diff --git a/api/current.txt b/api/current.txt
index 5bdb85330fd21..9588305c63b12 100644
--- a/api/current.txt
+++ b/api/current.txt
@@ -51422,6 +51422,7 @@ package java.lang {
     method public java.lang.StringBuffer insert(int, double);
     method public synchronized java.lang.StringBuffer replace(int, int, java.lang.String);
     method public synchronized java.lang.StringBuffer reverse();
+    method public synchronized java.lang.String toString();
   }
 
   public final class StringBuilder extends java.lang.AbstractStringBuilder implements java.lang.CharSequence java.io.Serializable {
@@ -51459,6 +51460,7 @@ package java.lang {
     method public java.lang.StringBuilder insert(int, double);
     method public java.lang.StringBuilder replace(int, int, java.lang.String);
     method public java.lang.StringBuilder reverse();
+    method public java.lang.String toString();
   }
 
   public class StringIndexOutOfBoundsException extends java.lang.IndexOutOfBoundsException {
diff --git a/api/system-current.txt b/api/system-current.txt
index 473ad12ffe8bc..e38d53ddbbf85 100644
--- a/api/system-current.txt
+++ b/api/system-current.txt
@@ -54778,6 +54778,7 @@ package java.lang {
     method public java.lang.StringBuffer insert(int, double);
     method public synchronized java.lang.StringBuffer replace(int, int, java.lang.String);
     method public synchronized java.lang.StringBuffer reverse();
+    method public synchronized java.lang.String toString();
   }
 
   public final class StringBuilder extends java.lang.AbstractStringBuilder implements java.lang.CharSequence java.io.Serializable {
@@ -54815,6 +54816,7 @@ package java.lang {
     method public java.lang.StringBuilder insert(int, double);
     method public java.lang.StringBuilder replace(int, int, java.lang.String);
     method public java.lang.StringBuilder reverse();
+    method public java.lang.String toString();
   }
 
   public class StringIndexOutOfBoundsException extends java.lang.IndexOutOfBoundsException {
diff --git a/api/test-current.txt b/api/test-current.txt
index 57d6f66698fb5..f80815f7b9613 100644
--- a/api/test-current.txt
+++ b/api/test-current.txt
@@ -49900,7 +49900,10 @@ 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);
@@ -49919,6 +49922,7 @@ 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();
   }
 
@@ -51423,6 +51427,8 @@ 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);
@@ -51461,7 +51467,7 @@ 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);
@@ -51469,7 +51475,7 @@ 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);
@@ -51496,9 +51502,10 @@ package java.lang {
     method public java.lang.StringBuffer insert(int, double);
     method public synchronized java.lang.StringBuffer replace(int, int, java.lang.String);
     method public synchronized java.lang.StringBuffer reverse();
+    method public synchronized java.lang.String toString();
   }
 
-  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);
@@ -51533,6 +51540,7 @@ package java.lang {
     method public java.lang.StringBuilder insert(int, double);
     method public java.lang.StringBuilder replace(int, int, java.lang.String);
     method public java.lang.StringBuilder reverse();
+    method public java.lang.String toString();
   }
 
   public class StringIndexOutOfBoundsException extends java.lang.IndexOutOfBoundsException {