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  1. /* package whatever; // don't place package name! */
  2.  
  3. import java.util.*;
  4. import java.lang.*;
  5. import java.io.*;
  6. import java.text.SimpleDateFormat;
  7. import java.time.temporal.ChronoUnit;
  8. import java.util.stream.Collectors;
  9.  
  10. /* Name of the class has to be "Main" only if the class is public. */
  11. class Ideone
  12. {
  13.     public static void main (String[] args) throws java.lang.Exception{
  14.  
  15.         SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm");
  16.         List<Entry> list = Arrays.asList(new Entry(sdf.parse("2011-03-21 09:00"), "VALUE1")
  17.                 , new Entry(sdf.parse("2011-03-21 09:00"), "VALUE2")
  18.                 , new Entry(sdf.parse("2011-03-22 09:00"), "VALUE3")
  19.                 , new Entry(sdf.parse("2011-03-22 09:00"), "VALUE4")
  20.                 , new Entry(sdf.parse("2011-03-21 09:00"), "VALUE5")
  21.         );
  22.  
  23.         Map<Date, List<Entry>> entries = list.stream().collect(Collectors.groupingBy(e ->
  24.                 // easier way to truncate the date
  25.                 Date.from(e.getDate().toInstant().truncatedTo(ChronoUnit.DAYS)),TreeMap::new, Collectors.toList()) //added treemap to sort by keys
  26.         );
  27.  
  28.         //--- printing---
  29.  
  30.         SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
  31.         format.setTimeZone(TimeZone.getTimeZone("GMT"));     // using GMT timezone to avoid inconsistencies when printing
  32.  
  33.         entries.forEach((date, entries1) -> {
  34.             System.out.println(format.format(date));
  35.             entries1.forEach(entry -> System.out.println("\t" + entry.getValue()));
  36.         });
  37.     }
  38. }
  39.  
  40. class Entry {
  41.     private final Date date;
  42.     private final String value;
  43.  
  44.     public Entry(Date date, String value) {
  45.         this.date = date;
  46.         this.value = value;
  47.     }
  48.  
  49.     public Date getDate() {
  50.         return date;
  51.     }
  52.  
  53.     public String getValue() {
  54.         return value;
  55.     }
  56.  
  57.     @Override
  58.     public String toString() {
  59.         return "Entry{" +
  60.                 "date=" + date +
  61.                 ", value='" + value + '\'' +
  62.                 '}';
  63.     }
  64. }
Success #stdin #stdout 0.26s 35996KB
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2011-03-21
	VALUE1
	VALUE2
	VALUE5
2011-03-22
	VALUE3
	VALUE4
https://ideone.com/4MwwHk
language:
Java (HotSpot 12)
created:
7 years ago
visibility:
public

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