diff --git a/java/java-impl/src/com/intellij/psi/impl/smartPointers/JavaAnchorProvider.java b/java/java-impl/src/com/intellij/psi/impl/smartPointers/JavaAnchorProvider.java
index 99cf1e82073b..aa7469e0851a 100644
--- a/java/java-impl/src/com/intellij/psi/impl/smartPointers/JavaAnchorProvider.java
+++ b/java/java-impl/src/com/intellij/psi/impl/smartPointers/JavaAnchorProvider.java
@@ -17,14 +17,15 @@ package com.intellij.psi.impl.smartPointers;
 
 import com.intellij.lang.java.JavaLanguage;
 import com.intellij.psi.*;
+import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
 /**
  * @author Dennis.Ushakov
  */
-public class JavaAnchorProvider implements SmartPointerAnchorProvider {
+public class JavaAnchorProvider extends SmartPointerAnchorProvider {
   @Override
-  public PsiElement getAnchor(PsiElement element) {
+  public PsiElement getAnchor(@NotNull PsiElement element) {
     if (!element.getLanguage().isKindOf(JavaLanguage.INSTANCE)) {
       return null;
     }
@@ -45,7 +46,7 @@ public class JavaAnchorProvider implements SmartPointerAnchorProvider {
 
   @Nullable
   @Override
-  public PsiElement getElement(PsiElement anchor) {
+  public PsiElement getElement(@NotNull PsiElement anchor) {
     if (anchor instanceof PsiIdentifier) {
       PsiElement parent = anchor.getParent();
       if (parent instanceof PsiJavaCodeReferenceElement) { // anonymous class, type
diff --git a/platform/core-impl/src/com/intellij/psi/impl/smartPointers/AnchorElementInfoFactory.java b/platform/core-impl/src/com/intellij/psi/impl/smartPointers/AnchorElementInfoFactory.java
index d14e4869b0e4..efdc031f1beb 100644
--- a/platform/core-impl/src/com/intellij/psi/impl/smartPointers/AnchorElementInfoFactory.java
+++ b/platform/core-impl/src/com/intellij/psi/impl/smartPointers/AnchorElementInfoFactory.java
@@ -53,7 +53,7 @@ public class AnchorElementInfoFactory implements SmartPointerElementInfoFactory
   }
 
   @Nullable
-  static PsiElement getAnchor(PsiElement element) {
+  static PsiElement getAnchor(@NotNull PsiElement element) {
     PsiUtilCore.ensureValid(element);
     PsiElement anchor = null;
     for (SmartPointerAnchorProvider provider : SmartPointerAnchorProvider.EP_NAME.getExtensions()) {
diff --git a/platform/core-impl/src/com/intellij/psi/impl/smartPointers/SmartPointerAnchorProvider.java b/platform/core-impl/src/com/intellij/psi/impl/smartPointers/SmartPointerAnchorProvider.java
index d32aaa44fe9f..412a8be49177 100644
--- a/platform/core-impl/src/com/intellij/psi/impl/smartPointers/SmartPointerAnchorProvider.java
+++ b/platform/core-impl/src/com/intellij/psi/impl/smartPointers/SmartPointerAnchorProvider.java
@@ -17,17 +17,28 @@ package com.intellij.psi.impl.smartPointers;
 
 import com.intellij.openapi.extensions.ExtensionPointName;
 import com.intellij.psi.PsiElement;
+import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
 /**
+ * Allows SmartPointer that points to stubbed psi element to survive stub-to-AST switch
+ *
  * @author Dennis.Ushakov
  */
-public interface SmartPointerAnchorProvider {
-  ExtensionPointName<SmartPointerAnchorProvider> EP_NAME = ExtensionPointName.create("com.intellij.smartPointer.anchorProvider");
+public abstract class SmartPointerAnchorProvider {
+  public static final ExtensionPointName<SmartPointerAnchorProvider> EP_NAME = ExtensionPointName.create("com.intellij.smartPointer.anchorProvider");
 
+  /**
+   * @param element
+   * @return anchor to be used when restoring element after stub-to-AST switch
+   */
   @Nullable
-  PsiElement getAnchor(PsiElement element);
+  public abstract PsiElement getAnchor(@NotNull PsiElement element);
 
+  /**
+   * @param anchor
+   * @return restored original element using anchor
+   */
   @Nullable
-  PsiElement getElement(PsiElement anchor);
+  public abstract PsiElement getElement(@NotNull PsiElement anchor);
 }
diff --git a/xml/impl/src/com/intellij/psi/impl/smartPointers/XmlAnchorProvider.java b/xml/impl/src/com/intellij/psi/impl/smartPointers/XmlAnchorProvider.java
index ed05e1dd47ab..10d35944ebab 100644
--- a/xml/impl/src/com/intellij/psi/impl/smartPointers/XmlAnchorProvider.java
+++ b/xml/impl/src/com/intellij/psi/impl/smartPointers/XmlAnchorProvider.java
@@ -20,15 +20,16 @@ import com.intellij.psi.xml.XmlTag;
 import com.intellij.psi.xml.XmlToken;
 import com.intellij.psi.xml.XmlTokenType;
 import com.intellij.xml.util.XmlTagUtil;
+import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
 /**
  * @author Dennis.Ushakov
  */
-public class XmlAnchorProvider implements SmartPointerAnchorProvider {
+public class XmlAnchorProvider extends SmartPointerAnchorProvider {
   @Nullable
   @Override
-  public PsiElement getAnchor(PsiElement element) {
+  public PsiElement getAnchor(@NotNull PsiElement element) {
     if (element instanceof XmlTag) {
       return XmlTagUtil.getStartTagNameElement((XmlTag)element);
     }
@@ -37,7 +38,7 @@ public class XmlAnchorProvider implements SmartPointerAnchorProvider {
 
   @Nullable
   @Override
-  public PsiElement getElement(PsiElement anchor) {
+  public PsiElement getElement(@NotNull PsiElement anchor) {
     if (anchor instanceof XmlToken) {
       XmlToken token = (XmlToken)anchor;
       return token.getTokenType() == XmlTokenType.XML_NAME ? token.getParent() : null;