莫不是这个? 看你的程序也没有考虑 trim(), 也没有考虑 分隔符替换啥的

@Test
public void test(){
    String ids1 = "4,1,2,3";
    String ids2 = "1,2,3,4,9,10,12,14,";
    System.out.println(ids1 + ids2.substring(ids1.length()));
    //or
    System.out.println(ids2.replaceFirst("(\\d+,){3}\\d+", ids1));
}

如果是按 ids1 中 数字个数来 进行替换. 首先 计算数字个数. 如果ids2中没有那么多数字, 则不做替换.

@Test
public void test(){
    String ids1 = "4,1,2,3";
    String ids2 = "1,2,3,4,9,10,12,14,";
    int count = ids1.split(",").length;
    //or count for , first
    //int count = ids1.length() - ids1.replace(",", "").length() + 1;
    System.out.println(ids2.replaceFirst("(\\d+,){" + (count-1) + "}\\d+", ids1));
}

可以用 StringBuffer 的 replace 函数啊。

class replace {
    public static void main(String[] args) {
        String ids1 = "4,1,2,3";
        String ids2 = "1,2,3,4,9,10,12,14";

        // String[] ids1s = ids1.split(",");
        // String[] ids2s = ids2.split(",");

        // for (int i = 0; i < ids1s.length; i++) {
        //     ids2s[i] = ids1s[i];
        // }

        // StringBuffer ids3 = new StringBuffer();
        // for (int i = 0; i < ids2s.length; i++) {
        //     if (i == 0) {
        //         ids3.append(ids2s[i]);
        //     }else{
        //         ids3.append(","+ids2s[i]);
        //     }
        // }

        // System.out.println(ids3.toString());
        StringBuffer ids4 = new StringBuffer(ids2);
        ids4.replace(0,7,ids1);
        System.out.println(ids4.toString());
    }
}