Java Immutable Collections

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Unmodifiable collections are usually read-only views (wrappers) of other collections. You can’t add, remove or clear them, but the underlying collection can change.

Immutable collections can’t be changed at all – they don’t wrap another collection – they have their own elements.

Here’s a quote from guava’s ImmutableList

Unlike Collections.unmodifiableList(java.util.List<? extends T>), which is a view of a separate collection that can still change, an instance of ImmutableList contains its own private data and will never change.

So, basically, in order to get an immutable collection out of a mutable one, you have to copy its elements to the new collection, and disallow all operations.

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