code: public StrBuilder insert(final int index, final char chars[]) {
          validateIndex(index);
          if (chars == null) {
              return insert(index, nullText);
          }
          final int len = chars.length;
          if (len > 0) {
              ensureCapacity(size + len);
              System.arraycopy(buffer, index, buffer, index + len, size - index);
              System.arraycopy(chars, 0, buffer, index, len);
              size += len;
          }
          return this;
      }
docs: Inserts the character array into this builder.
  Inserting null will use the stored null text value.
  
  @param index  the index to add at, must be valid
  @param chars  the char array to insert
  @return this, to enable chaining
  @throws IndexOutOfBoundsException if the index is invalid