related to this post on Twitter https://twitter.com/JosePaumard/status/864938907183480832

ArrayListDemo.java

import static java.lang.System.out;
import java.lang.reflect.Field;
import java.util.AbstractList;
import java.util.ArrayList;

/**
 * Related to an answer to this Twitter post
 * https://twitter.com/JosePaumard/status/864938907183480832
 *
 * @author Frank Dietrich <Frank.Dietrich@gmx.li>
 */
class ArrayListDemo {

    public static void main(String[] args) throws Exception {
        makeFieldsAccessible();

        /**
         * When we add elements to the ArrayList the backing array needs to be
         * increased in size at some point. When we add elements to the end and
         * the next element would be added on <code>array[array.size()]</code>
         * the backing array is copied into a new array with size
         * <code>array.size() + array.size() / 2</code>.
         *
         * For each change of the arrayList size an internal modification
         * counter is increased.
         *
         * In the output array elements which are <code>null</code>, are
         * displayed as a '#' character.
         */
        out.println("--- fill the list with latin characters");
        ArrayList<Object> list = new ArrayList<>();
        for (int loopCount = 1; loopCount <= 26; loopCount++) {
            list.add((char) ('a' + loopCount - 1));
            Character[] backingArray = getBackingArray(list);
            int modCount = getSizeModificationCounter(list);
            print(loopCount, list, backingArray, modCount);
        }

        /**
         * If we remove one element from the arrayList, all following elements
         * in the backing array are shifted to the left. And the previous last
         * element is set to <code>null</code>, so this object could be GCed.
         * The size of the backing array is not reduced.
         */
        out.printf("%n--- remove every third character from the list%n");
        int iteration = 0;
        for (int loopCount = 26 / 3; loopCount > 0; loopCount--) {
            iteration++;
            list.remove(loopCount * 3 - 1);

            Object[] backingArray = getBackingArray(list);
            int modCountValue = getSizeModificationCounter(list);
            print(iteration, list, backingArray, modCountValue);
        }

        /**
         * If we remove element from the list matching a predicate the removal
         * process is done in three steps:
         *
         * - create a BitSet which contains the index position of elements in
         * the backing array matching the prediacte, if one element in the
         * arrayList is <code>null</code> a NPE is thrown and the backing array
         * stays unaltered
         *
         * - shift the elements right from the removed ones to the left to fill
         * the gaps
         *
         * - set the elements which are now behind the right end of the new
         * arraylist size to <code>null</code>
         */
        out.printf("%n--- remove every character which comes before 'k' in the alphabet%n");
        list.removeIf((Object t) -> ((char) t) < 'k');
        Object[] backingArray = getBackingArray(list);
        int modCountValue = getSizeModificationCounter(list);
        print(1, list, backingArray, modCountValue);
    }

    static void makeFieldsAccessible() throws SecurityException, NoSuchFieldException {
        fieldElementData = ArrayList.class.getDeclaredField("elementData");
        fieldElementData.setAccessible(true);
        fieldModCount = AbstractList.class.getDeclaredField("modCount");
        fieldModCount.setAccessible(true);
    }

    static Character[] getBackingArray(ArrayList<Object> list) throws Exception {
        Object[] objects = (Object[]) fieldElementData.get(list);
        Character[] chars = new Character[objects.length];
        for (int i = 0; i < objects.length; i++) {
            chars[i] = (Character) objects[i];
        }
        return chars;
    }

    static int getSizeModificationCounter(ArrayList<Object> list) throws Exception {
        return (int) fieldModCount.get(list);
    }

    static void print(int loopCount, ArrayList<Object> list, Object[] backingArray, int modCountValue) {
        System.out.printf(
                "iteration: %2d  list.size(): %2d  backingArray.length: %s  size modification count: %2d  %s%n",
                loopCount, list.size(), backingArray.length,
                modCountValue, prettyArray(backingArray));
    }

    static String prettyArray(Object[] arr) {
        StringBuilder sb = new StringBuilder(arr.length);
        for (Object c : arr) {
            sb.append(c == null ? '#' : c);
        }
        return sb.toString();
    }

    static Field fieldModCount;
    static Field fieldElementData;
}

output

--- fill the list with latin characters
iteration:  1  list.size():  1  backingArray.length: 10  size modification count:  1  a#########
iteration:  2  list.size():  2  backingArray.length: 10  size modification count:  2  ab########
iteration:  3  list.size():  3  backingArray.length: 10  size modification count:  3  abc#######
iteration:  4  list.size():  4  backingArray.length: 10  size modification count:  4  abcd######
iteration:  5  list.size():  5  backingArray.length: 10  size modification count:  5  abcde#####
iteration:  6  list.size():  6  backingArray.length: 10  size modification count:  6  abcdef####
iteration:  7  list.size():  7  backingArray.length: 10  size modification count:  7  abcdefg###
iteration:  8  list.size():  8  backingArray.length: 10  size modification count:  8  abcdefgh##
iteration:  9  list.size():  9  backingArray.length: 10  size modification count:  9  abcdefghi#
iteration: 10  list.size(): 10  backingArray.length: 10  size modification count: 10  abcdefghij
iteration: 11  list.size(): 11  backingArray.length: 15  size modification count: 11  abcdefghijk####
iteration: 12  list.size(): 12  backingArray.length: 15  size modification count: 12  abcdefghijkl###
iteration: 13  list.size(): 13  backingArray.length: 15  size modification count: 13  abcdefghijklm##
iteration: 14  list.size(): 14  backingArray.length: 15  size modification count: 14  abcdefghijklmn#
iteration: 15  list.size(): 15  backingArray.length: 15  size modification count: 15  abcdefghijklmno
iteration: 16  list.size(): 16  backingArray.length: 22  size modification count: 16  abcdefghijklmnop######
iteration: 17  list.size(): 17  backingArray.length: 22  size modification count: 17  abcdefghijklmnopq#####
iteration: 18  list.size(): 18  backingArray.length: 22  size modification count: 18  abcdefghijklmnopqr####
iteration: 19  list.size(): 19  backingArray.length: 22  size modification count: 19  abcdefghijklmnopqrs###
iteration: 20  list.size(): 20  backingArray.length: 22  size modification count: 20  abcdefghijklmnopqrst##
iteration: 21  list.size(): 21  backingArray.length: 22  size modification count: 21  abcdefghijklmnopqrstu#
iteration: 22  list.size(): 22  backingArray.length: 22  size modification count: 22  abcdefghijklmnopqrstuv
iteration: 23  list.size(): 23  backingArray.length: 33  size modification count: 23  abcdefghijklmnopqrstuvw##########
iteration: 24  list.size(): 24  backingArray.length: 33  size modification count: 24  abcdefghijklmnopqrstuvwx#########
iteration: 25  list.size(): 25  backingArray.length: 33  size modification count: 25  abcdefghijklmnopqrstuvwxy########
iteration: 26  list.size(): 26  backingArray.length: 33  size modification count: 26  abcdefghijklmnopqrstuvwxyz#######

--- remove every third character from the list
iteration:  1  list.size(): 25  backingArray.length: 33  size modification count: 27  abcdefghijklmnopqrstuvwyz########
iteration:  2  list.size(): 24  backingArray.length: 33  size modification count: 28  abcdefghijklmnopqrstvwyz#########
iteration:  3  list.size(): 23  backingArray.length: 33  size modification count: 29  abcdefghijklmnopqstvwyz##########
iteration:  4  list.size(): 22  backingArray.length: 33  size modification count: 30  abcdefghijklmnpqstvwyz###########
iteration:  5  list.size(): 21  backingArray.length: 33  size modification count: 31  abcdefghijkmnpqstvwyz############
iteration:  6  list.size(): 20  backingArray.length: 33  size modification count: 32  abcdefghjkmnpqstvwyz#############
iteration:  7  list.size(): 19  backingArray.length: 33  size modification count: 33  abcdeghjkmnpqstvwyz##############
iteration:  8  list.size(): 18  backingArray.length: 33  size modification count: 34  abdeghjkmnpqstvwyz###############

--- remove every character which comes before 'k' in the alphabet
iteration:  1  list.size(): 11  backingArray.length: 33  size modification count: 35  kmnpqstvwyz######################