StringBuilder的append的方法

   public StringBuilder append(CharSequence s) {
        if (s == null)
            s = "null";
        if (s instanceof String)
            return this.append((String)s);
        if (s instanceof StringBuffer)
            return this.append((StringBuffer)s);
        if (s instanceof StringBuilder)
            return this.append((StringBuilder)s);
        return this.append(s, 0, s.length());
    }

    /**
     * @throws     IndexOutOfBoundsException {@inheritDoc}
     */
    public StringBuilder append(CharSequence s, int start, int end) {
        super.append(s, start, end);
        return this;
    }

  其中 CharSequence 是 String的父类，String继承与他

StringBuilder.append(添加的字符本身，从字符的第一个下标开始，到最后)