Confused about how object references work in Java

Question

I am confused about how object references work and was wondering if someone could help. Below is some example code which is supposed to deQueue a Queue based on a linked list for the general case:

Object head = listHead.datum;
listHead = listHead.next;
return head;

My understanding is that when you have a primitive variable the actual value is stored is assigned to it, but if the variable is an object then a reference to an object is stored in there. So in the above code a reference to listHead.datum is stored in head, but then the reference stored in listHead is changed to be listHead.next. When it comes time to return the Object called head I would have thought that it would follow it's assigned reference i.e. go to listHead (which now refers to a different place) and then to datum.

The above code should return the head of the Queue but following my logic it will return the second in the queue. Where am I going wrong?

Answer 1

We have:

Object head = listHead.datum;
listHead = listHead.next;
return head;

There are a number of references at play here:

• listHead is a reference
• head is a reference
• listHead.datum is a reference
• listHead.next is a reference

There are two actual object instances being referenced:

• whatever datum references (let's call that instance D)
• whatever next references (let's call that instance N)

Here's how things go down.

• listHead.datum has a reference to D
• listHead.next has a reference to N
• head is given a reference to D
• listHead is given a reference to N (note that head is not changed)
• you return head which still references D