codewars031: Reverse or rotate?

6 kyu https://www.codewars.com/kata/56b5afb4ed1f6d5fb0000991/train/java

//https://www.codewars.com/kata/56b5afb4ed1f6d5fb0000991/train/java
package codewars;
public class RevRot{
  private static Boolean divisible(StringBuilder s){
    long sum = 0;
    for(int i = 0; i < s.length(); i++){
      sum += Math.pow(Integer.valueOf(s.charAt(i)),3);
    }
    if(sum % 2 == 0){
      return true;
    }else{
      return false;
    }
  }
  private static String rotate(StringBuilder sb){
    sb.append(sb.charAt(0));
    sb.deleteCharAt(0);
    return sb.toString();
  }
  public static String revRot(String str, int sz){
    if(sz <= 0 || str == null || "".equals(str) || sz > str.length()){
      return "";
    }
    StringBuilder[] sb = new StringBuilder[str.length() / sz];
    for(int i = 0; i < sb.length; i++){
      sb[i] = new StringBuilder();
      for(int j = 0; j < sz; j++){
        sb[i].append(str.charAt(j + i * sz));
      }
    }
    for(int i = 0; i < sb.length; i++){
      if(RevRot.divisible(sb[i])){ 
        sb[i] = sb[i].reverse();
      }else{
        sb[i] = new StringBuilder(RevRot.rotate(sb[i]));
      }
    }
    StringBuilder re = new StringBuilder();
    for(String x : sb){
      re.append(x);
    }
    return re.toString();
  }
}