啊我眼殘弄錯了，下面不是java的寫法，是javascript的……
思路上應該是類似的，你可以參考看看

可以當字串用正規表達式玩，雖然感覺有點繞，當練習XD

// 原本的字串
let originString = '1234567899';

/**
 * 去掉後三碼:
 * $1 = (\d*): 任意數量的數字0-9
 * $2 = (\d{2}): 兩個數字0-9
 * $3 = (\d{3}): 三個數字0-9
 */
let newString = originString.replace(/(\d*)(\d{2})(\d{3})/, "$1.$2");
// 加 ,
let result = parseFloat(newString).toLocaleString();

console.log(result);

public class HelloWorld{

     public static void main(String []args){
        String sA = "1234567899";
        String sB = "12345678999";
        String sTA = String.format("%1$,.2f", Double.parseDouble(sA.substring(0, sA.length() - 3)) / 100);
        String sTB = String.format("%1$,.2f", Double.parseDouble(sB.substring(0, sB.length() - 3)) / 100);
        System.out.println(sTA);
        System.out.println(sTB);
     }
}