code: public StrBuilder del(int start, int end) {
  		if (start < 0) {
  			start = 0;
  		}
  		if (end > this.position) {
  			end = this.position;
  		}
  		if (start > end) {
  			throw new StringIndexOutOfBoundsException("Start is greater than End.");
  		}
  		if (end == this.position) {
  			this.position = start;
  		}
  
  		int len = end - start;
  		if (len > 0) {
  			System.arraycopy(value, start + len, value, start, this.position - end);
  			this.position -= len;
  		}
  		return this;
  	}
docs: 删除指定长度的字符
  
  @param start 开始位置（包括）
  @param end 结束位置（不包括）
  @return this