A hash table is a data structure that maps keys to values for highly efficient lookup.

HashMap:

HashMap<String, Integer> myFriends = new HashMap<String, Integer>();

• containsKey()
• put()
• size()
• get()
• keySet()

ArraryList & Resizeable Arrays:

ArrayList<String> merge(String[] words, String[] more) {
    ArrayList<String> sentence = new ArrayList<String>();
    for (String w : words) sentence.add(w);
    for (String w : more) sentence.add(w);