DiskBackedArrayList.java

package com.yourproject.datastructures;

import java.io.*;
import java.util.*;
import java.util.function.Consumer;

/**
 * Disk-backed array list
 * @param <E> element type
 */
public class DiskBackedArrayList<E> extends ArrayList<E> implements AutoCloseable {

    private int totalSize;
    private int threshold;
    private File backingFile;
    private ObjectOutputStream out;
    private List<E> prependedEntries = new ArrayList<>();

    public DiskBackedArrayList(int threshold) {
        this.threshold = threshold;
        try {
            this.backingFile = File.createTempFile(UUID.randomUUID().toString(), ".list");
            this.out = new ObjectOutputStream(new FileOutputStream(backingFile, true));
        } catch (IOException ex) {
            throw new RuntimeException(ex);
        }
    }

    @Override
    public int size() {
        return totalSize;
    }

    @Override
    public boolean add(E e) {
        boolean result = super.add(e);
        totalSize++;
        if (super.size() > threshold) {
            flushToDisk();
            super.clear();
        }
        return result;
    }

    @Override
    public void add(int index, E element) {
        if (index == 0) {
            prependedEntries.add(element);
            totalSize++;
        } else {
            throw new UnsupportedOperationException("Adding element at random index not supported");
        }
    }

    private void flushToDisk() {
        try {
            // using the super iterator which is memory-only
            for (Iterator<E> it = super.iterator(); it.hasNext();) {
                out.writeObject(it.next());
            }
        } catch (IOException ex) {
            throw new RuntimeException(ex);
        }
    }

    @Override
    public Iterator<E> iterator() {
        if (totalSize < threshold) {
            return super.iterator();
        } else {
            return new DiskBackedIterator(super.iterator());
        }
    }

    @Override
    public List<E> subList(int fromIndex, int toIndex) {
        if (totalSize < threshold) {
            return super.subList(fromIndex, toIndex);
        } else {
            throw new UnsupportedOperationException("Sublist not supported");
        }
    }

    @Override
    public boolean addAll(Collection<? extends E> c) {
        throw new UnsupportedOperationException("addAll not supported");
    }

    @Override
    public boolean addAll(int index, Collection<? extends E> c) {
        throw new UnsupportedOperationException("addAll not supported");
    }

    @Override
    public boolean remove(Object o) {
        throw new UnsupportedOperationException("Removing is not supported");
    }

    @Override
    public E remove(int index) {
        throw new UnsupportedOperationException("Removing is not supported");
    }

    @Override
    public void clear() {
        super.clear();
        prependedEntries.clear();
        try {
            out.close();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        backingFile.delete();
    }

    @Override
    public void close() throws Exception {
        clear();
    }

    /**
     * Iterator that fetches records from disk
     */
    private class DiskBackedIterator implements Iterator<E> {
        private int cursor;       // index of next element to return
        private int expectedModCount = modCount;
        private ObjectInputStream in;
        private Iterator<E> memoryIterator;
        private Iterator<E> prependedIterator;
        // prevent creating a synthetic constructor
        DiskBackedIterator(Iterator<E> memoryIterator) {
            try {
                in = new ObjectInputStream(new FileInputStream(backingFile));
                this.memoryIterator = memoryIterator;
                this.prependedIterator = prependedEntries.iterator();
            } catch (IOException ex) {
                throw new RuntimeException(ex);
            }
        }

        public boolean hasNext() {
            return cursor != totalSize;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= totalSize) {
                throw new NoSuchElementException();
            }
            cursor = i + 1;
            try {
                if (prependedIterator.hasNext()) {
                    return prependedIterator.next();
                } else if (in != null) {
                    try {
                        return (E) in.readObject();
                    } catch (EOFException ex) {
                        in.close();
                        in = null;
                        return memoryIterator.next();
                    }
                } else {
                    return memoryIterator.next();
                }
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }

        public void remove() {
            throw new UnsupportedOperationException("Removing from disk backed array is not allowed");
        }
      
        @Override
        public <T> T[] toArray(T[] a) {
           throw new UnsupportedOperationException("Converting to array not supported");
        }

        @Override
        public Object[] toArray() {
            throw new UnsupportedOperationException("Converting to array not supported");
        }

        @Override
        public void forEachRemaining(Consumer<? super E> action) {
            throw new UnsupportedOperationException("forEachRemaining not supported");
        }

        final void checkForComodification() {
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
        }
    }
}

It's really weird that no one but you have done it yet. And this ArrayList should be customizable and flexible. This ArrayList can replace MySql in most cases. The performance will be maximized. This needs to be developed more.