Provides high-performance collection classes and miscellaneous utilities; although this package provides very few collection classes, they are substitutes for most of `java.util.*` classes (for example, `java.util.IdentityHashMap` would be a {@link javolution.util.FastMap FastMap} with an {@link javolution.util.FastComparator#IDENTITY identity} key comparator).

## Overview:

Javolution collections are compliant with standard collections (generic when built with the ant target `1.5`) and they can safely be used with RTSJ virtual machines (e.g. if the capacity of a collection increases, the extension part is allocated from the same memory area as the collection itself).

They support direct iterations with the following advantages:

- Faster than iterators, see benchmark.
- No object creation not even the iterator object itself. For example, visiting a tree structure using iterators creates as many iterators as they are nodes in the tree:[code] public static void visit(Collection node) { for (Collection i : node) { // Creates iterator. visit(i); } }[/code] Not so with direct iterations:[code] public static void visit(FastCollection node) { for (FastCollection.Record r = node.head(), end = node.tail(); (r = r.getNext()) != end;) { visit(node.valueOf(r)); } }[/code]
- Used to implement most of {@link javolution.util.FastCollection FastCollection} base class methods (including {@link javolution.util.FastCollection#iterator iterator()}).
- Support forward/backward iterations from the start (head) or from the end (tail)
- Fully integrated with the JDK1.5+ generic framework (strong typing) and still compatible with other platforms (J2ME, 1.4, GCJ).

Here are few examples of direct iterations:[code] FastList list; for (FastList.Node n = list.head(), end = list.tail(); (n = n.getNext()) != end;) { String value = n.getValue(); // No typecast necessary. } ... FastMap map; for (FastMap.Entry e = map.head(), end = map.tail(); (e = e.getNext()) != end;) { String key = e.getKey(); // No typecast necessary. Thread value = e.getValue(); // No typecast necessary. }[/code]

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. public Set getTerms() { return _terms.unmodifiable(); } }[/code]

Fast collections (or maps) can be made thread-safe by marking them {@link javolution.util.FastCollection FastCollection#shared shared} Having a shared collection (or map) means that it can be safely used without synchronization. It does not mean that a change made by a thread is automatically viewed by another thread (that would require synchronizing). For example:[code] class Foo { private static final Collection INSTANCES = new FastTable().shared(); public Foo() { INSTANCES.add(this); } public static void showInstances() { for (Foo foo : INSTANCES) { System.out.println(foo); } } }[/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. try { tmp.addAll(persons); persons.clear(); persons.addAll(tmp); } finally { 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 when marked as {@link javolution.util.FastCollection#shared shared} 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} | Thread-safe when marked as {@link javolution.util.FastCollection#shared shared} 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#shared 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 the collections/maps are marked {@link javolution.util.FastCollection FastCollection#setShared shared}. 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 or removed (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 Collection getInstances() { return Collections.unmodifiableCollection(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 Collection INSTANCES = new FastTable().shared(); public static Collection 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#shared 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. Nevertheless, if your code use `ConcurrentHashMap` to "synchronize" between two threads then shared FastMap is not going to work.

   * 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 only if marked {@link javolution.util.FastCollection#shared shared}. In which case, the implementation may perform synchronization only when structural modification occurs.

6. What is the overhead in term of performance when FastMap are shared ?

   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).