Count of Each Alphabet using Array

You can use ASCII to assign the letters number values:

int[] letters = new int[128]; // There are 128 different possible characters.

for(int i = 0; i < input.length; i++) {
    char current = input.charAt(i);
    int index = Character.getNumericValue(char);
    letters[index]++;
}

ArrayList<Character> ch = new ArrayList<Character>();
ArrayList<Integer> count = new ArrayList<Integer>();

someMethod(String input) {
  for(char c : input.toCharArray()) {
    if(ch.indexOf(c) != -1) {
      i.set(ch.indexOf(c), i.get(ch.indexOf(c))+1);
    } else {
      ch.add(c);
      i.add(1);
    }
  }
}

doing it with a Map is easier, where the letters are keys and the values are counts. Using an array is more tricky; you could assign each letter a number, and use that number as an index into the array, and store the counts in the array. So 'A' is 1, 'B' is 2, etc....The algorithm is

1. Get next letter of string.
2. Get the index for the letter.
3. Increment the value at that index in the array by 1.

Of course you need to do null checking and whatever.

Note that this is logically a Map. It's just when you use a Map, the Map does step 2 above for you.

You should use a collection implementing Multiset iunterface, i.e. HashMultiset (both taken from Google Guava library). Multiset is designed to hold counts for objects in collection:

Multiset<String> m = HashMultiset.create(Arrays.asList("a", "a", "b", "c", "b", "a"));
// m.toString() prints "a x 3, b x 2, c x 1"
// m.count() gives 6

One way could be, you first create an array, then traverse string using charAt(index) method ,match the current char against those in the array.If you find the match ,increment the value there,or add it as a new entry in the array.