Iterating over a list, modifying each element: is there a faster method?

Question

I have an List of Strings and I would like to trim() each element of the list.

Currently, I'm using an ArrayList, doing a simple loop through the elements, and adding the trimmed element to a return list, like so:

int listLen = listToTrim.size();

List<String> trimmedList = new ArrayList<String>( listLen );

for ( int i = 0; i < listLen; i++ ) {
    trimmedList.add( listToTrim.get( i ).trim() );
}

return trimmedList;

For large lists, would there be a more efficient way of doing this?

Answer 1

Nope, you're good. That's about as efficient as it'll get. There's no magic that avoids iterating.

One point to keep in mind, 'though: If listToTrim is not a random-access-list (i.e. it does not implement RandomAccess), then using an Iterator (or an enhanced for-loop, which uses an Iterator internally) instead of a traditional for-loop is usually much more efficient. The most notable List that doesn't implement RandomAccess is the LinkedList. Calling l.get(300) on a LinkedList with 600 elements will have to iterate through ~300 elements to get the correct one!

Modifying your code to use an enhanced for-loop would look like this:

public List<String> trimStrings(Listy<String> listToTrim) {
    List<String> trimmedList = new ArrayList<String>(listToTrim.size());
    for (String str : listToTrim) {
      trimmedList.add(str.trim());
    }
    return trimmedList;
}

If you don't need the original list any more, then re-using the original list can save memory and improve performance:

public void trimStringsInPlace(List<String> listToTrim) {
    ListIterator<String> it = listToTrim.listIterator();
    while (it.hasNext()) {
      it.set(it.next().trim());
    }
}

Answer 2

Not really; you have to call trim on each element, and preallocating an ArrayList of the right size is as fast as you can get. Java 8 will allow you to make the syntax more compact, but iterating and trimming is the minimum amount of work here.