我有一个具有(java.util。)Date属性的项目列表,并且我想为最早的日期到现在的每一天创建一个DataSeriesItem 。它用于带有时间轴的图表系列。
该DataSeriesItem的创建将如下所示:DataSeriesItem seriesItem = new DataSeriesItem(Date, occurrenceCount);
其中occurrenceCount
是其Date属性与当天匹配的项的计数。第一个参数也可以是java.time.Instant
类型
我设法找到了一种可行的方法,但是我确信我的方法很糟糕,可以用一个流,也可以两个流来完成。但是,我是流媒体方面的初学者,但据我所知无法做到。
流可能吗?大概大概是什么样子?
我并不是要您真正地重新进行整个实现,而只是指出要使用的正确的流函数和映射,并给出一个奖励示例。
这是我的丑陋解决方案:List<?> items = myItems;
Collection<Date> foundDates = new HashSet<>();
for (Object item : items) {
foundDates.add((Date)getPropertyValueFromItem(item, configurator.getDateProperty()));
}
//====== This is the part I am asking about ======//
Map<Instant, Integer> foundInstants = new HashMap<>();
foundDates.stream().sorted(Date::compareTo).forEach(date -> {
Calendar c = Calendar.getInstance();
c.clear(); // clear nanoseconds, or else equals won't work!
c.set(date.getYear()+1900, date.getMonth(), date.getDate(), 0, 0, 0);
if(!foundInstants.containsKey(c.toInstant())){
foundInstants.put(c.toInstant(), 1);
} else {
// increment count of that entry
Integer value = foundInstants.get(c.toInstant());
foundInstants.remove(c.toInstant());
foundInstants.put(c.toInstant(), ++value);
}
});
//====== Leaving this code here for context ======//
// Could this maybe simplyfied too by using streams ?
// find oldest date
Date dateIndex = foundDates.stream().min(Date::compareTo).get();
Date now = new Date();
// starting from oldest date, add a seriesItem for each day until now
// if dateOccurrences contains the current/iterated date, use it's value, else 0
while(dateIndex.before(now)){
Calendar c = Calendar.getInstance();
c.clear();// clear nanoseconds, or else equals won't work!
c.set(dateIndex.getYear()+1900, dateIndex.getMonth(), dateIndex.getDate(), 0, 0, 0);
if(foundInstants.containsKey(c.toInstant())){
ExtendedDataSeriesItem seriesItem = new ExtendedDataSeriesItem(c.toInstant(), foundInstants.get(c.toInstant()));
seriesItem.setSeriesType("singleDataPoint");
series.add(seriesItem);
} else {
ExtendedDataSeriesItem seriesItem = new ExtendedDataSeriesItem(c.toInstant(), 0);
seriesItem.setSeriesType("singleDataPoint");
series.add(seriesItem);
}
c.add(Calendar.DATE, 1); // adding a day is complicated. Calendar gets it right. Date does not. This is why I don't use Date here
dateIndex = c.getTime();
}
最佳答案
您可以使用groupingBy()
,然后使用下游收集器counting()
。
Map<Date, Long> occurrances = dateList.stream().collect(
groupingBy(d -> yourTransformation(d), counting()));
从该 map 创建
DataSeriesItem
对象应该足够容易。