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  1. /* package whatever; // don't place package name! */
  2.  
  3. import java.util.*;
  4. import java.lang.*;
  5. import java.util.stream.Collectors;
  6.  
  7. /* Name of the class has to be "Main" only if the class is public. */
  8. class Ideone
  9. {
  10. 	public static void main (String[] args) throws java.lang.Exception
  11. 	{
  12. 		List<String> listKeys = new ArrayList<>(List.of("A", "B", "C"));
  13.  
  14.         Map<User, Long> mapUsers = new HashMap<>();
  15.         mapUsers.put(new User("Annie", "A"), 23L);
  16.         mapUsers.put(new User("Paul", "C"), 16L);
  17.  
  18.         List<UserCount> listRes = listKeys.stream()
  19.                 .map(key -> {
  20.                     User user = mapUsers.keySet().stream().filter(u -> u.getKey().equals(key)).findFirst().orElse(null);
  21.                     return new UserCount(key, user != null ? mapUsers.get(user) : 0L);
  22.                 })
  23.                 .collect(Collectors.toList());
  24.  
  25.         System.out.println(listRes);
  26. 	}
  27. }
  28.  
  29. class User {
  30.     private String name, key;
  31.  
  32.     public User(String name, String key) {
  33.         this.name = name;
  34.         this.key = key;
  35.     }
  36.  
  37.     public String getName() {
  38.         return name;
  39.     }
  40.  
  41.     public String getKey() {
  42.         return key;
  43.     }
  44.  
  45.     @Override
  46.     public boolean equals(Object obj) {
  47.         if (obj == this) return true;
  48.         if (obj == null) return false;
  49.         if (obj.getClass() != getClass()) return false;
  50.         User other = (User) obj;
  51.         return Objects.equals(name, other.getName()) && Objects.equals(key, other.getKey());
  52.     }
  53.  
  54.     @Override
  55.     public int hashCode() {
  56.         return Objects.hash(name, key);
  57.     }
  58.  
  59.     public String toString() {
  60.         return String.format("%s - %s", name, key);
  61.     }
  62. }
  63.  
  64. class UserCount {
  65.     private String key;
  66.     private long count;
  67.  
  68.     public UserCount(String key, long count) {
  69.         this.key = key;
  70.         this.count = count;
  71.     }
  72.  
  73.     public String getKey() {
  74.         return key;
  75.     }
  76.  
  77.     public long getCount() {
  78.         return count;
  79.     }
  80.  
  81.     @Override
  82.     public boolean equals(Object obj) {
  83.         if (obj == this) return true;
  84.         if (obj == null) return false;
  85.         if (obj.getClass() != getClass()) return false;
  86.         UserCount other = (UserCount) obj;
  87.         return Objects.equals(key, other.getKey()) && Objects.equals(count, other.getCount());
  88.     }
  89.  
  90.     @Override
  91.     public int hashCode() {
  92.         return Objects.hash(key, count);
  93.     }
  94.  
  95.     public String toString() {
  96.         return String.format("%s - %d", key, count);
  97.     }
  98. }
Success #stdin #stdout 0.12s 48920KB
comments ()
stdin
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Standard input is empty
stdout
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[A - 23, B - 0, C - 16]
https://ideone.com/FNm9XS
How to use Map.computeIfAbsent() in a stream?

https://stackoverflow.com/questions/72231281/how-to-use-map-computeifabsent-in-a-stream
language:
Java (HotSpot 12)
created:
2 years ago
visibility:
public

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