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