Bags or multisets (unordered collections that may contain
duplicate elements) should implement this interface directly.
All general-purpose Collection implementation classes (which
typically implement Collection indirectly through one of its
subinterfaces) should provide two "standard" constructors: a void (no
arguments) constructor, which creates an empty collection, and a
constructor with a single argument of type Collection, which
creates a new collection with the same elements as its argument. In
effect, the latter constructor allows the user to copy any collection,
producing an equivalent collection of the desired implementation type.
There is no way to enforce this convention (as interfaces cannot contain
constructors) but all of the general-purpose Collection
implementations in the Java platform libraries comply.
The "destructive" methods contained in this interface, that is, the
methods that modify the collection on which they operate, are specified to
throw UnsupportedOperationException if this collection does not
support the operation. If this is the case, these methods may, but are not
required to, throw an UnsupportedOperationException if the
invocation would have no effect on the collection. For example, invoking
the addAll(Collection extends E>) method on an unmodifiable collection may,
but is not required to, throw the exception if the collection to be added
Some collection implementations have restrictions on the elements that
they may contain. For example, some implementations prohibit null elements,
and some have restrictions on the types of their elements. Attempting to
add an ineligible element throws an unchecked exception, typically
NullPointerException or ClassCastException. Attempting
to query the presence of an ineligible element may throw an exception,
or it may simply return false; some implementations will exhibit the former
behavior and some will exhibit the latter. More generally, attempting an
operation on an ineligible element whose completion would not result in
the insertion of an ineligible element into the collection may throw an
exception or it may succeed, at the option of the implementation.
Such exceptions are marked as "optional" in the specification for this
It is up to each collection to determine its own synchronization
policy. In the absence of a stronger guarantee by the
implementation, undefined behavior may result from the invocation
of any method on a collection that is being mutated by another
thread; this includes direct invocations, passing the collection to
a method that might perform invocations, and using an existing
iterator to examine the collection.
Many methods in Collections Framework interfaces are defined in
terms of the equals method. For example,
the specification for the contains(Object o)
method says: "returns true if and only if this collection
contains at least one element e such that
(o==null ? e==null : o.equals(e))." This specification should
not be construed to imply that invoking Collection.contains
with a non-null argument o will cause o.equals(e) to be
invoked for any element e. Implementations are free to implement
optimizations whereby the equals invocation is avoided, for
example, by first comparing the hash codes of the two elements. (The
hashCode() specification guarantees that two objects with
unequal hash codes cannot be equal.) More generally, implementations of
the various Collections Framework interfaces are free to take advantage of
the specified behavior of underlying Object methods wherever the
implementor deems it appropriate.
This interface is a member of the
Java Collections Framework.