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Fastest way to remove the null elements in a list

Is there a quick way of removing the
null elements from a linked list?
The only way i know is to iterate on the
elements and remove the null ones.
I don't see how multithreading would help-- linked list doesn't have direct access to its members.
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EDIT: One I could think of is putting the elements into a set then back to the list again. This wouldn't retain the order, However would work every otherwise(?)

Unless you have an iterator pointing to the middle of the list, multithreading is not going to help* . A simple iteration with a ListIterator<T> should do the trick:
ListIterator<String> iter = list.listIterator();
while (iter.hasNext()) {
if (iter.next() == null) {
iter.remove();
}
}
* That is before we take into consideration that the linked list is not thread-safe without external synchronization.

What do you mean by fastest? The least number of operations is probably just going through one-by-one. If you use multithreading you would likely have to either chop up the list and put it back together or get the indexes and then go through the list in reverse and remove each one manually.

while(list.remove(null)); would work but might be slow on large list
Edit: In java 8 you can do list.parallelStream().filter((e)->e!=null) if you need the result as a linked list again you can do new LinkedList(Arrays.asList(stream.toArray));

This is pretty fast way. Worst case though it will be slower than just traversing once. In most cases it will be faster though
ArrayList a = new ArrayList<Integer>();
Collections.sort(a);
for(int i=a.size()-1; i>-1; i--)
{
if(a.get(i)==null)
a.remove(i);
else
break;
}

Iterator vs for

I was asked in an interview what is the advantage of using iterator over for loop or what is the advantage of using for loop over iterator?
Can any body please answer this?

First of all, there are 2 kinds of for loops, which behave very differently. One uses indices:
for (int i = 0; i < list.size(); i++) {
Thing t = list.get(i);
...
}
This kind of loop isn't always possible. For example, Lists have indices, but Sets don't, because they're unordered collections.
The other one, the foreach loop uses an Iterator behind the scenes:
for (Thing thing : list) {
...
}
This works with every kind of Iterable collection (or array)
And finally, you can use an Iterator, which also works with any Iterable:
for (Iterator<Thing> it = list.iterator(); it.hasNext(); ) {
Thing t = it.next();
...
}
So you in fact have 3 loops to compare.
You can compare them in different terms: performance, readability, error-proneness, capability.
An Iterator can do things that a foreach loop can't. For example, you can remove elements while you're iterating, if the iterator supports it:
for (Iterator<Thing> it = list.iterator(); it.hasNext(); ) {
Thing t = it.next();
if (shouldBeDeleted(thing) {
it.remove();
}
}
Lists also offer iterators that can iterate in both directions. A foreach loop only iterates from the beginning to an end.
But an Iterator is more dangerous and less readable. When a foreach loop is all you need, it's the most readable solution. With an iterator, you could do the following, which would be a bug:
for (Iterator<Thing> it = list.iterator(); it.hasNext(); ) {
System.out.println(it.next().getFoo());
System.out.println(it.next().getBar());
}
A foreach loop doesn't allow for such a bug to happen.
Using indices to access elements is slightly more efficient with collections backed by an array. But if you change your mind and use a LinkedList instead of an ArrayList, suddenly the performance will be awful, because each time you access list.get(i), the linked list will have to loop though all its elements until the ith one. An Iterator (and thus the foreach loop) doesn't have this problem. It always uses the best possible way to iterate through elements of the given collection, because the collection itself has its own Iterator implementation.
My general rule of thumb is: use the foreach loop, unless you really need capabilities of an Iterator. I would only use for loop with indices with arrays, when I need access to the index inside the loop.

Iterator Advantage:
Ability to remove elements from Collections.
Ability to move forward and backward using next() and previous().
Ability to check if there more elements or not by using hasNext().
Loop was designed only to iterate over a Collection, so if you want just to iterate over a Collection, its better to use loop such as for-Each, but if you want more that that you could use Iterator.

The main difference between Iterator and the classic for loop, apart from the obvious one of having or not having access to the index of the item you're iterating, is that using Iterator abstracts the client code from the underlying collection implementation, allow me to elaborate.
When your code uses an iterator, either in this form
for(Item element : myCollection) { ... }
this form
Iterator<Item> iterator = myCollection.iterator();
while(iterator.hasNext()) {
Item element = iterator.next();
...
}
or this form
for(Iterator iterator = myCollection.iterator(); iterator.hasNext(); ) {
Item element = iterator.next();
...
}
What your code is saying is "I don't care about the type of collection and its implementation, I just care that I can iterate through its elements". Which is usually the better approach, since it makes your code more decoupled.
On the other hand, if you're using the classic for loop, as in
for(int i = 0; i < myCollection.size(); i++) {
Item element = myCollection.get(i);
...
}
Your code is saying, I need to know the type of collection, because I need to iterate through its elements in a specific way, I'm also possibly going to check for nulls or compute some result based on the order of iteration. Which makes your code more fragile, because if at any point the type of collection you receive changes, it will impact the way your code works.
Summing it up, the difference is not so much about speed, or memory usage, is more about decoupling your code so that is more flexible to cope with change.

if you access to data by number (e.g. "i"), it is fast when you use array. because it goes to element directly
But, other data structure (e.g. tree, list), it needs more time, because it start from first element to target element. when you use list. It needs time O(n). so, it is to be slow.
if you use iterator, compiler knows that where you are. so It needs O(1)
(because, it start from current position)
finally, if you use only array or data structure that support direct access(e.g. arraylist at java). "a[i]" is good. but, when you use other data structure, iterator is more efficient

Unlike other answers, I want to point another things;
if you need to perform the iteration in more than one place in your code, you will likely end up duplicating the logic. This clearly isn’t a very extensible approach. Instead, what’s needed is a way to separate the logic for selecting the data from the code that actually processes it.
An iterator solves these problems by providing a generic interface for looping over a set of data so that the underlying data structure or storage mechanism — such as an array- is hidden.
Iterator is a concept not an implementation.
An iterator provides a number of operations for traversing and accessing data.
An iterator may wrap any datastructure like array.
One of the more interesting and useful advantages of using iterators is the capability to wrap or decorate another iterator to filter the return values
An iterator may be thread safe while a for loop alone cannot be as it is accessing elements directly. The only popular thread-safety iterator is CopyOnWriteArrayList but it is well known and used often so worth mentioning.
This is from the book that it is https://www.amazon.com/Beginning-Algorithms-Simon-Harris/dp/0764596748

I stumbled on this question. The answer lies to the problems Iterator tries to solve:
access and traverse the elements of an aggregate object without exposing its representation
define traversal operations for an aggregate object without changing its interface

Equivalence of for loop to List.get in java

I have an List with one element in it, but which may have more later. The list I am using is an ArrayList. I find that by calling List.get(0) instead of using a for loop I get faster results. Why is this? Shouldn't looping over 1 item be the same as getting 1 item? If not then how can I get similar performance? I know that my array will eventually be larger than one item. I am using this loop in the body of a opengl renderer. When I use the loop my fps drops by 45.
Edit: I have fixed the problem. My renderer was adding a new value to the list each time it was rendered.

Using the enhanced for loop (for-each) in Java results in compiled code that works like this:
Iterator<Thing> it = list.iterator();
while(it.hasNext()) {
yourLoop.loopBody(it.next());
}
You were probably expecting it to be equivalent to this ?
for (int i = 0; i < list.size(); i++) {
yourLoop.loopBody(list.get(i));
}
but this is not the case, and constructing the iterator takes additional time that you are observing.

Without knowing the List implementation, a specific answer can't really be given. For example, ArrayList is backed by an array, so calling get is essentially an array access.
Using the "foreach" version of a for loop on the other hand requires creating an Iterator. This is likely the cause for a slow down. Some implementations have complex implementation for an Iterator or ListIterator.
The answer then is a question of style and readability. Most Java programmers would say the foreach loop is much more readable and clear of your intent. And with some implementations of List (e.g. LinkedList) much faster.

I'm no expert in Collections, but setting up the iteration has got to be more expensive than simply getting a specific item.
By setting up a foreach, we don't know how many items there are in the collection, so it's going to set it up for iteration no matter how many items the list has.

IN general, if you're worried about speed, your fastest way to go if you know you're using ArrayList would be this:
int size = list.size();
for (int i=0;i<size;i++) {
Thing thing = list.get(i);
}
while the overhead of the "size" method is minimal, it is still overhead that need not be called in every loop. The get() method on ArrayList is O(1). This should give similar performance to an Iterator. If you look at the iterator code for ArrayList, it's quite simply and the only real extra overhead is from the Object creation, and concurrent modification checking.
For LinkedList, however, this type of loop would give horrible performance as the .get() method on LinkedList is O(n), and an Iterator would be much faster, since it's simply doing a pointer check and reassignment for each iteration which makes it O(1) for each call to next().

remove elements from CopyOnWriteArrayList

I am getting an exception when I try to remove elements from CopyOnWriteArrayList using an iterator.
I have noticed that it is documented
Element-changing operations on iterators themselves (remove, set, and add) are not supported. These methods throw UnsupportedOperationException.
(from http://download.oracle.com/javase/6/docs/api/java/util/concurrent/CopyOnWriteArrayList.html)
Now, surprisingly i can iterate it with foreach and use the remove() function . But then I get the famous bug - when trying to remove an item from a list using a for loop - you skip the element next to the removed element.
any suggestions then?

Iterate over the collection choosing all the elements you want to delete and putting those in a temporary collection. After you finish iteration remove all found elements from the original collection using method removeAll.
Would that work out for you? I mean, not sure if deletion logic is more complicated than that in your algorithm.

EDIT: I'm an idiot. I missed the fact that this is a copy-on-write list so every removal means a new copy. So my suggestions below are likely to be suboptimal if there's more than one removal.
Same as for any other list whose iterator doesn't support remove, or anything where you're not using an iterator. There are three basic techniques that come to mind to avoid this bug:
Decrement the index after removing something (being careful not to do anything with the index until the next iteration). For this you'll obviously have to use a for(int i=0; i < ... style of for loop, so that you can manipulate the index.
Somehow repeat what the inside of the loop is doing, without literally going back to the top of the loop. Bit of a hack - I would avoid this technique.
Iterate over the list in reverse (from end to start, instead of from start to end). I prefer this approach as it's the simplest.

Since this is a CopyOnWriteArrayList it is totally safe to remove elements while iterating with forEach. No need for fancy algorithms.
list.forEach(e -> {
if (shouldRemove(e))
list.remove(e);
});
EDIT: Well of course that works if you want to delete elements by reference, not by position.

Ususlly you would iterate first gathering elemenet to be deleted in a separate list then delete them outside the for each loop (which is disguised iterator based loop anyway)

Something like this:
int pos = 0;
while(pos < lst.size() ) {
Foo foo = lst.get(pos);
if( hasToBeRemoved(foo) ) {
lst.remove(pos);
// do not move position
} else {
pos++;
}
}

You could use Queue instead of List.
private Queue<Something> queue = new ConcurrentLinkedQueue<Something>();
It's thread safe and supports iterator.remove(). Be aware of the thread-safe behavior of Queue iterators, though (check the javadoc).

If you want to delete all use just clear(). If you want to keep elements put them in a temporary ArrayList and get them back from there.
List<Object> tKeepThese= new ArrayList<>();
for(ListIterator<Object> tIter = theCopyOnWriteArrayList; tIter.hasNext();)
{
tObject = tIter.next();
if(condition to keep element)
tKeepThese.add(tObject);
}
theCopyOnWriteArrayList.clear();
theCopyOnWriteArrayList.addAll(tKeepThese);

the shortest and most efficient way:
List<String> list = new CopyOnWriteArrayList<>();
list.removeIf(s -> s.length() < 1);
internally it creates an temporary array with the same length and copies all elements where the predicate returns true.
keep in mind that if you use this method to actually iterate over the elements to perform some action, these actions cannot be performed in paralell anymore since the removeIf-call is atomic and will lock the traversal for other threads

Below works fine with CopyOnWriteArrayList
for(String key : list) {
if (<some condition>) {
list.remove(key);
}
}

Performance of traditional for loop vs Iterator/foreach in Java

Is there any performance testing results available in comparing traditional for loop vs Iterator while traversing a ArrayList,HashMap and other collections?
Or simply why should I use Iterator over for loop or vice versa?

Assuming this is what you meant:
// traditional for loop
for (int i = 0; i < collection.size(); i++) {
T obj = collection.get(i);
// snip
}
// using iterator
Iterator<T> iter = collection.iterator();
while (iter.hasNext()) {
T obj = iter.next();
// snip
}
// using iterator internally (confirm it yourself using javap -c)
for (T obj : collection) {
// snip
}
Iterator is faster for collections with no random access (e.g. TreeSet, HashMap, LinkedList). For arrays and ArrayLists, performance differences should be negligible.
Edit: I believe that micro-benchmarking is root of pretty much evil, just like early optimization. But then again, I think it's good to have a feeling for the implications of such quite trivial things. Hence I've run a small test:
iterate over a LinkedList and an ArrayList respecively
with 100,000 "random" strings
summing up their length (just something to avoid that compiler optimizes away the whole loop)
using all 3 loop styles (iterator, for each, for with counter)
Results are similar for all but "for with counter" with LinkedList. All the other five took less than 20 milliseconds to iterate over the whole list. Using list.get(i) on a LinkedList 100,000 times took more than 2 minutes (!) to complete (60,000 times slower). Wow! :) Hence it's best to use an iterator (explicitly or implicitly using for each), especially if you don't know what type and size of list your dealing with.

The first reason to use an iterator is obvious correctness. If you use a manual index, there may be very innocuous off-by-one errors that you can only see if you look very closely: did you start at 1 or at 0? Did you finish at length - 1? Did you use < or <=? If you use an iterator, it is much easier to see that it is really iterating the whole array. "Say what you do, do what you say."
The second reason is uniform access to different data structures. An array can be accessed efficiently through an index, but a linked list is best traversed by remembering the last element accessed (otherwise you get a "Shlemiel the painter"). A hashmap is even more complicated. By providing a uniform interface from these and other data structures (e.g., you can also do tree traversals), you get obvious correctness again. The traversing logic has to be implemented only once, and the code using it can concisely "say what it does, and do what it says."

Performance is similar in most cases.
However, whenever a code receives a List, and loops on it, there is well-known case:
the Iterator is way better for all List implementations that do not implement RandomAccess (example: LinkedList).
The reason is that for these lists, accessing an element by index is not a constant time operation.
So you can also consider the Iterator as more robust (to implementation details).
As always, performance should not be hide readability issues.
The java5 foreach loop is a big hit on that aspect :-)

Yes, it does make a difference on collections which are not random access based like LinkedList. A linked list internally is implemented by nodes pointing to the next(starting at a head node).
The get(i) method in a linked list starts from the head node and navigates through the links all the way to the i'th node. When you iterate on the linked list using a traditional for loop, you start again from the head node each time, thus the overall traversal becomes quadratic time.
for( int i = 0; i< list.size(); i++ ) {
list.get(i); //this starts everytime from the head node instead of previous node
}
While the for each loop iterates over the iterator obtained from the linked list and calls its next() method. The iterator maintains the states of the last access and thus does not start all the way from head everytime.
for( Object item: list ) {
//item element is obtained from the iterator's next method.
}

One of the best reasons to use an iterator over the i++ syntax is that not all data structures will support random access let alone have it perform well. You should also be programming to the list or collection interface so that if you later decided that another data structure would be more efficient you'd be able to swap it out without massive surgery. In that case (the case of coding to an interface) you won't necessarily know the implementation details and it's probably wiser to defer that to the data structure itself.

One of the reasons I've learned to stick with the for each is that it simplifies nested loops, especially over 2+ dimensional loops. All the i's, j's, and k's that you may end up manipulating can get confusing very quickly.

Use JAD or JD-GUI against your generated code, and you will see that there is no real difference. The advantage of the new iterator form is that it looks cleaner in your codebase.
Edit: I see from the other answers that you actually meant the difference between using get(i) versus an iterator. I took the original question to mean the difference between the old and new ways of using the iterator.
Using get(i) and maintaining your own counter, especially for the List classes is not a good idea, for the reasons mentioned in the accepted answer.

I don't believe that
for (T obj : collection) {
calculates .size() each time thru the loop and is therefore faster than
for (int i = 0; i < collection.size(); i++) {

+1 to what sfussenegger said. FYI, whether you use an explicit iterator or an implicit one (i.e. for each) won't make a performance difference because they compile to the same byte code.