Type-safe flattening of nested collections/structures in Java

Question

I would like to flatten arbitrary deeply nested collections/structures of elements of some type T in Java, optimally with

• only having a live view and not a copied collection;
• not only handling Collections, but also Iterator, arrays of T of arbitrary dimension, Iterable, and all these structures arbitrarily mixed and nested;
• statical type-safety.

Is there a java library which can handle this?

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Guava seems to only handle one nesting level, i.e. Collection<Collection<T>> --flatten--> Collection<T>.

lambdaj looks promising: can I somehow combine on(), asIterator() and flattenIterator() to achieve this? In a statically type-safe manner?

Answer 1

Guava will probably support this eventually :

http://code.google.com/p/guava-libraries/issues/detail?id=174

(It might be easiest to read that from the bottom up, since the thinking on it has shifted a few times over its lifetime.)

Answer 2

Not based on either of the above, but perhaps a DeepIterator class that is constructed with a Collection whose next() method looks at the next Object and if it is a instanceof Collection then pushes the current iterator on Stack and recurses into that Collection's iterator.