问题描述
ImmutableSet 的JavaDoc说：
JavaDoc of ImmutableSet says:
但是 ImmutableSet 仍然存储元素的引用，我无法找出与 Collections.unmodifiableSet（）。示例：
But the ImmutableSet still stores reference of elements, I couldn't figure out the difference to Collections.unmodifiableSet(). Sample:
StringBuffer s=new StringBuffer("a");
ImmutableSet<StringBuffer> set= ImmutableSet.of(s);
s.append("b");//s is "ab", s is still changed here!
有人可以解释一下吗？
推荐答案
考虑一下：
Set<String> x = new HashSet<String>();
x.add("foo");
ImmutableSet<String> guava = ImmutableSet.copyOf(x);
Set<String> builtIn = Collections.unmodifiableSet(x);
x.add("bar");
System.out.println(guava.size()); // Prints 1
System.out.println(builtIn.size()); // Prints 2
换句话说， ImmutableSet 尽管它有可能通过改变而构建的任何集合，但它是不可变的 - 因为它创建了一个副本。 Collections.unmodifiableSet 可以防止返回的集合被直接更改，但它仍然是可能更改的后备集的视图。
In other words, ImmutableSet is immutable despite whatever collection it's built from potentially changing - because it creates a copy. Collections.unmodifiableSet prevents the returned collection from being directly changed, but it's still a view on a potentially-changing backing set.
请注意，如果您开始通过任何一组更改引用对象的内容，则无论如何都会关闭所有投注。不要那样做。实际上，首先使用可变元素类型创建集合并不是一个好主意。 （使用可变密钥类型进行Ditto映射。）
Note that if you start changing the contents of the objects referred to by any set, all bets are off anyway. Don't do that. Indeed, it's rarely a good idea to create a set using a mutable element type in the first place. (Ditto maps using a mutable key type.)
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