复制集 - 开卷题库

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最佳答案

Another way to do this is to use the copy constructor:

Collection<E> oldSet = ...
TreeSet<E> newSet = new TreeSet<E>(oldSet);

Or create an empty set and add the elements:

Collection<E> oldSet = ...
TreeSet<E> newSet = new TreeSet<E>();
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<K,V> copy = new HashSet<K,V>(map.size());
for (Entry<K,V> e : map.entrySet())
copy.add(new java.util.AbstractMap.SimpleEntry<K,V>(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<Item> newSet = oldSet.stream().collect(Collectors.toSet());

Or you can collect to an ImmutableSet (if you know that the set should not change):

Set<Item> newSet = oldSet.stream().collect(ImmutableSet.toImmutableSet());

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Starting from Java 10:

Set<E> oldSet = Set.of();
Set<E> 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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Java 8+:

Set<String> copy = new HashSet<>(mySet);