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8231977: Discuss serializability of collections

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Reviewed-by: darcy, plevart, chegar
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Stuart Marks 2019-10-07 17:13:59 -07:00
parent 381e90eb6b
commit ba63c30a94
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src/java.base/share/classes/java/util/Collection.java
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@ -188,6 +188,38 @@ import java.util.stream.StreamSupport;
* or if the only reference to the backing collection is through an
* unmodifiable view, the view can be considered effectively immutable.
*
* <h2><a id="serializable">Serializability of Collections</a></h2>
*
* <p>Serializability of collections is optional. As such, none of the collections
* interfaces are declared to implement the {@link java.io.Serializable} interface.
* However, serializability is regarded as being generally useful, so most collection
* implementations are serializable.
*
* <p>The collection implementations that are public classes (such as {@code ArrayList}
* or {@code HashMap}) are declared to implement the {@code Serializable} interface if they
* are in fact serializable. Some collections implementations are not public classes,
* such as the <a href="#unmodifiable">unmodifiable collections.</a> In such cases, the
* serializability of such collections is described in the specification of the method
* that creates them, or in some other suitable place. In cases where the serializability
* of a collection is not specified, there is no guarantee about the serializability of such
* collections. In particular, many <a href="#view">view collections</a> are not serializable.
*
* <p>A collection implementation that implements the {@code Serializable} interface cannot
* be guaranteed to be serializable. The reason is that in general, collections
* contain elements of other types, and it is not possible to determine statically
* whether instances of some element type are actually serializable. For example, consider
* a serializable {@code Collection<E>}, where {@code E} does not implement the
* {@code Serializable} interface. The collection may be serializable, if it contains only
* elements of some serializable subtype of {@code E}, or if it is empty. Collections are
* thus said to be <i>conditionally serializable,</i> as the serializability of the collection
* as a whole depends on whether the collection itself is serializable and on whether all
* contained elements are also serializable.
*
* <p>An additional case occurs with instances of {@link SortedSet} and {@link SortedMap}.
* These collections can be created with a {@link Comparator} that imposes an ordering on
* the set elements or map keys. Such a collection is serializable only if the provided
* {@code Comparator} is also serializable.
*
* <p>This interface is a member of the
* <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
* Java Collections Framework</a>.