Deleting objects from an ArrayList in Java

Question

I need to delete some objects from an ArrayList if they meet a condition and I\'m wondering which way could be more efficient.

Here\'s the situation: I

Answer 1

I have found an alternative faster solution:

  int j = 0;
  for (Iterator i = list.listIterator(); i.hasNext(); ) {
    j++;

    if (campo.getNome().equals(key)) {
       i.remove();
       i = list.listIterator(j);
    }
  }

Answer 2

Most performant would, I guess, be using the listIterator method and do a reverse iteration:

for (ListIterator<E> iter = list.listIterator(list.size()); iter.hasPrevious();){
    if (weWantToDelete(iter.previous()))  iter.remove();
}

Edit: Much later, one might also want to add the Java 8 way of removing elements from a list (or any collection!) using a lambda or method reference. An in-place filter for collections, if you like:

list.removeIf(e -> e.isBad() && e.shouldGoAway());

This is probably the best way to clean up a collection. Since it uses internal iteration, the collection implementation could take shortcuts to make it as fast as possible (for ArrayLists, it could minimize the amount of copying needed).

Answer 3

With an iterator you can handle always the element which comes to order, not a specified index. So, you should not be troubled with the above matter.

Iterator itr = list.iterator();
String strElement = "";
while(itr.hasNext()){

  strElement = (String)itr.next();
  if(strElement.equals("2"))
  {
    itr.remove();
  }

Answer 4

Maybe Iterator’s remove() method? The JDK’s default collection classes should all creator iterators that support this method.

Answer 5

Unless you're positive that the issue you're facing is indeed a bottleneck, I would go for the readable

public ArrayList filterThings() {

    ArrayList pileOfThings;
    ArrayList filteredPileOfThings = new ArrayList();

    for (Thing thingy : pileOfThings) {
        if (thingy.property != 1) {
            filteredPileOfThings.add(thingy);
        }            
    }
    return filteredPileOfThings;
}

Answer 6

First, I'd make sure that this really is a performance bottleneck, otherwise I'd go with the solution that is cleanest and most expressive.

If it IS a performance bottleneck, just try the different strategies and see what's the quickest. My bet is on creating a new ArrayList and puting the desired objects in that one, discarding the old ArrayList.