The following does not work in all cases. so solution has to be better than this:

    `
    StringBuilder compressed = new StringBuilder();
    int len = input.length();

    for(int i=0; i < len; i  ) {
        int counter = 1;
        while(i < len-1 && input.charAt(i) == input.charAt(i 1)) {
            i  ;
            counter  ;
        }
        compressed.append(counter);
        compressed.append(input.charAt(i));
    }
    return compressed.toString();

`

The solution that is immediately obvious to me is to only append the counter variable to the string if counter > 1 (there was a run).

if(counter > 1) compressed.append(counter);
compressed.append(input.charAt(i));

If there are no runs in a string, the output will simply be the input string.

Without much context, I'm not sure if this fits the criteria of the problem, because it's not truly RLE, but it is a potential solution.