Linked Lists - Building Java Programs: A Back to Basics Approach - page 955

Just calling the constructor leaves the computer's memory looking like this:

data
10

next

data
2

next data

5

next data

12

next
/ front

current

But now you have to link this into the list by changing current.next :

current.next = new ListNode(value, current.next);

Now the computer's memory looks like this:

data
10

next

data
2

next data

5

next data

12

next
/

front

current

This diagram isn't the easiest picture to read, but if you follow the links carefully,
you'll see that, starting at the front, the sequence of values is 2, 5, 10, 12, which is
what you want.
The code works for most cases, but it is incomplete. What if you want to insert the
value 42 ? Remember your loop test:

while (current.next.data < value)

This code depends on finding a value in the list that is less than the value you are
trying to insert. What if there is no such value, as in the case of 42 ? This code keeps
moving current forward until current.next is null . At that point, when you try
to ask for the value of current.next.data , you are asking for null.data , which
throws a NullPointerException because Java is looking for the data field of a
nonexistent object.
If the value that you wish to insert is greater than everything else in the list, then it
belongs after the last node in the list. So you want to stop when current gets to the
last node in the list. Thus, a second (still incorrect) attempt at writing the code for the
test would be

while (current.next.data < value && current.next != null)