Hash map using an ArrayList

HM.java

import java.util.ArrayList;

class HM<K, V> {
    private static class HashNode<K, V> {
        private K key;
        private V value;
        private HashNode<K, V> next;

        HashNode(K key, V value) {
            this.key = key;
            this.value = value;
        }
    }

    private ArrayList<HashNode<K, V>> buckets = new ArrayList<>();
    private int size;
    private int numBuckets;

    public HM(int size) {
        numBuckets = size;

        for (int i = 0; i < size; i++) {
            buckets.add(i, null);
        }
    }

    private int getHashIndex(K key) {
        return key.hashCode() % numBuckets;
    }

    public void put(K key, V value) {
        int idx = getHashIndex(key);
        HashNode<K, V> head = buckets.get(idx);
        while (head != null) {
            if (head.key.equals(key)) {
                head.value = value;
                return;
            }
            head = head.next;
        }

        head = buckets.get(idx);
        HashNode<K, V> newNode = new HashNode<>(key, value);
        newNode.next = head;
        buckets.set(idx, newNode);
        size++;
    }

    public V get(K key) {
        int idx = getHashIndex(key);
        HashNode<K, V> head = buckets.get(idx);
        while (head != null) {
            if (head.key.equals(key)) {
                return head.value;
            }
            head = head.next;
        }

        return null;
    }

    public V remove(K key) {
        int idx = getHashIndex(key);
        HashNode<K, V> head = buckets.get(idx);
        HashNode<K, V> prev = null;
        while (head != null && !head.key.equals(key)) {
            prev = head;
            head = head.next;
        }

        if (head == null) {
            return null;
        }

        V value = head.value;
        if (prev != null) {
            prev.next = head.next;
        } else {
            buckets.set(idx, head.next);
        }

        size--;

        return value;
    }

    public int size() {
        return size;
    }

    public boolean isEmpty() {
        return size == 0;
    }

}