Strings.java

package modern.challenge;

public final class Strings {

    private Strings() {
        throw new AssertionError("Cannot be instantiated");
    }

    public static int countOccurrencesOfACertainCharacterV1(String str, char ch) {

        if (str == null || str.isEmpty()) {
            // or throw IllegalArgumentException            
            return -1;
        }

        return str.length() - str.replace(String.valueOf(ch), "").length();
    }

    public static int countOccurrencesOfACertainCharacterVCP1(String str, String ch) {

        if (str == null || ch == null || str.isEmpty() || ch.isEmpty()) {
            // or throw IllegalArgumentException            
            return -1;
        }

        if (ch.codePointCount(0, ch.length()) > 1) {
            return -1; // there is more than 1 Unicode character in the given String
        }

        int result = str.length() - str.replace(ch, "").length();

        // if ch.length() return 2 then this is a Unicode surrogate pair
        return ch.length() == 2 ? result / 2 : result;
    }

    public static int countOccurrencesOfACertainCharacterV2(String str, char ch) {

        if (str == null || str.isEmpty()) {
            // or throw IllegalArgumentException
            return -1;
        }

        int count = 0;
        for (int i = 0; i < str.length(); i++) {
            if (str.charAt(i) == ch) {
                count++;
            }
        }
        return count;
    }

    public static int countOccurrencesOfACertainCharacterVCP2(String str, String ch) {

        if (str == null || ch == null || str.isEmpty() || ch.isEmpty()) {
            // or throw IllegalArgumentException            
            return -1;
        }

        if (ch.codePointCount(0, ch.length()) > 1) {
            return -1; // there is more than 1 Unicode character in the given String
        }

        int count = 0;
        int codePoint = ch.codePointAt(0);
        for (int i = 0; i < str.length(); i++) {
            if (str.codePointAt(i) == codePoint) {
                count++;
            }
        }

        return count;
    }

    public static long countOccurrencesOfACertainCharacterV3(String str, char ch) {

        if (str == null || str.isEmpty()) {
            // or throw IllegalArgumentException
            return -1;
        }

        return str.chars()
                .filter(c -> c == ch)
                .count();
    }
    
    public static long countOccurrencesOfACertainCharacterVCP3(String str, String ch) {

        if (str == null || ch == null || str.isEmpty() || ch.isEmpty()) {
            // or throw IllegalArgumentException            
            return -1;
        }

        if (ch.codePointCount(0, ch.length()) > 1) {
            return -1; // there is more than 1 Unicode character in the given String
        }

        int codePoint = ch.codePointAt(0);        
        
        return str.codePoints()
                .filter(c -> c == codePoint)
                .count();
    }
}