Find occurrences of characters in a Java String [duplicate]

Answer 1

Guava's CharMatcher API is quite powerful and concise:

CharMatcher.is('a').countIn("aaaab"); //returns 4

Answer 2

Try using Apache Commons' StringUtils:

int count = StringUtils.countMatches("aaaab", "a");
// count = 4 

Answer 3

Java 8

Approach 1 - Gets the occurrence of a single character

    String sentence = "Aaron ate apples upon a rock";

    long counted = IntStream.range(0, sentence.length())
            .filter(i->sentence.charAt(i) == 'a')
            .count();
    System.out.println("First approach: " + counted);

Approach 2 - Allows the character to be specified

    String sentence = "Aaron ate apples upon a rock";

    BiFunction<String, Character, Long> counter = (s,c) -> {
        return IntStream.range(0, s.length())
                .filter(i->s.charAt(i) == c)
                .count();
    };
    System.out.println("Second approach (with 'a'): " + counter.apply(sentence, 'a'));
    System.out.println("Second approach (with 'o'): " + counter.apply(sentence, 'o'));

Approach 3 - Counts occurrences of all characters

     String sentence = "Aaron ate apples upon a rock";

     Map<Character, Long> counts = IntStream.range(0, sentence.length())
             .mapToObj(i->sentence.charAt(i))
             .collect(Collectors.groupingBy(o->o, Collectors.counting()));      

     System.out.println("Third approach for every character... ");
     counts.keySet().stream()
        .forEach(key -> System.out.println("'" + key + "'->" + counts.get(key)));

Answer 4

String string = "aaab";
int count = string.length() - string.replaceAll("a", "").length();

instead of "a" use a regex like "[a-zA-Z]" to count all word characters