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javacollectionsguava
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Reputation: 11559

Guava ImmutableList... what exactly is backing it?

I downloaded the source code for Google Guava just out of curiosity to see what is backing the immutable collections.

I was going through the ImmutableList and I noticed that it was still backed by an old-fashioned array, as in Object[]

So I'm just curious, the Object[] array is probably mutable by its nature, and it is not threadsafe. I already knew that ArrayList and CopyOnWriteArrayList were backed by an Object[] array, but they are mutable.

So is the ImmutableList only immutable and threadsafe because its internal properties are well encapsulated and protected? Is it also immutable and threadsafe because the encapsulation ensures nothing will modify it after construction? Will there ever be a day where low-level arrays like this will be switched out for something better and not of a legacy, and be inherently immutable and final rather than immutable by careful encapsulation?

Upvotes: 0

Views: 257

Answers (1)

Thilo
Thilo

Reputation: 262534

Yes, the ImmutableList is "only" immutable because it does not allow its internal backing array to be modified.

That is exactly the same situation as for a java.lang.String, which also wraps a private char[]. Along the same lines, the very useful concurrency libraries are largely implemented as regular Java classes relying on only very few (and very basic) JVM synchronization primitives.

So that should be good enough. Obviously, people writing these library classes have to be careful and knowledgable, but this requirement does not magically go away by moving the code even more "low-level" into JVM primitives. (Agreed, you can shoot yourself in the foot using dark voodoo like reflection now, but that hardly happens by accident, and in regular usage, a "user-land" implementation works just as well).

Will there ever be a day

That is pure speculation. But there is apparently work on "value types", which is a related topic.

Upvotes: 10

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