OOP Java

class RandomizedSet {
  Map<Integer, Integer> dict;
  List<Integer> list;
  Random rand = new Random();

  /** Initialize your data structure here. */
  public RandomizedSet() {
    dict = new HashMap();
    list = new ArrayList();
  }

  /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
  public boolean insert(int val) {
    if (dict.containsKey(val)) return false;

    dict.put(val, list.size());
    list.add(list.size(), val);
    return true;
  }

  /** Removes a value from the set. Returns true if the set contained the specified element. */
  public boolean remove(int val) {
    if (! dict.containsKey(val)) return false;

    // move the last element to the place idx of the element to delete
    int lastElement = list.get(list.size() - 1);
    int idx = dict.get(val);
    list.set(idx, lastElement);
    dict.put(lastElement, idx);
    // delete the last element
    list.remove(list.size() - 1);
    dict.remove(val);
    return true;
  }

  /** Get a random element from the set. */
  public int getRandom() {
    return list.get(rand.nextInt(list.size()));
  }
}