How to count frequency of characters in a string?

Using the stream API as of JDK-8:

Map<Character, Long> frequency =
            str.chars()
               .mapToObj(c -> (char)c)
               .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

or if you want the keys as Integers:

Map<Character, Integer> frequency =
            str.chars()
               .mapToObj(c -> (char)c)
               .collect(Collectors.groupingBy(Function.identity(), Collectors.summingInt(c -> 1)));

Another variant:

Map<Character, Integer> frequency = 
            str.chars()
               .mapToObj(c -> (char)c)
               .collect(Collectors.toMap(Function.identity(), c -> 1, Math::addExact));

You can use a java Map and map a char to an int. You can then iterate over the characters in the string and check if they have been added to the map, if they have, you can then increment its value.

For example:

HashMap<Character, Integer> map = new HashMap<Character, Integer>();
String s = "aasjjikkk";
for (int i = 0; i < s.length(); i++) {
    char c = s.charAt(i);
    Integer val = map.get(c);
    if (val != null) {
        map.put(c, new Integer(val + 1));
    }
    else {
       map.put(c, 1);
   }
}

At the end you will have a count of all the characters you encountered and you can extract their frequencies from that.

Alternatively, you can use Bozho's solution of using a Multiset and counting the total occurences.

A concise way to do this is:

Map<Character,Integer> frequencies = new HashMap<>();
for (char ch : input.toCharArray()) 
   frequencies.put(ch, frequencies.getOrDefault(ch, 0) + 1);

We use a for-each to loop through every character. The frequencies.getOrDefault() gets value if key is present or returns(as default) its second argument.