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HashSet order and difference with JDK 7/8

This is a two-part question:

  • Does a HashSet implement some sort of hidden ordering mechanic or just to quote the documentation: It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time.does it tell me that the MIGHT order sometimes changes in the future and / or depending on memory usage?
  • Why do I get completely different “orders” (I dare say) when I switch between JDKs?

Take this example:

for (int i = 0; i < 1000; i++) {
        Set<String> stringSet = new HashSet<>();
        stringSet.add("qwe");
        stringSet.add("rtz");
        stringSet.add("123");
        stringSet.add("qwea");
        stringSet.add("12334rasefasd");
        stringSet.add("asdxasd");
        stringSet.add("arfskt6734");
        stringSet.add("123121");
        stringSet.add("");
        stringSet.add("qwr");
        stringSet.add("rtzz");
        stringSet.add("1234");
        stringSet.add("qwes");
        stringSet.add("1234rasefasd");
        stringSet.add("asdxasdq");
        stringSet.add("arfskt6743");
        stringSet.add("123121 ");
        stringSet.add(" ");
        System.out.println(stringSet);
    }

will, regardless of how many times I run it, output the following output:

JDK 7: [, , 123, qwea, asdxasdq, qwe, qwr, 123121 , arfskt6743, 1234rasefasd, qwes, rtz, rtzz, 1234, 12334rasefasd, asdxasd, arfskt6734, 123121]

JDK 8: [, , qwes, arfskt6743, asdxasdq, 123121, 123121 , arfskt6734, qwr, 123, 1234, qwea, rtzz, rtz, 12334rasefasd, 1234rasefasd, qwe, asdxasd]

Obviously, an empty line and a line only for spaces lead both times, but the rest is completely different.

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4 answers

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public static void main(String... args) {
  System.out.println(System.getProperty("java.version"));
  List<String> strings=Arrays.asList("qwe", "rtz", "123", "qwea",
      "12334rasefasd", "asdxasd", "arfskt6734", "123121", "", "qwr",
      "rtzz", "1234", "qwes", "1234rasefasd", "asdxasdq", "arfskt6743",
      "123121 ", " ");

  for (int i = 5; i < 26; i++) {
      Set<String> stringSet = new HashSet<>(1<<i);
      stringSet.addAll(strings);
      System.out.println(stringSet);
  }
}

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1.7.0_51
[,  , qwea, 123, asdxasdq, qwe, qwr, 123121 , arfskt6743, 1234rasefasd, qwes, rtzz, rtz, 1234, 12334rasefasd, arfskt6734, asdxasd, 123121]
[, qwr, arfskt6743, rtzz, 12334rasefasd,  , qwea, 123, asdxasdq, qwe, 123121 , 1234rasefasd, qwes, rtz, 1234, arfskt6734, asdxasd, 123121]
[, qwr, arfskt6743, rtzz, 12334rasefasd,  , 123, rtz, 1234, arfskt6734, asdxasd, 123121, qwea, asdxasdq, qwe, 123121 , 1234rasefasd, qwes]
[, qwr, arfskt6743, rtzz, 12334rasefasd,  , arfskt6734, asdxasd, 123121, 123121 , 1234rasefasd, 123, rtz, 1234, qwea, asdxasdq, qwe, qwes]
[, rtzz,  , 123121, 123121 , 1234rasefasd, 123, rtz, qwea, asdxasdq, qwe, qwes, qwr, arfskt6743, 12334rasefasd, arfskt6734, asdxasd, 1234]
[, rtzz,  , 123121, 123, asdxasdq, 123121 , 1234rasefasd, rtz, qwea, qwe, qwes, qwr, arfskt6743, 12334rasefasd, arfskt6734, asdxasd, 1234]
[,  , 123121, asdxasdq, 1234rasefasd, rtz, qwea, qwes, arfskt6743, 12334rasefasd, arfskt6734, asdxasd, rtzz, 123, 123121 , qwe, qwr, 1234]
[,  , 123121, 1234rasefasd, rtz, qwea, qwes, asdxasd, rtzz, 123, 1234, asdxasdq, arfskt6743, 12334rasefasd, arfskt6734, 123121 , qwe, qwr]
[,  , rtz, asdxasd, rtzz, arfskt6743, 12334rasefasd, arfskt6734, 123121 , qwe, qwr, 123121, 1234rasefasd, qwea, qwes, 123, 1234, asdxasdq]
[,  , arfskt6743, 12334rasefasd, arfskt6734, qwea, qwes, 1234, asdxasdq, rtz, asdxasd, rtzz, 123121 , qwe, qwr, 123121, 1234rasefasd, 123]
[,  , qwea, qwes, rtz, asdxasd, rtzz, 123121 , qwe, qwr, 123, arfskt6743, 12334rasefasd, arfskt6734, 1234, asdxasdq, 123121, 1234rasefasd]
[,  , qwea, qwes, asdxasd, 1234, asdxasdq, 123121, 1234rasefasd, rtz, rtzz, 123121 , qwe, qwr, 123, arfskt6743, 12334rasefasd, arfskt6734]
[,  , qwea, qwes, asdxasd, 1234, 1234rasefasd, rtzz, 123121 , 123, arfskt6743, arfskt6734, asdxasdq, 123121, rtz, qwe, qwr, 12334rasefasd]
[,  , 1234rasefasd, 123, asdxasdq, rtz, qwe, qwr, 12334rasefasd, qwea, qwes, asdxasd, 1234, rtzz, 123121 , arfskt6743, arfskt6734, 123121]
[,  , 123, asdxasdq, rtz, qwe, qwr, 12334rasefasd, 123121 , 123121, 1234rasefasd, qwea, qwes, asdxasd, 1234, rtzz, arfskt6743, arfskt6734]
[,  , 123, asdxasdq, rtz, qwe, qwr, 12334rasefasd, 1234rasefasd, qwea, qwes, asdxasd, 1234, rtzz, 123121 , 123121, arfskt6743, arfskt6734]
[,  , 123, rtz, qwe, qwr, 12334rasefasd, asdxasd, asdxasdq, 1234rasefasd, qwea, qwes, 1234, rtzz, 123121 , 123121, arfskt6743, arfskt6734]
[,  , 123, rtz, qwe, qwr, asdxasd, asdxasdq, 1234, arfskt6743, arfskt6734, 12334rasefasd, 1234rasefasd, qwea, qwes, rtzz, 123121 , 123121]
[,  , 123, rtz, qwe, qwr, asdxasdq, 1234, 12334rasefasd, 1234rasefasd, qwea, qwes, rtzz, 123121 , 123121, asdxasd, arfskt6743, arfskt6734]
[,  , 123, rtz, qwe, qwr, 1234, 12334rasefasd, qwea, qwes, rtzz, 123121 , asdxasdq, 1234rasefasd, 123121, asdxasd, arfskt6743, arfskt6734]
[,  , 123, rtz, qwe, qwr, 1234, qwea, qwes, rtzz, 12334rasefasd, 123121 , asdxasdq, 1234rasefasd, 123121, asdxasd, arfskt6743, arfskt6734]

1.8.0_111
[,  , qwes, arfskt6743, asdxasdq, 123121, 123121 , arfskt6734, qwr, 123, 1234, qwea, rtzz, rtz, 12334rasefasd, 1234rasefasd, qwe, asdxasd]
[, 123121, arfskt6734, qwr, 1234, asdxasd,  , qwes, arfskt6743, asdxasdq, 123121 , 123, qwea, rtzz, rtz, 12334rasefasd, 1234rasefasd, qwe]
[, arfskt6734, qwr,  , arfskt6743, asdxasdq, 123, rtzz, 123121, 1234, asdxasd, qwes, 123121 , qwea, rtz, 12334rasefasd, 1234rasefasd, qwe]
[, qwr,  , 123, rtzz, 1234, asdxasd, qwes, 123121 , qwea, rtz, 1234rasefasd, arfskt6734, arfskt6743, asdxasdq, 123121, 12334rasefasd, qwe]
[,  , 123, 1234, rtz, arfskt6734, arfskt6743, asdxasdq, 123121, 12334rasefasd, qwe, qwr, rtzz, asdxasd, qwes, 123121 , qwea, 1234rasefasd]
[,  , 1234, asdxasdq, 123121, 12334rasefasd, rtzz, asdxasd, qwes, 123121 , qwea, 123, rtz, arfskt6734, arfskt6743, qwe, qwr, 1234rasefasd]
[,  , 1234, 12334rasefasd, qwes, qwea, rtz, asdxasdq, 123121, rtzz, asdxasd, 123121 , 123, arfskt6734, arfskt6743, qwe, qwr, 1234rasefasd]
[,  , asdxasdq, rtzz, 123121 , arfskt6734, arfskt6743, qwe, qwr, 1234, 12334rasefasd, qwes, qwea, rtz, 123121, asdxasd, 123, 1234rasefasd]
[,  , asdxasdq, 123121 , arfskt6734, arfskt6743, 1234, 12334rasefasd, qwes, qwea, 123121, asdxasd, 1234rasefasd, rtzz, qwe, qwr, rtz, 123]
[,  , asdxasdq, 1234, rtzz, 123121 , arfskt6734, arfskt6743, 12334rasefasd, qwes, qwea, 123121, asdxasd, 1234rasefasd, qwe, qwr, rtz, 123]
[,  , 1234, rtzz, 123121 , arfskt6734, arfskt6743, 12334rasefasd, qwes, qwea, 123121, asdxasd, qwe, qwr, rtz, 123, asdxasdq, 1234rasefasd]
[,  , 1234, 123121 , arfskt6734, arfskt6743, qwes, qwea, asdxasd, rtzz, 12334rasefasd, 123121, qwe, qwr, rtz, 123, asdxasdq, 1234rasefasd]
[,  , arfskt6734, arfskt6743, asdxasd, 123, asdxasdq, 1234rasefasd, 1234, 123121 , qwes, qwea, rtzz, 12334rasefasd, 123121, qwe, qwr, rtz]
[,  , arfskt6734, arfskt6743, 123, asdxasdq, 123121 , qwes, qwea, rtzz, 12334rasefasd, 123121, qwe, qwr, rtz, asdxasd, 1234rasefasd, 1234]
[,  , 123, 123121 , qwe, qwr, rtz, asdxasd, 1234rasefasd, arfskt6734, arfskt6743, asdxasdq, qwes, qwea, rtzz, 12334rasefasd, 123121, 1234]
[,  , 123, qwe, qwr, rtz, asdxasd, 1234rasefasd, arfskt6734, arfskt6743, qwes, qwea, rtzz, 123121, 1234, 123121 , asdxasdq, 12334rasefasd]
[,  , 123, qwe, qwr, rtz, arfskt6734, arfskt6743, 123121 , asdxasdq, asdxasd, 1234rasefasd, qwes, qwea, rtzz, 123121, 1234, 12334rasefasd]
[,  , 123, qwe, qwr, rtz, arfskt6734, arfskt6743, 123121 , 1234, 12334rasefasd, asdxasdq, asdxasd, 1234rasefasd, qwes, qwea, rtzz, 123121]
[,  , 123, qwe, qwr, rtz, arfskt6734, arfskt6743, 1234, asdxasdq, asdxasd, 1234rasefasd, qwes, qwea, rtzz, 123121 , 12334rasefasd, 123121]
[,  , 123, qwe, qwr, rtz, 1234, asdxasdq, asdxasd, qwes, qwea, rtzz, 123121 , 12334rasefasd, 123121, arfskt6734, arfskt6743, 1234rasefasd]
[,  , 123, qwe, qwr, rtz, 1234, qwes, qwea, rtzz, 123121 , 123121, arfskt6734, arfskt6743, asdxasdq, asdxasd, 12334rasefasd, 1234rasefasd]

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 h ^= (h >>> 20) ^ (h >>> 12);
 return h ^ (h >>> 7) ^ (h >>> 4);

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return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);

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Ans 1: Hashset , , , .

Ans 2: the result of the iteration may differ in versions of java, since it depends on the implementation of this version in hashcode. but the Hashset provides one guarantee that the iteration order will never get a means of changing after inserting the elements if you repeat it every time you get the same order in this version of Java.

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Source: https://habr.com/ru/post/1694129/

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