@GwtCompatible(emulated=true)
public abstract class ImmutableCollection<E>
extends AbstractCollection<E>
implements Serializable

A Collection whose contents will never change, and which offers a few additional guarantees detailed below.

Warning: avoid direct usage of ImmutableCollection as a type (just as with Collection itself). Prefer subtypes such as ImmutableSet or ImmutableList, which have well-defined Object.equals(java.lang.Object) semantics, thus avoiding a common source of bugs and confusion.

About all Immutable- collections

The remainder of this documentation applies to every public Immutable- type in this package, whether it is a subtype of ImmutableCollection or not.

Guarantees

Each makes the following guarantees:

• Shallow immutability. Elements can never be added, removed or replaced in this collection. This is a stronger guarantee than that of Collections.unmodifiableCollection(java.util.Collection<? extends T>), whose contents change whenever the wrapped collection is modified.
• Null-hostility. This collection will never contain a null element.
• Deterministic iteration. The iteration order is always well-defined, depending on how the collection was created (see the appropriate factory method for details). View collections such as Multiset.elementSet() iterate in the same order as the parent, except as noted.
• Thread safety. It is safe to access this collection concurrently from multiple threads.
• Integrity. This type cannot be subclassed outside this package (which would allow these guarantees to be violated).

"Interfaces", not implementations

Each public class, such as ImmutableSet, is a type offering meaningful behavioral guarantees -- not merely a specific implementation as in the case of, say, ArrayList. You should treat them as interfaces in every important sense of the word.

For field types and method return types, you should generally use the immutable type (such as ImmutableList) instead of the general collection interface type (such as List). This communicates to your callers all of the semantic guarantees listed above, which is almost always very useful information.

On the other hand, a parameter type of ImmutableList is generally a nuisance to callers. Instead, accept Iterable and have your method or constructor body pass it to the appropriate copyOf method itself.

Creation

Except for logically "abstract" types like ImmutableCollection itself, each Immutable type provides the static operations you need to obtain instances of that type. These usually include:

• Static methods named of, accepting an explicit list of elements or entries.
• Static methods named copyOf (or copyOfSorted), accepting an existing collection whose contents should be copied.
• A static nested Builder class which can be used to populate a new immutable instance.

Warnings

• Warning: as with any collection, it is almost always a bad idea to modify an element (in a way that affects its Object.equals(java.lang.Object) behavior) while it is contained in a collection. Undefined behavior and bugs will result. It's generally best to avoid using mutable objects as elements at all, as many users may expect your "immutable" object to be deeply immutable.

Performance notes

• Implementations can be generally assumed to prioritize memory efficiency, then speed of access, and lastly speed of creation.
• The copyOf methods will sometimes recognize that the actual copy operation is unnecessary; for example, copyOf(copyOf(anArrayList)) should copy the data only once. This reduces the expense of habitually making defensive copies at API boundaries. However, the precise conditions for skipping the copy operation are undefined.
• Warning: a view collection such as ImmutableMap.keySet or ImmutableList.subList(int, int) may retain a reference to the entire data set, preventing it from being garbage collected. If some of the data is no longer reachable through other means, this constitutes a memory leak. Pass the view collection to the appropriate copyOf method to obtain a correctly-sized copy.
• The performance of using the associated Builder class can be assumed to be no worse, and possibly better, than creating a mutable collection and copying it.
• Implementations generally do not cache hash codes. If your element or key type has a slow hashCode implementation, it should cache it itself.

Example usage

   class Foo {
     private static final ImmutableSet<String> RESERVED_CODES =
         ImmutableSet.of("AZ", "CQ", "ZX");

     private final ImmutableSet<String> codes;

     public Foo(Iterable<String> codes) {
       this.codes = ImmutableSet.copyOf(codes);
       checkArgument(Collections.disjoint(this.codes, RESERVED_CODES));
     }
   }

See also

See the Guava User Guide article on immutable collections.