What's the difference between Collections.unmodifiableSet() and ImmutableSet of Guava?

java immutability guava
卢声远 Shengyuan Lu picture 卢声远 Shengyuan Lu · Apr 10, 2011 · Viewed 28.2k times · Source

JavaDoc of ImmutableSet says:

Unlike Collections.unmodifiableSet, which is a view of a separate collection that can still change, an instance of this class contains its own private data and will never change. This class is convenient for public static final sets ("constant sets") and also lets you easily make a "defensive copy" of a set provided to your class by a caller.

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!

Could anyone explain it?

Answer

Jon Skeet picture Jon Skeet · Apr 10, 2011

Consider this:

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

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.

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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