Linked Lists - Building Java Programs: A Back to Basics Approach - page 928

Constructing a List
Let's begin by constructing a series of nodes that store the sequence of values [3, 7,
12]. There are three values, which means you'll need three nodes that are linked
together. When you're creating linked lists, if you keep a reference to the front of the
list, then you can get to anything in the list. You'll usually use a single variable of
type ListNode that refers to (or points to) the front of the list. You begin with this
declaration:

ListNode list;

The variable list is not itself a node. It's a variable that is capable of referring to
a node. You haven't yet given it a value, so you would draw the following picture for
memory at this point in the program:

list ?

The “?” will be replaced with a reference to a node, which means that this box
does not have a data field or a next field. It's a box where you can store a reference
to such an object. You don't have an actual node yet. To understand this distinction
between an object and a variable that stores a reference to an object, you might want
to review the section on reference semantics in Chapter 7.
The way to get an actual node is to call new :

list = new ListNode();

This call constructs a new node and tells Java to have the variable list refer to it:

data
0

next
/

list

Recall that when objects are constructed, Java initializes the fields to the zero-
equivalent for the type. That is why in the node above the data field has the value 0
and the next field has the value null . Notice that we use a slash through the box to
indicate a null value.
What do you want to do with the node you have constructed? You want to store 3
in its data field ( list.data ) and you want its next field to point to a new node:

list.data = 3;
list.next = new ListNode();

The addition of these lines of code makes our linked list appear as follows:

data
3

next data

0

next
/

list