Users may provide a read-only view of any {@link javolution.util.FastCollection FastCollection} (or {@link javolution.util.FastMap FastMap}) instance using the {@link javolution.util.FastCollection#unmodifiable() FastCollection.unmodifiable()} (or {@link javolution.util.FastMap#unmodifiable FastMap.unmodifiable()}) method. For example:[code] public class Polynomial { private final FastSet _terms = new FastSet(); // Read-only view (also thread-safe as terms are not "deleted"). public Set getTerms() { return _terms.unmodifiable(); } }[/code]

Collection/maps of primitive types can be created using the {@link javolution.util.Index Index} class. It avoids the overhead of wrapping primitives types (for reasonably small int values). For example:[code] public class SparseVector { FastMap _elements = new FastMap(); ... }[/code]

Although all collections capacity increases smoothly (no resizing/copy or rehashing ever performed), it is nevertheless possible to specify an initial capacity; in which case, all necessary storage is allocated at creation. For RTSJ VMs, all collections/maps can reside in ImmortalMemory (e.g. static) and be used by all threads (including NoHeapRealtimeThread) without resulting into memory leaks or illegal access errors. For example:[code] public class XmlFormat { // RTSJ Unsafe! Memory leaks (when entries removed) or IllegalAssignmentError (when new entries while in ScopedArea). static HashMap ClassToFormat = HashMap(); // RTSJ Safe! Removed entries are internally recycled, new entries are in ImmortalMemory. static FastMap ClassToFormat = FastMap(); }[/code] For more details, please read Javolution-Collection.pdf

Temporary collection classes can be recycled (e.g. throw-away collections) to avoid the creation cost. For example:[code] static void removeDuplicate(List persons) { FastSet tmp = FastSet.newInstance(); // Possibly recycled instance. tmp.addAll(persons); persons.clear(); persons.addAll(tmp); FastSet.recycle(tmp); // Recycles the temporary instance. }[/code]

Here is a summary of the collection classes with their defining characteristics:

Javolution Collections Classes

	Ordering	Duplication Allowed	Custom Comparators	Record Type	Miscellaneous
{@link javolution.util.FastTable FastTable}	Insertion Order	Yes	{@link javolution.util.FastTable#setValueComparator setValueComparator(FastComparator)}	{@link javolution.util.Index Index}	Thread-safe random access collection No array resize/copy ever performed
{@link javolution.util.FastList FastList}	Insertion Order	Yes	{@link javolution.util.FastList#setValueComparator setValueComparator(FastComparator)}	{@link javolution.util.FastList.Node Node}	Recycle their own nodes (no adverse effect on GC)
{@link javolution.util.FastSet FastSet}	Insertion Order	No	{@link javolution.util.FastSet#setValueComparator setValueComparator(FastComparator)}	{@link javolution.util.FastCollection.Record Record}	Based on {@link javolution.util.FastSet FastMap} (same characteristics)
FastTree	Comparator	No	setValueComparator(FastComparator)	TreeNode	(not implemented)
{@link javolution.util.FastMap FastMap}	Insertion Order	Key: No Value: Yes	{@link javolution.util.FastMap#setKeyComparator setKeyComparator(FastComparator)} {@link javolution.util.FastMap#setValueComparator setValueComparator(FastComparator)}	{@link javolution.util.FastMap.Entry Entry}	Thread-safe when marked as {@link javolution.util.FastMap#setShared shared} No rehash/resize ever performed Recycle their own entries (no adverse effect on GC)

FAQ:

1. ArrayList may throw ConcurrentModificationException, but Javolution FastTable does not, why?

   FastTable (or any Javolution collection/map) do support concurrent modifications as long as these are not insertions at an arbitrary position or deletions (Note: Shared FastMap does support concurrent deletions). In other words you can safely iterate (using iterators or not) through a FastList, FastMap (entries, keys values), FastTable, etc. while new elements/entries are being added (by you or another thread). You can also export a {@link javolution.util.FastCollection#unmodifiable() read-only} view over your collection and still add more elements to it.

   Disallowing concurrent modifications (standard java util) has proven to be a performance killer for many (forcing users to work with copies of their whole collections). Furthermore the additional checks required directly impact performance (e.g. ArrayList iterations about 3x slower than FastTable iterations).

2. Do you have a test case showing any scenario of concurrent modification where ArrayList "fails" and FastTable doesn't?

   Let's say that you have a collection of "Units", and you want to provide users with a read-only view of these units. The following code will fail miserably:[code] public class Unit { static ArrayList INSTANCES = new ArrayList(); public static List getInstances() { return Collections.unmodifiableList(INSTANCES); } }[/code] Why? Because, it the user iterates on the read-only list of units while a new unit is added to the collection (by another thread) a ConcurrentModificationException is automatically raised. In other words, it is almost impossible to provide a "read-only" view of non-fixed size collections with the current java.util classes (e.g. you will have to replace the whole collection each time a new unit is added).

   Now with FastTable the following is completely safe even when new units are added:[code] public class Unit { static FastTable INSTANCES = new FastTable(); public static List getInstances() { return INSTANCES.unmodifiable(); } }[/code]

3. Do checks for concurrent modifications make your code safer?

   Not really. The current checks for concurrent modifications do not "guarantee" that concurrent modifications will not occur! You can imagine two threads one updating a collection and the other one iterating the collection. As long as the update is not performed while the other thread is iterating, everything is fine (no ConcurrentModificationException)! But, if for a reason or another the timing changes (e.g. in the user environment) and iterations are performed at the wrong time then your application crashes... Not a good thing and very high probability for this to happen!

4. Are {@link javolution.util.FastMap#setShared shared maps} valid substitutes for ConcurrentHashMap?

   Unlike ConcurrentHashMap access to a shared FastMap never blocks. Retrieval reflects the map state not older than the last time the accessing threads have been synchronized^* (for multi-processors systems synchronizing ensures that the CPU internal cache is not stale).

   In practice, it means that most well-written concurrent programs should be able to use shared FastMap in place of ConcurrentHashMap as threads are already synchronized to ensure proper behavior.

   ^* It is important for both threads to synchronize on the same monitor in order to set up the happens-before relationship properly. It is not the case that everything visible to thread A when it synchronizes on object X becomes visible to thread B after it synchronizes on object Y. The release and acquire have to "match" (i.e., be performed on the same monitor) to have the right semantics. Otherwise, the code has a data race.

5. Are all Javolution collection thread-safe?

   Collections/Maps are thread-safe with regard to access (no need to synchronize reading even if the collection is modified concurrently). But the modifications themselves require either the collection/map to be marked shared or synchronization to be used.

6. What is the overhead in term of performance when FastMap.setShared is set to true?

   Marking the map shared avoid synchronizing when possible (e.g. put when entry already exists or remove when entry does not exist), if a new entry is created and added, synchronization is performed internally. In all cases there is no impact on reading (never synchronized).