Java8-如何将List转变为逗号分隔的字符串

List<String> cities = Arrays.asList("Milan", 
                                    "London", 
                                    "New York", 
                                    "San Francisco");
String citiesCommaSeparated = String.join(",", cities);
System.out.println(citiesCommaSeparated);
//Output: Milan,London,New York,San Francisco

String citiesCommaSeparated = cities.stream()
                                    .collect(Collectors.joining(","));
System.out.println(citiesCommaSeparated);

String citiesCommaSeparated = cities.stream()
                                    .map(String::toUpperCase)
                                    .collect(Collectors.joining(","));
//Output: MILAN,LONDON,NEW YORK,SAN FRANCISCO