Class ImmutableCollection<E>

  • All Implemented Interfaces:
    Serializable, Iterable<E>, Collection<E>
    Direct Known Subclasses:
    ImmutableList, ImmutableMultiset, ImmutableSet

    @DoNotMock("Use ImmutableList.of or another implementation")
    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.


    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. Typically this is insertion order unless an explicit ordering is otherwise specified (e.g. ImmutableSortedSet.naturalOrder()). See the appropriate factory method for details. View collections such as ImmutableMultiset.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

    These are classes instead of interfaces to prevent external subtyping, but should be thought of as interfaces in every important sense. Each public class such as ImmutableSet is a type offering meaningful behavioral guarantees. This is substantially different from the case of (say) HashSet, which is an implementation, with semantics that were largely defined by its supertype.

    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.

    Expressing the immutability guarantee directly in the type that user code references is a powerful advantage. Although Java offers certain immutable collection factory methods, such as Collections.singleton(Object) and Set.of, we recommend using these classes instead for this reason (as well as for consistency).


    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.


    • 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) { = ImmutableSet.copyOf(codes);
         checkArgument(Collections.disjoint(, RESERVED_CODES));

    See also

    See the Guava User Guide article on immutable collections.

    See Also:
    Serialized Form
    • Method Detail

      • spliterator

        public Spliterator<Espliterator()
        Description copied from interface: java.util.Collection
        Creates a Spliterator over the elements in this collection. Implementations should document characteristic values reported by the spliterator. Such characteristic values are not required to be reported if the spliterator reports Spliterator.SIZED and this collection contains no elements.

        The default implementation should be overridden by subclasses that can return a more efficient spliterator. In order to preserve expected laziness behavior for the and Collection.parallelStream() methods, spliterators should either have the characteristic of IMMUTABLE or CONCURRENT, or be late-binding. If none of these is practical, the overriding class should describe the spliterator's documented policy of binding and structural interference, and should override the and Collection.parallelStream() methods to create streams using a Supplier of the spliterator, as in:

             Stream<E> s = -> spliterator(), spliteratorCharacteristics)

        These requirements ensure that streams produced by the and Collection.parallelStream() methods will reflect the contents of the collection as of initiation of the terminal stream operation.

        Specified by:
        spliterator in interface Collection<E>
        Specified by:
        spliterator in interface Iterable<E>
        a Spliterator over the elements in this collection
      • toArray

        public final Object[] toArray()
        Description copied from class: java.util.AbstractCollection
        Returns an array containing all of the elements in this collection. If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order. The returned array's runtime component type is Object.

        The returned array will be "safe" in that no references to it are maintained by this collection. (In other words, this method must allocate a new array even if this collection is backed by an array). The caller is thus free to modify the returned array.

        Specified by:
        toArray in interface Collection<E>
        toArray in class AbstractCollection<E>
        an array, whose runtime component type is Object, containing all of the elements in this collection
      • toArray

        public final <T extends @Nullable Object> T[] toArray​(T[] other)
        Description copied from class: java.util.AbstractCollection
        Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this collection.

        If this collection fits in the specified array with room to spare (i.e., the array has more elements than this collection), the element in the array immediately following the end of the collection is set to null. (This is useful in determining the length of this collection only if the caller knows that this collection does not contain any null elements.)

        If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

        Specified by:
        toArray in interface Collection<E>
        toArray in class AbstractCollection<E>
        Type Parameters:
        T - the component type of the array to contain the collection
        other - the array into which the elements of this collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
        an array containing all of the elements in this collection
      • contains

        public abstract boolean contains​(@CheckForNull
                                         Object object)
        Description copied from class: java.util.AbstractCollection
        Returns true if this collection contains the specified element. More formally, returns true if and only if this collection contains at least one element e such that Objects.equals(o, e).
        Specified by:
        contains in interface Collection<E>
        contains in class AbstractCollection<E>
        object - element whose presence in this collection is to be tested
        true if this collection contains the specified element
      • asList

        public ImmutableList<EasList()
        Returns an ImmutableList containing the same elements, in the same order, as this collection.

        Performance note: in most cases this method can return quickly without actually copying anything. The exact circumstances under which the copy is performed are undefined and subject to change.