您无需将 ArrayList 重新添加回您的 Map.如果 ArrayList 已经存在，那么只需将您的值添加到它.

改进的实现可能如下所示:

Map>map = new HashMap>();

在处理每一行时:

String user = 行中的用户字段字符串值 = 行中的值字段集合<字符串>值 = map.get(user);如果(值==空){值 = 新的 ArrayList<String>();map.put(用户，值)}values.add(value);

2014 年 4 月跟进 - 我在 2009 年写了原始答案，当时我对 Google Guava 的了解有限.鉴于 Google Guava 所做的一切，我现在建议使用它的 Multimap 而不是重新发明它.

Multimap值 = HashMultimap.create();values.put("user1", "value1");values.put("user2", "value2");values.put("user3", "value3");values.put("user1", "value4");System.out.println(values.get("user1"));System.out.println(values.get("user2"));System.out.println(values.get("user3"));

输出:

[value4, value1][值2][值3]

I have one million rows of data in .txt format. the format is very simple. For each row:

user1,value1
user2,value2
user3,value3
user1,value4
...

You know what I mean. For each user, it could appear many times, or appear only once (you never know). I need to find out all the values for each user. Because user may appear randomly, I used Hashmap to do it. That is: HashMap(key: String, value: ArrayList). But to add data to the arrayList, I have to constantly use HashMap get(key) to get the arrayList, add value to it, then put it back to HashMap. I feel it is not that very efficient. Anybody knows a better way to do that?

You don't need to re-add the ArrayList back to your Map. If the ArrayList already exists then just add your value to it.

An improved implementation might look like:

Map<String, Collection<String>> map = new HashMap<String, Collection<String>>();

while processing each line:

String user = user field from line
String value = value field from line

Collection<String> values = map.get(user);
if (values==null) {
    values = new ArrayList<String>();
    map.put(user, values)
}
values.add(value);

Follow-up April 2014 - I wrote the original answer back in 2009 when my knowledge of Google Guava was limited. In light of all that Google Guava does, I now recommend using its Multimap instead of reinvent it.

Multimap<String, String> values = HashMultimap.create();
values.put("user1", "value1");
values.put("user2", "value2");
values.put("user3", "value3");
values.put("user1", "value4");

System.out.println(values.get("user1"));
System.out.println(values.get("user2"));
System.out.println(values.get("user3"));

Outputs:

[value4, value1]
[value2]
[value3]