Efficient way to count characters or find duplicate characters in String

There are multiple ways to count the characters or find the duplicate characters in a String. Here we will see the two approaches for the same which may be useful in same or different scenarios.

Count or find duplicate characters using Hash Map

With hash map we can keep the character count in the map against given character which is used as map key. Map uses hashcode to store the keys. To learn more on map storage, you can refer the link Why hashmap keys should be immutable.

    public void countCharUsingMap(String str){
        Map<Character, Integer> countMap = new HashMap<>();
        for(Character ch:str.toCharArray()){
            int count = countMap.get(ch)==null?1:countMap.get(ch)+1;
            countMap.put(ch, count);
        }
        countMap.forEach((k,v)->{
            System.out.println("'"+k+"' : "+v+(v>1?"[DUP]":""));
        });
    }

This code has stored the count for each character in Map, which we will use to iterate through map entries to print the key, value which is character and it's count in given string.
Above code executed with string "This is a test for character count!" and below is the output.

' ' : 6[DUP]
'a' : 3[DUP]
'!' : 1
'c' : 3[DUP]
'e' : 2[DUP]
'f' : 1
'h' : 2[DUP]
'i' : 2[DUP]
'n' : 1
'o' : 2[DUP]
'r' : 3[DUP]
's' : 3[DUP]
'T' : 1
't' : 4[DUP]
'u' : 1

Count or find duplicate characters using ASCII numbers

If we are having only ascii characters in our string, it is very easy and comparatively faster to count the characters using ascii number. Please note that this method will not work with unicode characters. Here we are using ascii number of each character to store the count in fixed array of int. As we know that there are total 256 characters in ascii, so we need to create and array of size 256 to store the count. In java each character already represents it's ascii number if we typecast it to int and that way we can store the count at particular index in array which is the ascii number of character.

    public void countUsingAsciiCode(String str){
        int[] countArr = new int[256];

        for(char ch:str.toCharArray()){
            int count = 1;
            int index = (int)ch;
            countArr[index]++;
        }

        for(int idx=0;idx<countArr.length;idx++){
            int count = countArr[idx];
            if(count>0){
                System.out.println("'"+((char)idx)+"' : "+count+(count>1?"[DUP]":""));
            }
        }
    }

This code has stored the count of each character in given int array which is used latter to print the count and duplicate for same. Since it is primitive array, it will have 0 as default value so we will skip the array elements with 0 value.
Above code executed with string "This is a test for character count!" and below is the output.

' ' : 6[DUP]
'!' : 1
'T' : 1
'a' : 3[DUP]
'c' : 3[DUP]
'e' : 2[DUP]
'f' : 1
'h' : 2[DUP]
'i' : 2[DUP]
'n' : 1
'o' : 2[DUP]
'r' : 3[DUP]
's' : 3[DUP]
't' : 4[DUP]
'u' : 1