Copying sets Java

Question

Is there a way to copy a TreeSet  That is  is it possible to go  Set  lt Item gt  itemList  Set  lt Item gt  tempList   tempList   itemList    or do you have to physically iterate through the sets and copy them one by one

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Java 8    Set lt String gt  copy   new HashSet lt  gt  mySet

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Starting from Java 10   Set lt E gt  oldSet   Set of    Set lt E gt  newSet   Set copyOf oldSet     Set copyOf   returns an unmodifiable Set containing the elements of the given Collection   The given Collection must not be null  and it must not contain any null elements

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Another way to do this is to use the copy constructor   Collection lt E gt  oldSet       TreeSet lt E gt  newSet   new TreeSet lt E gt  oldSet     Or create an empty set and add the elements   Collection lt E gt  oldSet       TreeSet lt E gt  newSet   new TreeSet lt E gt     newSet addAll oldSet     Unlike clone these allow you to use a different set class  a different comparator  or even populate from some other  non-set  collection type     Note that the result of copying a Set is a new Set containing references to the objects that are elements if the original Set   The element objects themselves are not copied or cloned   This conforms with the way that the Java Collection APIs are designed to work  they don t copy the element objects

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The copy constructor given by  Stephen C is the way to go when you have a Set you created  or when you know where it comes from   When it comes from a Map entrySet    it will depend on the Map implementation you re using   findbugs says     The entrySet   method is allowed to return a view of the underlying   Map in which a single Entry object is reused and returned during the   iteration  As of Java 1 6  both IdentityHashMap and EnumMap did so    When iterating through such a Map  the Entry value is only valid until   you advance to the next iteration  If  for example  you try to pass   such an entrySet to an addAll method  things will go badly wrong    As addAll   is called by the copy constructor  you might find yourself with a Set of only one Entry  the last one   Not all Map implementations do that though  so if you know your implementation is safe in that regard  the copy constructor definitely is the way to go  Otherwise  you d have to create new Entry objects yourself   Set lt K V gt  copy   new HashSet lt K V gt  map size     for  Entry lt K V gt  e   map entrySet        copy add new java util AbstractMap SimpleEntry lt K V gt  e      Edit  Unlike tests I performed on Java 7 and Java 6u45  thanks to Stephen C   the findbugs comment does not seem appropriate anymore  It might have been the case on earlier versions of Java 6  before u45  but I don t have any to test

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With Java 8 you can use stream and collect to copy the items   Set lt Item gt  newSet   oldSet stream   collect Collectors toSet       Or you can collect to an ImmutableSet  if you know that the set should not change    Set lt Item gt  newSet   oldSet stream   collect ImmutableSet toImmutableSet

[java] Copying sets Java

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