CODEWITHSUNDEEP
javalinked-list
nemesis
nemesis

Reputation: 263

could someone help explain how this indexOf() implementation works?

This is a sample implementation of indexOf() in LinkedList on Oracle's website. I'm a little confused on how the if loop works here:

public int indexOf(E e) {
    for (ListIterator<E> it = listIterator(); it.hasNext(); )
        if (e == null ? it.next() == null : e.equals(it.next()))
            return it.previousIndex();
    // Element not found
    return -1;
}

So the ListIterator object is created at the head of the list. for loop goes on until the iterator reaches the list's end, and the if loop checks if the target object is found. The part I did not understand though, is that why does the if loop check it.next() == null when e == null? Could someone help walk me through how it's done when input e is null?

Upvotes: 0

Views: 328

Answers (4)

fvarg00
fvarg00

Reputation: 220

if (e == null ? it.next() == null : e.equals(it.next())) - this is a java ternary operator

it.next() == null is a null check(returning true or false to see if this is the last element) if that is true

return it.previousIndex(); executes

if e is not null, then

e.equals(it.next())

executes and if thats true,

return it.previousIndex(); executes

which is the index of the element your want the index of. Hope my explanation isn't confusing.

Upvotes: 0

jrd1
jrd1

Reputation: 10726

I think you've misunderstood what is going on with regards to the ternary operator.

This line:

if (e == null ? it.next() == null : e.equals(it.next()))

Checks to see that e is null, if it is, then it will check that the iterator's next element is null (this effectively stops the iteration operation so as to avoid a NullPointerException), otherwise: (if e is not null - i.e. it has some value in it), perform the comparison:

e.equals(it.next())

Upvotes: 0

amit
amit

Reputation: 178461

The loop checks if it.next() == null only if e == null this is done to avoid NullPointerException when evaluating e.equals(it.next()).

if e != null, then the regular e.equals() method is invoked.

null is a valid "element" that can be inserted to a LinkedList, so this must be taken into consideration.

The position of the last element is not inserted. Note that unlike the text book data structure where the last element in a Linked List is null, in here - when you get to the last element - it.hasNext() will be evaluated to false, without letting you see this "junk" element.

Upvotes: 5

nickolayratchev
nickolayratchev

Reputation: 1206

If e is null, the indexOf() method will iterate until it finds another null element in the iterable object you have passed and return its index.

Upvotes: 0

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