Point
Rectangle *rectangle2 = [[Rectangle alloc] init];
Point *pt = [[Point alloc] init];
[pt setX:1 andY:2];
rectangle2.origin = pt;
NSLog(#"Point's x= %i, y=%i",pt.x,pt.y); // result: Point's x=1,y=2
NSLog(#"Rectangle's origin x= %i, y=%i",rectangle2.origin.x,rectangle2.origin.y); //Rectangle's origin x=1,y=2
Now if I change the x,y of the object pt(following line of code), the rectangle2 object's origin will change as well, because it does not own its object, instead just points to where pt pointing to
[pt setX:10 andY:12];
NSLog(#"Point's x= %i, y=%i",pt.x,pt.y); // result: Point's x=10,y=12
NSLog(#"Rectangle's origin x= %i, y=%i",rectangle2.origin.x,rectangle2.origin.y); //Rectangle's origin x=10,y=12
This is completely logical, with no issue.
Issue seems to be happening in a similar scenario.
AddressCard class has an object of type NSString,
#implementation AddressCard{
NSString *phone;
}
-(void) setPhone:(NSString *) p {
phone = p;
}
-(NSString *) name {
return name;
}
#end
I instantiate AddressCard, assign an NSString pointer to its phone object.
AddressCard *addressCard1 = [[AddressCard alloc] init];
NSString *phoneObject = #"(111) 111 - 1111";
addressCard1.phone = phoneObject;
NSLog(#"addressCard1 phone: %#",addressCard1.phone); // result: (111) 111 - 1111
NSLog(#"phone object: %#",phoneObject); // result: (111) 111 - 1111
But now if I change phoneObject, addressCard1.phone won't change despite this way I'm setting the phone object in AddressCard class (setPhone method in class implementation)
phoneObject = [NSString stringWithString:#"PHONE IS CHANGED"];
NSLog(#"%#",phoneObject); //result : PHONE IS CHANGED
NSLog(#"Phone after change in the AddressCard is: %#",addressCard1.phone); // result: (111) 111 - 1111
Could some objective-c ninja tells me what's the difference between two snippets, and the reason of this?

Answer 1

In the first case, you modify an object: changing the x, y that are in the point.
In the second case, you create a new string, and point to that with phoneObject. You didn't modify the string itself (and you couldn't anyway - NSString is immutable). That means the original string you gave to the address card still exists and is still valid, so the address card still uses that string. If you want it to use the new one, you need to tell it to, putting:
addressCard1.phone = phoneObject;
again after reassigning phoneObject.