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  1. import java.util.*;
  2.  
  3. class Example {
  4.     public static void main(String[] args) {
  5.         Set<String> identityA = newIdentityHashSet();
  6.         Set<String> identityB = newIdentityHashSet();
  7.  
  8.         identityA.add("X");
  9.         identityB.add(new String("X"));
  10.  
  11.         System.out.println("This should be [X, X]:");
  12.         System.out.println("    " + unionInHashSet(identityA, identityB));
  13.  
  14.         System.out.println("Returning an identity hash set solves this:");
  15.         System.out.println("    " + unionInIdentityHashSet(identityA, identityB));
  16.  
  17.         Set<String> a = new HashSet<>(identityA);
  18.         Set<String> b = new HashSet<>(identityB);
  19.  
  20.         System.out.println("But now the union of regular sets is messed up:");
  21.         System.out.println("    " + unionInIdentityHashSet(a, b));
  22.     }
  23.  
  24.     static <T> Set<T> unionInHashSet(Set<T> a, Set<T> b) {
  25.         Set<T> union = new HashSet<>();
  26.         union.addAll(a);
  27.         union.addAll(b);
  28.         return union;
  29.     }
  30.  
  31.     static <T> Set<T> unionInIdentityHashSet(Set<T> a, Set<T> b) {
  32.         Set<T> union = newIdentityHashSet();
  33.         union.addAll(a);
  34.         union.addAll(b);
  35.         return union;
  36.     }
  37.  
  38.     static <T> Set<T> newIdentityHashSet() {
  39.         return Collections.newSetFromMap(new IdentityHashMap<>());
  40.     }
  41. }
Success #stdin #stdout 0.04s 2184192KB
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stdin
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Standard input is empty
stdout
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This should be [X, X]:
    [X]
Returning an identity hash set solves this:
    [X, X]
But now the union of regular sets is messed up:
    [X, X]
https://ideone.com/mRlF0k
language:
Java (HotSpot 12)
created:
6 years ago
visibility:
public

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