left shift string implementations in Java

LeftShift

//One implementation that not consider the
//time and space complexity and use string
//as input
public final class LeftShift {
  
  //brute-force algorithm
  private String leftShiftOne(String s) {
    if(s == null)
      throw new NullPointerException("null");
    if(s.length() <= 1)
      return s;
      
    StringBuilder sb = new StringBuilder(s.subString(1, s.length()));
    sb.append(s.charAt(0));
    
    return sb.toString();
  }
  
  private String leftShiftM(String s, int m) {
    while(--m >= 0) {
      str = leftShiftOne(str);
    }
    
    return str;
  }
  
  //The third-step algorithm
  private String reverse(String s) {
    if(s == null) 
      throw new NullPointerException("null");
    if(s.length <= 1)
      return s;
      
    return new StringBuilder(s).reverse().toString();
  }
  
  private String leftShiftN(String s, int n) {
    return reverse(reverse(str.substring(0, n)).concat(reverse(str.substring(n, str.length()))));
  }
}

//Another implementation that use brute-force algorithm
//and accept char[] as input
public final class LeftShiftTwo {
  
  private void leftShiftOne(char[] arr) {
    if(arr == null)
      throw new NullPointerException("null");
    if(arr.length <= 1)
      return;
    
    char c = arr[0];
    for(int i = 1; i < arr.length; i++) {
      arr[i - 1] = arr[i];
    }
    arr[arr.length - 1] = c;
  }
  
  private void leftShiftM(char[] arr, int m) {
    while(--m >= 0) {
      leftShiftOne(arr);
    }
  }
}

//Another implementation that use three-steps algorithm
//and accept char[] as input
public final class LeftShiftThree {
  
  private void reverse(char[] arr, int begin, int end) {
    if(begin < 0 || end > arr.length || begin > end)
      throw new IndexOutOfBoundsException();

    while(begin < end) {
      char t = arr[begin];
      arr[begin++] = arr[end];
      arr[end--] = t;
    }
  }
  
  //Actually there is a bug: if the m is bigger
  //than the char array length, then how to do
  //with the algorithm
  private void leftShiftM(char[] arr, int m) {
    if(m == null)
      throw new NullPointerException("null");
    if(m.length <= 1)
      return;
      
    m %= arr.length;
    
    reverse(arr, 0, m - 1);
    reverse(arr, m, arr.length - 1);
    reverse(arr, 0, arr.length - 1);
  }
}