Question

Let’s say I have two ArrayList objects. The user will enter some letters, but in this example I will just hardcode some letters to an ArrayList to make life easier. It seems a simple problem but I am getting so confused over it!

ArrayList<String> letters = new ArrayList<String>();
ArrayList<String> duplicateLetters = new ArrayList<String>();

letters.add("z");
letters.add("a");
letters.add("z");
letters.add("z");
letters.add("b");

My aim, is to remove the duplicates from the ArrayList letters, so it will ultimately only contain ["z"], ["a"] and ["b"].

However, I want the ArrayList duplicateLetters to store any letters which was entered more than once. Because "z" was duplicated twice, I want duplicateLetters to contain ["z"] and ["z"] in this example.

If possible, I would also like to keep track of the amount of times each letter was duplicated.

I have already tried using a HashSet which works very effectively when removing the duplicates in a List (See this question: How do I remove repeated elements from ArrayList?). However, Sets just simply disregard duplicates and in this case I want to keep track of them.

I’m begging for ideas! :/

Answer 1

Why not maintain a Map that maps letters to frequencies. Each time the user enters a letter, you would update this map:

Map<String, Integer> map = new HashMap<String, Integer>();

if (map.containsKey(input)) {
    map.put(input, map.get(input) + 1);
} else {
    map.put(input, 1);
}

To create letters, you would then have:

List<String> letters = new ArrayList<String>(map.keySet());

To create duplicateLetters you could loop over the keys and only add those with a value greater than 1:

List<String> duplicateLetters = new ArrayList<String>();

for (String key : map.keySet())
    if (map.get(key) > 1)
        duplicateLetters.add(key);