Utility-class for marking several ArrayList elements as removed and removing them all then.

ArrayLists.java

package net.aquadc.util;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;

/**
 * For lists with O(n) random-deletion time (array-based lists, like {@link ArrayList}),
 * it's faster to mark items as removed and them sweep them all together.
 *
 * In general, this is a bit slower than removing from LinkedList,
 * but memory footprint, adding to end and reading performance is better.
 *
 * After {@link #markAsRemoved} call(s), list is in inconsistent state (heap pollution),
 * so no one should read it before {@link #commitRemovals} called.
 *
 * This mechanism is borrowed from {@link android.util.SparseArray}.
 * <a href="https://gist.github.com/Miha-x64/d2b2156f67e5c27b9061969d2d44b756">This Gist location</a>
 */
@SuppressWarnings({"unchecked", "SuspiciousMethodCalls", "rawtypes"})
public final class ArrayLists {

    private ArrayLists() {}

    private static final Object REMOVED = new Object();
    private static final Collection<Object> COLLECTION_OF_REMOVED = Collections.singleton(REMOVED);

    /**
     * Marks element at {@param idx} as removed.
     * Don't show this list to anyone until you call {@link #commitRemovals}!
     */
    public static void markAsRemoved(ArrayList<?> list, int idx) {
        ((List) list).set(idx, REMOVED);
    }

    /**
     * Actually removes all items which were marked as removed by {@link #markAsRemoved}.
     */
    public static boolean commitRemovals(ArrayList<?> list) {
        return list.removeAll(COLLECTION_OF_REMOVED);
    }

}