java - 计算 ArrayList 中特定变量的出现次数

Question

在我的类Feeds中，我与其他成员一起有一个名为“Date”的成员变量，它是String类型。我有一个包含 Feeds 对象的 ArrayList。我想查找具有相同日期字符串的对象的出现情况。然后，可以将出现的次数放入 HashMap 中，其中包含 String 日期作为键，出现次数作为值。

大致如下:

List<Feeds> m_feeds = new ArrayList<Feeds>();

//add all feeds objects
m_feeds.add(...);

int occurrences = 0;
HashMap<String, Integer> repeatedDatabase = new HashMap<String, Integer>();

for (Feeds f : m_feeds){
     occurrences = Collections.frequency(m_feeds, f.date);
     // i know this method compares objects but i want to know how 
    // only a single variable can be done
    repeatedDatabase.put(f.date, occurrences);
}

Answer 1

除了给您一个简单的解决方案之外，我还冒昧地修复了您代码中的一些问题，请看一下:

List<Feeds> mFeeds = new ArrayList<>(); //If you are using Java 7+ you do not need to declare explicitly the Type in Diamonds. If you aren't, ignore this. Also fixed name to adapt to Java standards.

//add all feeds objects
m_feeds.add(...);

HashMap<String, Integer> repeatedDatabase = new HashMap<>(); //See Above.

for (Feeds f : m_feeds){
    String s = f.date; //Suggestion: use a getter method, do not make public variables accessible outside class
    Integer i = repeatedDatabase.get(s);
    if (i == null){
        repeatedDatabase.put(s, 1);
    } else {
        repeatedDatabase.put(s, i+1);
    } 
}