String compresstion

public class Main {

    public static void main(String[] args) {
        System.out.println(compress("aaaabbsddddddd"));

    }

    // String compression
    // aaaabbsddddddd    ==    a4b2sd7
    static String compress(String str) {
        String result = "";
        int i = 0;

        int c;
        char prev;

        if (str.length() > 0) {
            while(i < str.length()) {
                c = 1;
                prev = str.charAt(i);
                i++;
                while ((i < str.length()) && (str.charAt(i) == prev)) {
                    c++;
                    i++;
                }
                if(c == 1){
                    result += prev;
                } else{
                    result += prev + "" + c;
                }
            }
        }
        return result;
    }
}