1 files changed, 32 insertions, 2 deletions
diff --git a/src/main/java/com/fasterxml/jackson/databind/util/IgnorePropertiesUtil.java b/src/main/java/com/fasterxml/jackson/databind/util/IgnorePropertiesUtil.java
index 4fd701874..88984289f 100644
--- a/src/main/java/com/fasterxml/jackson/databind/util/IgnorePropertiesUtil.java
+++ b/src/main/java/com/fasterxml/jackson/databind/util/IgnorePropertiesUtil.java
@@ -1,6 +1,8 @@
 package com.fasterxml.jackson.databind.util;
 
 import java.util.Collection;
+import java.util.HashSet;
+import java.util.Set;
 
 /**
  * @since 2.12
@@ -9,8 +11,6 @@ public class IgnorePropertiesUtil
 {
     /**
      * Decide if we need to ignore a property or not, given a set of field to ignore and a set of field to include.
-     *
-     * @since 2.12
      */
     public static boolean shouldIgnore(Object value, Collection<String> toIgnore, Collection<String> toInclude) {
         if (toIgnore == null && toInclude ==null) {
@@ -28,4 +28,34 @@ public class IgnorePropertiesUtil
         // NOTE: conflict between both, JsonIncludeProperties will take priority.
         return !toInclude.contains(value) || toIgnore.contains(value);
     }
+
+    /**
+     * Helper that encapsulates logic for combining two sets of "included names":
+     * default logic is to do intersection (name must be in both to be included
+     * in result)
+     * 
+     * @param prevToInclude Existing set of names to include, if defined; null means "not defined"
+     * @param newToInclude New set of names to included, if defined; null means "not defined"
+     *
+     * @return Resulting set of names, using intersection if neither {@code null}; or the
+     *    non-null one (if only one is {@code null}); or {@code null} if both arguments {@code null}.
+     */
+    public static Set<String> combineNamesToInclude(Set<String> prevToInclude,
+            Set<String> newToInclude) {
+        if (prevToInclude == null) {
+            return newToInclude;
+        }
+        if (newToInclude == null) {
+            return prevToInclude;
+        }
+        final Set<String> result = new HashSet<>();
+
+        // Make the intersection with the previously included properties
+        for (String prop : newToInclude) {
+            if (prevToInclude.contains(prop)) {
+                result.add(prop);
+            }
+        }
+        return result;
+    }
 }