Summary

Change the java.nio.Buffer hierarchy to be sealed.

Problem

Buffers are not sealed which it would be appropriate for them to be as they are a closed abstraction.

Solution

In the java.nio.Buffer class tree, convert the subclassed buffer classes to sealed and the non-subclassed classes to final.

Specification

--- a/src/java.base/share/classes/java/nio/Buffer.java
+++ b/src/java.base/share/classes/java/nio/Buffer.java
@@ -189,11 +189,13 @@
  * @author Mark Reinhold
  * @author JSR-51 Expert Group
  * @since 1.4
  */

-public abstract class Buffer {
+public abstract sealed class Buffer
+    permits ByteBuffer, CharBuffer, DoubleBuffer, FloatBuffer, IntBuffer,
+        LongBuffer, ShortBuffer {
     // Cached unsafe-access object
     static final Unsafe UNSAFE = Unsafe.getUnsafe();

     static final ScopedMemoryAccess SCOPED_MEMORY_ACCESS = ScopedMemoryAccess.getScopedMemoryAccess();

--- a/src/java.base/share/classes/java/nio/ByteBufferAs-X-Buffer.java.template
+++ b/src/java.base/share/classes/java/nio/ByteBufferAs-X-Buffer.java.template
@@ -29,12 +29,20 @@

 import java.util.Objects;
 import jdk.internal.access.foreign.MemorySegmentProxy;
 import jdk.internal.misc.Unsafe;

+#if[rw]
+sealed
+#else[rw]
+final
+#end[rw]
 class ByteBufferAs$Type$Buffer$RW$$BO$                  // package-private
     extends {#if[ro]?ByteBufferAs}$Type$Buffer{#if[ro]?$BO$}
+#if[rw]
+    permits ByteBufferAs$Type$BufferR$BO$
+#end[rw]
 {

 #if[rw]

     protected final ByteBuffer bb;

--- a/src/java.base/share/classes/java/nio/Direct-X-Buffer.java.template
+++ b/src/java.base/share/classes/java/nio/Direct-X-Buffer.java.template
@@ -35,17 +35,25 @@
 import jdk.internal.misc.VM;
 import jdk.internal.ref.Cleaner;
 import sun.nio.ch.DirectBuffer;


+#if[rw]
+sealed
+#else[rw]
+final
+#end[rw]
 class Direct$Type$Buffer$RW$$BO$
 #if[rw]
     extends {#if[byte]?Mapped$Type$Buffer:$Type$Buffer}
 #else[rw]
     extends Direct$Type$Buffer$BO$
 #end[rw]
     implements DirectBuffer
+#if[rw]
+    permits Direct$Type$BufferR$BO$
+#end[rw]
 {

 #if[rw]

     // Cached unaligned-access capability

--- a/src/java.base/share/classes/java/nio/Heap-X-Buffer.java.template
+++ b/src/java.base/share/classes/java/nio/Heap-X-Buffer.java.template
@@ -38,12 +38,20 @@
  * read/write class, overriding the mutation methods to throw a {@link
  * ReadOnlyBufferException} and overriding the view-buffer methods to return an
  * instance of this class rather than of the superclass.
 #end[rw]
  */
+#if[rw]
+sealed
+#else[rw]
+final
+#end[rw]
 class Heap$Type$Buffer$RW$
     extends {#if[ro]?Heap}$Type$Buffer
+#if[rw]
+    permits Heap$Type$BufferR
+#end[rw]
 {
 #if[rw]
     // Cached array base offset
     private static final long ARRAY_BASE_OFFSET = UNSAFE.arrayBaseOffset($type$[].class);

--- a/src/java.base/share/classes/java/nio/MappedByteBuffer.java
+++ b/src/java.base/share/classes/java/nio/MappedByteBuffer.java
@@ -68,12 +68,13 @@
  * @author Mark Reinhold
  * @author JSR-51 Expert Group
  * @since 1.4
  */

-public abstract class MappedByteBuffer
+public abstract sealed class MappedByteBuffer
     extends ByteBuffer
+    permits DirectByteBuffer
 {

     // This is a little bit backwards: By rights MappedByteBuffer should be a
     // subclass of DirectByteBuffer, but to keep the spec clear and simple, and
     // for optimization purposes, it's easier to do it the other way around.
--- a/src/java.base/share/classes/java/nio/StringCharBuffer.java

+++ b/src/java.base/share/classes/java/nio/StringCharBuffer.java
@@ -27,11 +27,11 @@

 import java.util.Objects;

 // ## If the sequence is a string, use reflection to share its array

-class StringCharBuffer                                  // package-private
+final class StringCharBuffer                                  // package-private
     extends CharBuffer
 {
     CharSequence str;

     StringCharBuffer(CharSequence s, int start, int end) { // package-private

--- a/src/java.base/share/classes/java/nio/X-Buffer.java.template
+++ b/src/java.base/share/classes/java/nio/X-Buffer.java.template
@@ -262,13 +262,23 @@
  * @author Mark Reinhold
  * @author JSR-51 Expert Group
  * @since 1.4
  */

-public abstract class $Type$Buffer
+public abstract sealed class $Type$Buffer
     extends Buffer
     implements Comparable<$Type$Buffer>{#if[char]?, Appendable, CharSequence, Readable}
+    permits
+#if[byte]
+    Heap$Type$Buffer, MappedByteBuffer
+#else[byte]
+#if[char]
+    StringCharBuffer,
+#end[char]
+    Heap$Type$Buffer, Direct$Type$BufferS, Direct$Type$BufferU,
+    ByteBufferAs$Type$BufferB, ByteBufferAs$Type$BufferL
+#end[byte]
 {
     // Cached array base offset
     private static final long ARRAY_BASE_OFFSET = UNSAFE.arrayBaseOffset($type$[].class);

     // These fields are declared here rather than in Heap-X-Buffer in order to