使用ArrayList作为Java中的值进行映射-为什么为此使用第三方库？

Question

I've recently found the need to use a Map with a Long as the key and an ArrayList of objects as the value, like this:

Map<Long, ArrayList<Object>>

But I just read here that using a third-party library like Google Guava is recommended for this. In particular, a Multimap is recommended instead of the above.

What are the main benefits of using a library to do something so simple?

Answer 1

I like the ArrayList analogy given above. Just as ArrayList saves you from array-resizing boilerplate, Multimap saves you from list-creation boilerplate.

Before:

Map<String, List<Connection>> map =
    new HashMap<>();
for (Connection connection : connections) {
  String host = connection.getHost();
  if (!map.containsKey(host)) {
    map.put(host, new ArrayList<Connection>());
  }
  map.get(host).add(connection);
}

After:

Multimap<String, Connection> multimap =
    ArrayListMultimap.create();
for (Connection connection : connections) {
  multimap.put(connection.getHost(), connection);
}

And that leads into the next advantage: Since Guava has committed to using Multimap , it includes utilities built around the class. Using them, the "after" in "before and after" should really be:

Multimap<String, Connection> multimap =
    Multimaps.index(connections, Connection::getHost);

Multimaps.index is one of many such utilities. Plus, the Multimap class itself provides richer methods than Map<K, List<V>> .

Answer 2

What is Guava details some reasoning and benefits.

For me the biggest reason would be reliability and testing. As mentioned it has been battle-tested at Google, is now very widely used elsewhere and has extensive unit testing.