Finding the middle element of Singly Linked List
I'm no Java programmer, so this merely regards the algorithm.

while(pointerOne.next.next != null) {
    pointerOne = pointerOne.next.next;
    pointerTwo = pointerTwo.next;
    pos++;
}


What happens if pointerOne.next is null? I'm guessing you'd need this:

while(pointerOne != null &amp;&amp; pointerOne.next != null) {


I don't know if there is some shorthand for this in Java.

I also fail to see the purpose of the final if. Assume that you have a list with 3 elements. Then, after the loop, you'll have

pointerOne = first.next.next; // i.e., pointerOne = last
pointerTwo = first.next;
pos = 2;


So, pointerTwo points to the second (i.e., middle) element. But then your if will printout the successor of pointerTwo, which is the third element of a three-element list, so definitely not the middle one.

Lastly, I'd suggest using more descriptive names. For example, ptrLast and ptrMiddle, instead of pointerOne and pointerTwo. It makes your code more readable to others (which also includes you, after a few months).

One comment on your previous attempt:


  Is it an efficient way? Also, I am doubtful about this line while(pointerOne.next.next != null). Previously, I was just using this condition in while loop: pointerOne.next != null, but I was getting Null pointer exception so shifter to earlier one.


If the length of your list is even, you'll get pointerOne == null at some point, so pointerOne.next will result in an error. But, similarly, if the length of your list is odd, you'll get pointerOne.next == null at some point, so pointerOne.next.next will result in an error.

Since you're jumping by two nodes at a time, you need to make sure that they are both non-null. Hence the condition I've put above.