java object assignment between three references

Question

I've been studying for coding interviews and I have a question about java object assignments.

say I have a Node class and I create three instances.

Node a = new Node(1);
Node b = new Node(2);
Node c = new Node(3);

Now let's say I do an assignmet

a = b;

At this point I know that any change I make to the properties of Node a or b will result in a change to both a and b because this is a shallow copy.

i.e.

a.data = 99 //then b.data will become 99

or

b.data = 99 //then a.data will become 99

however if I do

b = c;

now any changes I make to the property of Node b won't have any affect on Node a.

b.data = 99;//then b.data will become 99 and c.data will become 99 but a.data will not change

I don't understand this behavior. I understand that Node b is assigned the value of Node c's address, but why doesn't this affect Node a?

"At this point I know that any change I make to the properties of Node a or b will result in a change to both a and b because this is a shallow copy." No, this is not correct. Your only making changes to the object itself to which each refernece points. — Kon, Jan 23 '16 at 00:57
Welcome to StackOverflow. Please take a few minutes to visit the [help] and read [ask]. You are requested to do sufficient online research before posting to ensure your question is not trivially answered with a simple Google search (i.e. Java reference assignment). — Jim Garrison, Jan 23 '16 at 01:02

score 2 · Accepted Answer · answered Jan 23 '16 at 00:57

Assigning b to the object referenced by c doesn't affect a because b no longer references the same object referred to by a. Here's what is happening:

a -> (1)
b -> (2)
c -> (3)

Now a = b; is executed. Now 2 references are referring the same object. The Node containing 1 no longer has any references to it and it can be garbage collected at any time.

a -\   (1)
    \
b ---> (2)
c ---> (3)

Now b = c; is executed. This does not affect how a references its object.

a --\   (1)
     \
b -\  > (2)
    \
c ---->  (3)

Now b and c are pointing to the Node containing 3 while a continues to refer to the node containing (2). This is why a.data is not affected when b.data is changed. They are referring to different objects again.

java object assignment between three references

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