Comparing two strings in java character by character

I am beginner in java, I am trying to compare two strings in java char by char and find how many different chars they have by the following code but it doesn't work,

     min is the min between the 2 strings

     for(int i=0; i<min-1; i++){
            s1 = w1.substring(j,j++);
            s2 = w2.substring(j,j++);

            if (! s1.equalsIgnoreCase(s2) ){
                counter++;    
            }
      }`

Any tips?