从ArrayList中删除项目 <ArrayList<T> &gt;&gt;

Question

In following code, I want to remove items which are null or size of arraylist is zero from ArrayList in first "for loop". After running first loop, again I have a second loop. In second loop, I have tested whether it contains any item which is null or size of item is zero. Although I erase in first loop, ArrayList BS contains items which was deleted from list.

ArrayList<ArrayList<Record>> BS = new ArrayList<>(); 
.
.// some codes
.

for (int j = 0; j < BS.size(); j++) {
    if(BS.get(j) == null || BS.get(j).size() == 0){
        BS.remove(j);
    }
}


for (int j = 0; j < BS.size(); j++) {
    if(BS.get(j) == null || BS.get(j).size() == 0){
            System.out.println("Again, fall into if condition");
    }
}

Answer 1

The problem you've got here is that you won't check the item immediately after the one you removed.

Either:

BS.removeIf(a -> a == null || a.isEmpty());

Or iterate the list in reverse:

for (int j = BS.size() - 1; j >= 0; j--) { ... }

Or use an Iterator , and use the remove() method.

Iterator<? extends List<Record>> it = BS.iterator();
while (it.hasNext()) {
  List<Record> list = it.next();
  if (list == 0 || list.isEmpty()) it.remove();
}

removeIf is the best approach, as it is optimized to know how to remove many elements efficiently: removing anything but the last element of an ArrayList requires all of the elements with greater index to be shifted along in the internal array, so removing elements by simply calling remove is quadratic in time complexity.

However, it is available in Java 8+ only. (You can implement the efficient removal quite easily in earlier versions of Java; it's just a little more work).

Answer 2

When removing items from an Array, usually you want to go from the last item to the first one, otherwwise your index will jump one item each time you do a remove.

This because removing an item shifts the remaining ones back of one position, namely from position "j + 1" to position j. So the next loop you skip the item a position j (that's the item that was in position j+1 in the previous loop).

Try this:

for (int j = BS.size() - 1; j >= 0; j--) {
    if(BS.get(j) == null || BS.get(j).size() == 0){
        BS.remove(j);
    }
}

Answer 3

Problem is when a item is removed next item will not be removed if the condition is true. For that you should decrement the index when you remove an item.

Change as follow.

ArrayList<ArrayList<Record>> BS = new ArrayList<>(); 
.
.// some codes
.

for (int j = 0; j < BS.size(); j++) {
    if(BS.get(j) == null || BS.get(j).size() == 0){
        BS.remove(j);
        j--;
    }
}