package string;

public class CompressChar {

    int[] seq = new int [256];


    public String compressString(String str){

        StringBuffer strComp = new StringBuffer();
        for( char c : str.toCharArray()){
            seq[c]++;
        }
        for (char c : str.toCharArray()){
            if(seq[c]>0){
                strComp.append(c).append(seq[c]);
                seq[c]=0;//so that it does not enter , when char occurs again
            }
        }

        if(str.length()<strComp.length()){
            return str;
        }
        return strComp.toString();

    }
    public static void main(String[] args) {
        CompressChar ch = new CompressChar();
        System.out.println(ch.compressString("abbcdrfac"));
    }
}

a11b5c6d1r1f1

a5b5c1d1r1f1a6c5

StringBuilder sb = new StringBuilder();

char[] charArray = str.toCharArray()
for(char c: charArray){
...
}
// reuse the same array again