How can I count how often an element from an ArrayList occurs in the another ArrayList?

Question

I want to count how often an element from ArrayList "list1" occurs in the other ArrayList "list2".

I want this output:

A 2
B 0
C 1
D 2

I get this output:

A 0
B 0
C 0
D 69

Coud you please help me to do this? Thank you!

enter code here

    HashMap<Character, Integer> map = new HashMap<Character, Integer>();
    ArrayList<Character> list1 = new ArrayList<Character>();
    ArrayList<Character> list2 = new ArrayList<Character>();

    Collections.addAll(list1, 'A', 'B', 'C', 'D');
    Collections.addAll(list2, 'D', 'A', 'C', 'A', 'D');

    for (int i = 0; i < list1.size(); i++) {

        for (int j = 0; j < list2.size(); j++) {
            

            if (list1.get(i) == list2.get(j)) {
                map.put(list1.get(i), 1);
            } 
            
            if (list1.get(i) == list2.get(j) && (map.containsKey(list1.get(i)))) {
                map.replace(list1.get(i), list1.get(i) + 1);
            } 
            
            if (list1.get(i) != list2.get(j)) {
                map.put(list1.get(i), 0);
            }
            
        } 
    }

    
    System.out.println("Map: ");
    for (Map.Entry<Character, Integer> entry : map.entrySet()) {
        System.out.println(entry.getKey() + " " + entry.getValue());
    }

Answer 1

Use the method frequency, from Collection, namely:

    for (Character c : list1)
       map.put(c, Collections.frequency(list2, c));

    System.out.println("Map: ");
    for (Map.Entry<Character, Integer> entry : map.entrySet()) {
        System.out.println(entry.getKey() + " " + entry.getValue());
    }

If you are a fan of one liners :

    list1.forEach(c -> map.put(c, Collections.frequency(list2, c)));

Including the printing of the elements:

list1.forEach(c ->  System.out.printf("%s %d%n", c, Collections.frequency(list2, c)));

By the way, your original answer was almost correct, you just had to rethink a bite the conditionals, and their order:

 for (Character c1 : list1) {
    for (Character c2 : list2) {
        if(map.containsKey(c1) && c1 == c2 ){
           map.put(c1, map.get(c1) + 1);
        }
        else if (!map.containsKey(c1) && c1 != c2 ) {
            map.put(c1, 0);
        } 
        else if (!map.containsKey(c1) && c1 == c2) {
            map.put(c1, 1);
        } 
    } 
}

Another advice is, if you do not explicitly need the loop index, it is better to use the idiom for (Character c1 : list1) rather than for(int i = 0; i < list1.size(); i++). The first is cleaner, and less error-prone than the second version. Moreover, you can use the variable c1 instead of having to do list1.get(i) all the time.