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");
}