Counting unique characters in a String given by the user

Question

I have to write a program that counts the uniques characters in a String given by the user. For example "abc" returns 3 and "aabbccd" returns 4. I am not allow to use advanced Java classes like Map, Set, etc. I can only use arrays, Strings, for loops, while loops, if statements. I am trying to use a nested loop but am getting confused about how to write the algorithm for the second for loop.

public static int countUniqueCharacters(String input){

String orgInput = input.toLowerCase();
        int count = 0;
        int stringLength = input.length();
        for( int i = 0; i<stringLength; i++){
            for(int j = 2; j > j-i-1; j--){
                char temp = orgInput.charAt(i);
                if (temp == orgInput.charAt(j)){
                    count++;

Answer 1

Using Java 8 you could do the following:

public static long countUniqueCharacters(String input) {
    return input.chars()
            .distinct()
            .count();
}

This creates an IntStream of chars, then takes only distincts values and then counts the number of occurences.

Answer 2

It is extremely easy :)

public static int countUniqueCharacters(String input) {
    boolean[] isItThere = new boolean[Character.MAX_VALUE];
    for (int i = 0; i < input.length(); i++) {
        isItThere[input.charAt(i)] = true;
    }

    int count = 0;
    for (int i = 0; i < isItThere.length; i++) {
        if (isItThere[i] == true){
            count++;
        }
    }

    return count;
}

Example for input "aab"

First for-cycle goes 3 times, each time for one char.

Value of "a" is 97, so it turns isItThere[97] to true, then second "a" is involved, which is doing the same, isItThere[97] is set to true again (hence changing nothing).

After that "b" is involved, value of char "b" is 98, therefore isItThere[98] is set to true.

And then you have second for-cycle, where you cycle through the all isItThere array. If you find any true statement, you increment count. In our case, you find isItThere[97] and isItThere[98] as true statement, it means you increment twice and returning 2.

Answer 3

Here another solution:

public static int countUniqueCharacters(String input) {
    String buffer = "";
    for (int i = 0; i < input.length(); i++) {
        if (!buffer.contains(String.valueOf(input.charAt(i)))) {
            buffer += input.charAt(i);
        }
    }
    return buffer.length();
}

The first occurance of each character is stored in buffer. Therefore you have of all characters one in buffer, therefore buffer.length() delivers the count you need.