对对象列表进行分组并使用Java集合进行计数 [英] grouping List of Objects and counting using Java collection

问题描述

哪个Java Collection类最好对对象列表进行分组？

Which Java Collection class is better to group the list of objects?

我有来自以下用户的消息列表：

I have a list of messages from users like below:

aaa hi
bbb hello
ccc Gm
aaa  Can?
CCC   yes
ddd   No

从此消息对象列表中我想计算并显示 aaa（2）+ bbb（1）+ ccc（2）+ ddd（1）。有代码帮助吗？

From this list of message object I want to count and display aaa(2)+bbb(1)+ccc(2)+ddd(1). Any code help?

推荐答案

将其他几个答案拼凑在一起，以适应另一个问题中的代码，并且修复一些琐碎的错误：

Putting the pieces together from a couple of the other answers, adapting to your code from the other question and fixing a few trivial errors:

    // as you want a sorted list of keys, you should use a TreeMap
    Map<String, Integer> stringsWithCount = new TreeMap<>();
    for (Message msg : convinfo.messages) {
        // where ever your input comes from: turn it into lower case,
        // so that "ccc" and "CCC" go for the same counter
        String item = msg.userName.toLowerCase();
        if (stringsWithCount.containsKey(item)) {
            stringsWithCount.put(item, stringsWithCount.get(item) + 1);
        } else {
            stringsWithCount.put(item, 1);
        }
    }
    String result = stringsWithCount
            .entrySet()
            .stream()
            .map(entry -> entry.getKey() + '(' + entry.getValue() + ')')
            .collect(Collectors.joining("+"));
    System.out.println(result);

此打印：

aaa(2)+bbb(1)+ccc(2)+ddd(1)

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