Cracking the Coding Interview Q1.5

Implement an method  to compress the string according to the number of occurrences of each char in the string. For example, "aabcccaa" is compressed as "a2b1c3a2". If the string after the compression is not shorter, return the original string.  

package Question1_5;

public class Question {

    public static int setChar(char[] array, char c, int index, int count) {
        array[index] = c;
        index++;
        char[] cnt = String.valueOf(count).toCharArray();
        for (char x : cnt) {
            array[index] = x;
            index++;
        }
        return index;
    }
    
    public static int countCompression(String str) {
        if (str == null || str.isEmpty()) return 0;
        char last = str.charAt(0);
        int size = 0;
        int count = 1;
        for (int i = 1; i < str.length(); i++) {
            if (str.charAt(i) == last) {
                count++;
            } else {
                last = str.charAt(i);
                size += 1 + String.valueOf(count).length();
                count = 1;
            } 
        }
        size += 1 + String.valueOf(count).length();
        return size;
    }
    
    public static String compressBad(String str) {
        int size = countCompression(str);
        if (size >= str.length()) {
            return str;
        }
        String mystr = "";
        char last = str.charAt(0);
        int count = 1;
        for (int i = 1; i < str.length(); i++) {
            if (str.charAt(i) == last) {
                count++;
            } else {
                mystr += last + "" + count;
                last = str.charAt(i);
                count = 1;
            }
        }
        return mystr + last + count;
    }
    
    public static String compressBetter(String str) {
        int size = countCompression(str);
        if (size >= str.length()) {
            return str;
        }
        StringBuffer mystr = new StringBuffer();
        char last = str.charAt(0);
        int count = 1;
        for (int i = 1; i < str.length(); i++) {
            if (str.charAt(i) == last) {
                count++;
            } else {
                mystr.append(last);
                mystr.append(count);
                last = str.charAt(i);
                count = 1;
            }
        }
        mystr.append(last);
        mystr.append(count);
        return mystr.toString();
    }    
    
    public static String compressAlternate(String str) {
        int size = countCompression(str);
        if (size >= str.length()) {
            return str;
        }
        char[] array = new char[size];
        int index = 0;
        char last = str.charAt(0);
        int count = 1;
        for (int i = 1; i < str.length(); i++) {
            if (str.charAt(i) == last) {
                count++;
            } else {
                index = setChar(array, last, index, count);
                last = str.charAt(i);
                count = 1;
            }
        }
        index = setChar(array, last, index, count);
        return String.valueOf(array);
    }
    
    public static void main(String[] args) {
        String str = "abbccccccde";
        int c = countCompression(str);
        String str2 = compressAlternate(str);
        String t = compressBetter("");
        System.out.println("Compression: " + t);
        System.out.println("Old String (len = " + str.length() + "): " + str);
        System.out.println("New String (len = " + str2.length() + "): " + str2);
        System.out.println("Potential Compression: " + c);
    }
}