list中的数据用string接收

List<String> testList=new ArrayList<>();
            testList.add("a");
            testList.add("b");
            testList.add("c");

        StringBuilder sb = new StringBuilder();
        boolean flag = false;
        for (String string : testList) {
            if (flag) {
                sb.append(",");
            } else {
                flag = true;
            }
            sb.append(string);
        }
        String test = sb.toString();
        System.out.println(test);

        String num = "a";
        if(test.indexOf(num)!=-1) {
            System.out.println("包含");
        }else {
            System.out.println("不包含");
        }