@Documented @Target(value=TYPE) @Retention(value=RUNTIME) @Inherited public @interface Immutable
An object is immutable if its state cannot be observed to change after construction. Immutable objects are inherently thread-safe.
A class is immutable if all instances of that class are immutable. The immutability of a class can only be fully guaranteed if the class is final, otherwise one must ensure all subclasses are also immutable.
A conservative definition of object immutability is:
thisreference does not escape the constructor).
The requirement that all reference fields be immutable ensures deep immutability, meaning all contained state is also immutable. A weaker property, common with container classes, is shallow immutability, which allows some of the object's fields to point to mutable objects. One example of shallow immutability is guava's ImmutableList, which may contain mutable elements.
It is possible to implement immutable classes with some internal mutable state, as long as callers can never observe changes to that state. For example, some state may be lazily initialized to improve performance.
It is also technically possible to have an immutable object with non-final fields (see the
String.hashCode() for an example), but doing this correctly requires
subtle reasoning about safe data races and deep knowledge of the Java Memory Model.
Use of this annotation is validated by Error Prone's immutability analysis, which
ensures that all
@Immutable-annotated classes are deeply immutable according to the
conservative definition above. Non-final classes may be annotated with
any code compiled by Error Prone will be checked to ensure that no mutable subtypes of
@Immutable-annotated classes exist.
For more information about immutability, see:
public abstract String containerOf
containerOfspecifies which type parameters must be instantiated with immutable types for the container to be deeply immutable.
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