Collider

Collider is a tiny library that provides immutable persistent collections for Java. It does this by wrapping Clojure's collections in an object-oriented and type-safe facade. Browse the Javadoc online.

Examples

Collider collections can be created by calling one of the factory methods in the Collider class. The resulting collection can be used like any other unmodifiable Java collection--for instance, ClojureList<T> implements the standard java.util.List<T> interface.

List<String> myStrings = clojureList("a", "b", "c");
assertEquals(3, myStrings.size());
assertEquals("a", myStrings.get(0));

Collider collections can also be created by transforming another collection. The technique of persistent modification is used to efficiently create modified copies while leaving the immutable original untouched.

ClojureMap<String, String> emptyMap = clojureMap();

ClojureMap<String, String> assoc = emptyMap.assoc("key", "value");
assertTrue(assoc.containsKey("key"));
assertEquals(assoc.size(), 1);
assertEquals(assoc.get("key"), "value");

ClojureMap<String, String> dissoc = assoc.dissoc("key");
assertEquals(dissoc, emptyMap);

Since Collider specifically targets Java 8, not only can its collections be used with the Stream API, but they also include some convenience methods to make common use cases (such as mapping a function over a list) more concise.

ClojureList<Integer> evens = clojureList(0, 2, 4, 6, 8);  // [0, 2, 4, 6, 8]
ClojureList<Integer> odds = evens.map(x -> x + 1);        // [1, 3, 5, 7, 9]

Collider also provides collectors that work with the Stream API.

ClojureList<Integer> singles = IntStream.range(0, 100).boxed().collect(toClojureList());

Internally, these collectors use Clojure's transient collections to efficiently accumulate a result. Transients are generally used as follows:

1. Obtain a transient version of a collection from a persistent collection in O(1) time
2. Mutate the transient collection
3. Turn the transient collection into a persistent collection in O(1) time

Collider makes transients available directly. Continuing the above example:

TransientList<Integer> tr = singles.asTransient();
range(100, 200).forEach(tr::append);
ClojureList<Integer> moreSingles = tr.toPersistent();

assertEquals(moreSingles.size(), 200);
assertEquals(moreSingles, range(0, 200).boxed().collect(toClojureList()));