public static String formatAmount(String amountStr) {
        try {
            // 将字符串金额解析为双精度类型
            double amount = Double.parseDouble(amountStr);
            System.out.println("格式化之前: " + amount);

            // 创建一个格式化器，设置千分位分隔符和保留两位小数
            NumberFormat formatter = new DecimalFormat("#,##0.00");
            return formatter.format(amount);
        } catch (NumberFormatException e) {
            // 处理无效的金额字符串
            e.printStackTrace();
            return amountStr;
        }
    }

    public static void main(String[] args) {
        String amountStr = "645642";
        String formattedAmount = formatAmount(amountStr);
        System.out.println("Formatted Amount: " + formattedAmount);  // 输出：Formatted Amount: 1,234,567.89

        //将数组用/字符拼接成字符串
        List<String> list = Arrays.asList("Google", "Taobao");
        Collections.reverse(list);
        String str = String.join("/", list);
        System.out.println("str ====== " + str);
    }

执行之后的结果：