diff --git a/api/current.txt b/api/current.txt
index 311625e6f67bc..59548aa29d94c 100644
--- a/api/current.txt
+++ b/api/current.txt
@@ -49823,7 +49823,10 @@ package java.lang {
     ctor public AbstractMethodError(java.lang.String);
   }
 
-   abstract class AbstractStringBuilder implements java.lang.CharSequence {
+   abstract class AbstractStringBuilder implements java.lang.Appendable java.lang.CharSequence {
+    method public java.lang.AbstractStringBuilder append(java.lang.CharSequence);
+    method public java.lang.AbstractStringBuilder append(java.lang.CharSequence, int, int);
+    method public java.lang.AbstractStringBuilder append(char);
     method public int capacity();
     method public char charAt(int);
     method public int codePointAt(int);
@@ -49842,6 +49845,7 @@ package java.lang {
     method public java.lang.CharSequence subSequence(int, int);
     method public java.lang.String substring(int);
     method public java.lang.String substring(int, int);
+    method public abstract java.lang.String toString();
     method public void trimToSize();
   }
 
@@ -51346,6 +51350,8 @@ package java.lang {
     method public int indexOf(java.lang.String, int);
     method public java.lang.String intern();
     method public boolean isEmpty();
+    method public static java.lang.String join(java.lang.CharSequence, java.lang.CharSequence...);
+    method public static java.lang.String join(java.lang.CharSequence, java.lang.Iterable<? extends java.lang.CharSequence>);
     method public int lastIndexOf(int);
     method public int lastIndexOf(int, int);
     method public int lastIndexOf(java.lang.String);
@@ -51384,7 +51390,7 @@ package java.lang {
     field public static final java.util.Comparator<java.lang.String> CASE_INSENSITIVE_ORDER;
   }
 
-  public final class StringBuffer extends java.lang.AbstractStringBuilder implements java.lang.Appendable java.lang.CharSequence java.io.Serializable {
+  public final class StringBuffer extends java.lang.AbstractStringBuilder implements java.lang.CharSequence java.io.Serializable {
     ctor public StringBuffer();
     ctor public StringBuffer(int);
     ctor public StringBuffer(java.lang.String);
@@ -51392,7 +51398,7 @@ package java.lang {
     method public synchronized java.lang.StringBuffer append(java.lang.Object);
     method public synchronized java.lang.StringBuffer append(java.lang.String);
     method public synchronized java.lang.StringBuffer append(java.lang.StringBuffer);
-    method public java.lang.StringBuffer append(java.lang.CharSequence);
+    method public synchronized java.lang.StringBuffer append(java.lang.CharSequence);
     method public synchronized java.lang.StringBuffer append(java.lang.CharSequence, int, int);
     method public synchronized java.lang.StringBuffer append(char[]);
     method public synchronized java.lang.StringBuffer append(char[], int, int);
@@ -51421,7 +51427,7 @@ package java.lang {
     method public synchronized java.lang.StringBuffer reverse();
   }
 
-  public final class StringBuilder extends java.lang.AbstractStringBuilder implements java.lang.Appendable java.lang.CharSequence java.io.Serializable {
+  public final class StringBuilder extends java.lang.AbstractStringBuilder implements java.lang.CharSequence java.io.Serializable {
     ctor public StringBuilder();
     ctor public StringBuilder(int);
     ctor public StringBuilder(java.lang.String);
diff --git a/api/system-current.txt b/api/system-current.txt
index 2b97388f854a0..b60e4972df1b9 100644
--- a/api/system-current.txt
+++ b/api/system-current.txt
@@ -53179,7 +53179,10 @@ package java.lang {
     ctor public AbstractMethodError(java.lang.String);
   }
 
-   abstract class AbstractStringBuilder implements java.lang.CharSequence {
+   abstract class AbstractStringBuilder implements java.lang.Appendable java.lang.CharSequence {
+    method public java.lang.AbstractStringBuilder append(java.lang.CharSequence);
+    method public java.lang.AbstractStringBuilder append(java.lang.CharSequence, int, int);
+    method public java.lang.AbstractStringBuilder append(char);
     method public int capacity();
     method public char charAt(int);
     method public int codePointAt(int);
@@ -53198,6 +53201,7 @@ package java.lang {
     method public java.lang.CharSequence subSequence(int, int);
     method public java.lang.String substring(int);
     method public java.lang.String substring(int, int);
+    method public abstract java.lang.String toString();
     method public void trimToSize();
   }
 
@@ -54702,6 +54706,8 @@ package java.lang {
     method public int indexOf(java.lang.String, int);
     method public java.lang.String intern();
     method public boolean isEmpty();
+    method public static java.lang.String join(java.lang.CharSequence, java.lang.CharSequence...);
+    method public static java.lang.String join(java.lang.CharSequence, java.lang.Iterable<? extends java.lang.CharSequence>);
     method public int lastIndexOf(int);
     method public int lastIndexOf(int, int);
     method public int lastIndexOf(java.lang.String);
@@ -54740,7 +54746,7 @@ package java.lang {
     field public static final java.util.Comparator<java.lang.String> CASE_INSENSITIVE_ORDER;
   }
 
-  public final class StringBuffer extends java.lang.AbstractStringBuilder implements java.lang.Appendable java.lang.CharSequence java.io.Serializable {
+  public final class StringBuffer extends java.lang.AbstractStringBuilder implements java.lang.CharSequence java.io.Serializable {
     ctor public StringBuffer();
     ctor public StringBuffer(int);
     ctor public StringBuffer(java.lang.String);
@@ -54748,7 +54754,7 @@ package java.lang {
     method public synchronized java.lang.StringBuffer append(java.lang.Object);
     method public synchronized java.lang.StringBuffer append(java.lang.String);
     method public synchronized java.lang.StringBuffer append(java.lang.StringBuffer);
-    method public java.lang.StringBuffer append(java.lang.CharSequence);
+    method public synchronized java.lang.StringBuffer append(java.lang.CharSequence);
     method public synchronized java.lang.StringBuffer append(java.lang.CharSequence, int, int);
     method public synchronized java.lang.StringBuffer append(char[]);
     method public synchronized java.lang.StringBuffer append(char[], int, int);
@@ -54777,7 +54783,7 @@ package java.lang {
     method public synchronized java.lang.StringBuffer reverse();
   }
 
-  public final class StringBuilder extends java.lang.AbstractStringBuilder implements java.lang.Appendable java.lang.CharSequence java.io.Serializable {
+  public final class StringBuilder extends java.lang.AbstractStringBuilder implements java.lang.CharSequence java.io.Serializable {
     ctor public StringBuilder();
     ctor public StringBuilder(int);
     ctor public StringBuilder(java.lang.String);