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  1. /* package whatever; // don't place package name! */
  2.  
  3. import java.io.*;
  4. import java.lang.*;
  5. import java.util.*;
  6. import java.util.Comparator;
  7. import java.util.concurrent.*;
  8. import java.util.function.*;
  9. import java.util.stream.*;
  10.  
  11. class Car {
  12.   int num;
  13.   String engine;
  14.  
  15.   public Car(int num, String engine) {
  16.     this.num = num;
  17.     this.engine = engine;
  18.   }
  19.  
  20.   public Integer getNum() {
  21.     return this.num;
  22.   }
  23.  
  24.   public String getEngine() {
  25.     return this.engine;
  26.   }
  27.  
  28.   public String toString() {
  29.     return String.format("Car: %d, %s", num, engine);
  30.   }
  31. }
  32.  
  33. /* Name of the class has to be "Main" only if the class is public. */
  34. class Ideone {
  35.   public static <T> Predicate<T> distinctByKey(
  36.       Function<? super T, ?> keyExtractor) {
  37.     Set<Object> seen = ConcurrentHashMap.newKeySet();
  38.     return t -> seen.add(keyExtractor.apply(t));
  39.   }
  40.  
  41.   public static Comparator<Car> withNullEnginesLast() {
  42.     return Comparator.comparing(Car::getNum)
  43.         .thenComparing(
  44.             Car::getEngine, Comparator.nullsLast(Comparator.naturalOrder()));
  45.   }
  46.  
  47.   public static void main(String[] args) throws java.lang.Exception {
  48.     // your code goes here
  49.  
  50.     var carList = List.of(
  51.         // num, engine
  52.         new Car(1, "S02K"), new Car(1, null), new Car(2, null),
  53.         new Car(2, "S9K"), new Car(3, null));
  54.  
  55.     var result =
  56.         carList.stream()
  57.             .sorted(withNullEnginesLast()) // sort first by num, then engine
  58.                                            // where null-engines last
  59.             .filter(distinctByKey(Car::getNum))
  60.             .collect(Collectors.toList());
  61.  
  62.     System.out.println(result);
  63.   }
  64. }
Success #stdin #stdout 0.13s 42952KB
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[Car: 1, S02K, Car: 2, S9K, Car: 3, null]
https://ideone.com/69nfTx
language:
Java (HotSpot 12)
created:
2 years ago
visibility:
secret

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