How to efficiently (performance) remove many items from List in Java?

Question

I have quite large List named items (>= 1,000,000 items) and some condition denoted by that selects items to be deleted and is true for many (maybe hal

Answer 1

Rather than muddying my first answer, which is already rather long, here's a second, related option: you can create your own ArrayList, and flag things as "removed". This algoritm makes the assumptions:

• it's better to waste time (lower speed) during construction than to do the same during the removal operation. In other words, it moves the speed penalty from one location to another.
• it's better to waste memory now, and time garbage collecting after the result is computeed rather than spend the time up front (you're always stuck with time garbage collecting...).
• once removal begins, elements will never be added to the list (otherwise there are issues with re-allocating the flags object)

Also, this is, again, not tested so there's prlolly syntax errors.

public class FlaggedList extends ArrayList {
  private Vector flags = new ArrayList();
  private static final String IN = Boolean.TRUE;  // not removed
  private static final String OUT = Boolean.FALSE; // removed
  private int removed = 0;

  public MyArrayList(){ this(1000000); }
  public MyArrayList(int estimate){
    super(estimate);
    flags = new ArrayList(estimate);
  }

  public void remove(int idx){
    flags.set(idx, OUT);
    removed++;
  }

  public boolean isRemoved(int idx){ return flags.get(idx); }
}

and the iterator - more work may be needed to keep it synchronized, and many methods are left out, this time:

public class FlaggedListIterator implements ListIterator
{
  int idx = 0;

  public FlaggedList list;
  public FlaggedListIterator(FlaggedList list)
  {
    this.list = list;
  }
  public boolean hasNext() {
    while(idx