java - 如何通过键列表对 Hashmap 中的列表元素进行分组

Question

我正在尝试使用 HashMap 将事件列表分组为日期，但我在实现它时遇到问题。我希望日期成为关键，而事件成为值。因此，每个与键匹配的事件都将存储在 HashMap 的列表中。到目前为止，我有一个事件列表和一个日期列表......我的代码如下所示，提前致谢。

代码

public class TestMap {

public static void main(String[] args){
    TestMap test = new TestMap();

    List<Date> mDays = new ArrayList<Date>();
    SimpleDateFormat mFormat = new SimpleDateFormat("EEEE d MMMM yyyy");
    GregorianCalendar mCalendar = (GregorianCalendar)GregorianCalendar.getInstance();
    mCalendar.setTime(new Date());
     for (int i = 0; i < 7; i++){
            mDays.add(mCalendar.getTime());
            mCalendar.add(Calendar.DAY_OF_WEEK, 1);
           System.out.println("Date: " + mFormat.format(mDays.get(i)));

      }


    List<Events> mEvents = new ArrayList<Events>();
    mEvents.add( new Events("1", "boat party", "Sunday 7 December 2014"));
    mEvents.add( new Events("2", "church meeting", "Monday 8 December 2014"));
    mEvents.add( new Events("3", "shopping", "Saturday 13 December 2014"));
    mEvents.add( new Events("4", "clubbing", "Friday 12 December 2014"));
    mEvents.add( new Events("5", "visiting", "Thursday 11 December 2014"));
    mEvents.add( new Events("6", "tour", "Wednesday 10 December 2014"));
    mEvents.add( new Events("7", "work", "Tuesday 9 December 2014"));
    mEvents.add( new Events("1", "beckie birthday", "Sunday 7 December 2014"));
    mEvents.add( new Events("2", "camping", "Monday 8 December 2014"));
    mEvents.add( new Events("3", "gym", "Saturday 13 December 2014"));
    mEvents.add( new Events("4", "working", "Friday 12 December 2014"));
    mEvents.add( new Events("5", "farming", "Thursday 11 December 2014"));
    mEvents.add( new Events("6", "attend sales", "Wednesday 10 December 2014"));
    mEvents.add( new Events("7", "work", "Tuesday 9 December 2014"));

    for(Events evt: mEvents){

        System.out.println("ID: " +evt.getEventId()+ " Title: "+evt.getEventTitle()+ "   Date:    
  "+evt.getStartDate());
    }
}

}

事件类:

class Events{

private String eventId;
private String eventTitle;
private String startDate;

public Events(String eventId, String eventTitle, String startDate) {
    super();
    this.eventId = eventId;
    this.eventTitle = eventTitle;
    this.startDate = startDate;
}
public String getEventId() {
    return eventId;
}
public void setEventId(String eventId) {
    this.eventId = eventId;
}
public String getEventTitle() {
    return eventTitle;
}
public void setEventTitle(String eventTitle) {
    this.eventTitle = eventTitle;
}
public String getStartDate() {
    return startDate;
}
public void setStartDate(String startDate) {
    this.startDate = startDate;
}

}

Answer 1

假设您按 ID 字段对项目进行分组，

// create a map which relates String-List of Event mapping
HashMap<String, List<Event>> listMap=new HashMap<String,List<Event>>();

// loop through the events
for(Events evt: mEvents){
    if(!listMap.containsKey(evt.getID())){
        listMap.put(evt.getID(), new ArrayList<Event>());
    }
    listMap.get(evt.getID()).add(evt);
}

// if you want to get a list of all events with the same ID,
Collection<Event> eventWithSpecificId=listMap.get("certain ID");