Summary
-------

Update the class hierarchy in `java.lang.ref` that could not be user-extended to use sealed classes.

Problem
-------

The classes in `java.lang.ref`, the root class `Reference` and its subclasses `PhantomReference`, `SoftReference`, and `WeakReference`, form a set of related classes where `Reference` cannot be directly extended by users but its subclasses can. This intention can be represented using sealed classes.

Solution
--------

Update the classes in question so that  `Reference` is sealed but its subclasses are non-sealed, retaining their subclassability.

Specification
-------------

(Note that as an implementation detail, the non-public class `FinalReference` can also subclass `Reference`, but this is not a visible change to the public API.)

    diff --git a/src/java.base/share/classes/java/lang/ref/PhantomReference.java b/src/java.base/share/classes/java/lang/ref/PhantomReference.java
    index 2c1532b8eb2..71947101e9c 100644
    --- a/src/java.base/share/classes/java/lang/ref/PhantomReference.java
    +++ b/src/java.base/share/classes/java/lang/ref/PhantomReference.java
    @@ -50,7 +50,7 @@
      * @since    1.2
      */
     
    -public class PhantomReference<T> extends Reference<T> {
    +public non-sealed class PhantomReference<T> extends Reference<T> {
     
         /**
          * Returns this reference object's referent.  Because the referent of a
    diff --git a/src/java.base/share/classes/java/lang/ref/Reference.java b/src/java.base/share/classes/java/lang/ref/Reference.java
    index 17d8c7b669c..75c8f3544dc 100644
    --- a/src/java.base/share/classes/java/lang/ref/Reference.java
    +++ b/src/java.base/share/classes/java/lang/ref/Reference.java
    @@ -41,7 +41,8 @@
      * @since    1.2
      */
     
    -public abstract class Reference<T> {
    +public abstract sealed class Reference<T>
    +    permits PhantomReference, SoftReference, WeakReference, FinalReference {
     
         /* The state of a Reference object is characterized by two attributes.  It
          * may be either "active", "pending", or "inactive".  It may also be
    diff --git a/src/java.base/share/classes/java/lang/ref/SoftReference.java b/src/java.base/share/classes/java/lang/ref/SoftReference.java
    index 8ce74a67d0a..b57e52bf766 100644
    --- a/src/java.base/share/classes/java/lang/ref/SoftReference.java
    +++ b/src/java.base/share/classes/java/lang/ref/SoftReference.java
    @@ -61,7 +61,7 @@
      * @since    1.2
      */
     
    -public class SoftReference<T> extends Reference<T> {
    +public non-sealed class SoftReference<T> extends Reference<T> {
     
         /**
          * Timestamp clock, updated by the garbage collector
    diff --git a/src/java.base/share/classes/java/lang/ref/WeakReference.java b/src/java.base/share/classes/java/lang/ref/WeakReference.java
    index fdf2b5c9f02..966f7f1b26c 100644
    --- a/src/java.base/share/classes/java/lang/ref/WeakReference.java
    +++ b/src/java.base/share/classes/java/lang/ref/WeakReference.java
    @@ -45,7 +45,7 @@
      * @since    1.2
      */
     
    -public class WeakReference<T> extends Reference<T> {
    +public non-sealed class WeakReference<T> extends Reference<T> {
     
         /**
          * Creates a new weak reference that refers to the given object.  The new