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  1. import java.time.LocalDateTime;
  2. import java.time.format.DateTimeFormatter;
  3. import java.util.List;
  4. import java.util.Map;
  5. import java.util.stream.Collectors;
  6.  
  7. class Ideone {
  8.   private static final DateTimeFormatter FORMATTER =
  9.       DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
  10.  
  11.   public static void main(String[] args) {
  12.     List<X> list = List.of(
  13.         new X(LocalDateTime.parse("2022-01-01 01:00:00", FORMATTER)),
  14.         new X(LocalDateTime.parse("2022-01-01 01:01:00", FORMATTER)),
  15.         new X(LocalDateTime.parse("2022-01-01 02:00:00", FORMATTER)),
  16.         new X(LocalDateTime.parse("2022-01-01 03:00:00", FORMATTER)));
  17.     Map<Integer, List<X>> map = list.stream()
  18.         .collect(Collectors.groupingBy(x -> x.getT().getHour()));
  19.     System.out.println(map);
  20.   }
  21. }
  22.  
  23. class X {
  24.   private final LocalDateTime t;
  25.  
  26.   public X(LocalDateTime t) {
  27.     this.t = t;
  28.   }
  29.  
  30.   public LocalDateTime getT() {
  31.     return t;
  32.   }
  33.  
  34.   @Override
  35.   public String toString() {
  36.     return "X{t=" + t + '}';
  37.   }
  38. }
Success #stdin #stdout 0.16s 54112KB
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Standard input is empty
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{1=[X{t=2022-01-01T01:00}, X{t=2022-01-01T01:01}], 2=[X{t=2022-01-01T02:00}], 3=[X{t=2022-01-01T03:00}]}
https://ideone.com/CWLnx3
language:
Java (HotSpot 12)
created:
1 year ago
visibility:
secret

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