Summary

Explicitly add Appendable to the list of implemented interfaces of StringBuffer and StringBuilder.

Problem

StringBuffer and StringBuilder "mysteriously" are Appendable by virtue of inheriting from their non-public superclass.

Solution

Explicitly add Appendable to the implements clauses of StringBuffer and StringBuilder.

Specification

diff --git a/src/java.base/share/classes/java/lang/StringBuffer.java b/src/java.base/share/classes/java/lang/StringBuffer.java
index b0ee2a5c2e3..d62696d9300 100644
--- a/src/java.base/share/classes/java/lang/StringBuffer.java
+++ b/src/java.base/share/classes/java/lang/StringBuffer.java
@@ -111,7 +111,7 @@ import jdk.internal.vm.annotation.IntrinsicCandidate;
  */
  public final class StringBuffer
     extends AbstractStringBuilder
-    implements Serializable, Comparable<StringBuffer>, CharSequence
+    implements Appendable, Serializable, Comparable<StringBuffer>, CharSequence
 {

     /**
diff --git a/src/java.base/share/classes/java/lang/StringBuilder.java b/src/java.base/share/classes/java/lang/StringBuilder.java
index 17fd105ad3f..bffbd14ef50 100644
--- a/src/java.base/share/classes/java/lang/StringBuilder.java
+++ b/src/java.base/share/classes/java/lang/StringBuilder.java
@@ -90,7 +90,7 @@ import java.io.StreamCorruptedException;
  */
 public final class StringBuilder
     extends AbstractStringBuilder
-    implements java.io.Serializable, Comparable<StringBuilder>, CharSequence
+    implements Appendable, java.io.Serializable, Comparable<StringBuilder>, CharSequence
 {

     /** use serialVersionUID for interoperability */