ExampleLoopString.java

public class ExampleLoopString {

    public static int frequencyOf(String input, char c) {
        if (input == null) {
            return 0;
        }
        int ret = 0;
        for (int i = 0; i < input.length(); i++) {
            if (input.charAt(i) == c) {
                ret += 1;
            }
        }
        return ret;
    }

    public static void main(String[] args) {
        System.out.println(frequencyOf("abcd", 'd'));
    }
}