graphql-java
diff --git a/src/main/java/graphql/schema/PropertyFetchingImpl.java b/src/main/java/graphql/schema/PropertyFetchingImpl.java
@@ -244,12 +244,51 @@ private Method findPubliclyAccessibleMethod(CacheKey cacheKey, Class<?> rootClas
                     return method;
                 }
             }
+            // Check public interfaces implemented by this class (handles non-public classes
+            // like TreeMap.Entry that implement public interfaces like Map.Entry)
+            Method method = findMethodOnPublicInterfaces(cacheKey, currentClass.getInterfaces(), methodName, dfeInUse, allowStaticMethods);
+            if (method != null) {
+                return method;
+            }
             currentClass = currentClass.getSuperclass();
         }
         assert rootClass != null;
         return rootClass.getMethod(methodName);
     }
 
+    private Method findMethodOnPublicInterfaces(CacheKey cacheKey, Class<?>[] interfaces, String methodName, boolean dfeInUse, boolean allowStaticMethods) {
+        for (Class<?> iface : interfaces) {
+            if (Modifier.isPublic(iface.getModifiers())) {
+                if (dfeInUse) {
+                    try {
+                        Method method = iface.getMethod(methodName, singleArgumentType);
+                        if (isSuitablePublicMethod(method, allowStaticMethods)) {
+                            METHOD_CACHE.putIfAbsent(cacheKey, new CachedMethod(method));
+                            return method;
+                        }
+                    } catch (NoSuchMethodException e) {
+                        // ok try the next approach
+                    }
+                }
+                try {
+                    Method method = iface.getMethod(methodName);
+                    if (isSuitablePublicMethod(method, allowStaticMethods)) {
+                        METHOD_CACHE.putIfAbsent(cacheKey, new CachedMethod(method));
+                        return method;
+                    }
+                } catch (NoSuchMethodException e) {
+                    // continue searching
+                }
+            }
+            // Also search super-interfaces of non-public interfaces
+            Method method = findMethodOnPublicInterfaces(cacheKey, iface.getInterfaces(), methodName, dfeInUse, allowStaticMethods);
+            if (method != null) {
+                return method;
+            }
+        }
+        return null;
+    }
+
     private boolean isSuitablePublicMethod(Method method, boolean allowStaticMethods) {
         int methodModifiers = method.getModifiers();
         if (Modifier.isPublic(methodModifiers)) {

diff --git a/src/test/groovy/graphql/schema/PropertyDataFetcherTest.groovy b/src/test/groovy/graphql/schema/PropertyDataFetcherTest.groovy
@@ -7,6 +7,7 @@ import graphql.schema.fetching.ConfusedPojo
 import graphql.schema.somepackage.ClassWithDFEMethods
 import graphql.schema.somepackage.ClassWithInterfaces
 import graphql.schema.somepackage.ClassWithInteritanceAndInterfaces
+import graphql.schema.somepackage.InterfaceInheritanceHolder
 import graphql.schema.somepackage.RecordLikeClass
 import graphql.schema.somepackage.RecordLikeTwoClassesDown
 import graphql.schema.somepackage.TestClass
@@ -788,6 +789,124 @@ class PropertyDataFetcherTest extends Specification {
 
     class OtherObject extends BaseObject {}
 
+    def "fetch via public interface method on non-public class - issue 4278"() {
+        given:
+        // TreeMap.Entry is a package-private class implementing the public Map.Entry interface
+        // On Java 16+, setAccessible fails on JDK internal classes, so the only way to invoke
+        // getValue() is by finding it through the public Map.Entry interface
+        PropertyDataFetcherHelper.setUseLambdaFactory(false)
+        PropertyDataFetcher.clearReflectionCache()
+
+        def treeMap = new TreeMap<String, String>()
+        treeMap.put("testKey", "testValue")
+        def entry = treeMap.entrySet().iterator().next()
+        def environment = env("value", entry)
+
+        when:
+        def result = fetcher.get(environment)
+
+        then:
+        result == "testValue"
+
+        where:
+        fetcher                                  | _
+        new PropertyDataFetcher("value")         | _
+        SingletonPropertyDataFetcher.singleton() | _
+    }
+
+    def "fetch via public interface method on non-public class for key - issue 4278"() {
+        given:
+        PropertyDataFetcherHelper.setUseLambdaFactory(false)
+        PropertyDataFetcher.clearReflectionCache()
+
+        def treeMap = new TreeMap<String, String>()
+        treeMap.put("testKey", "testValue")
+        def entry = treeMap.entrySet().iterator().next()
+        def environment = env("key", entry)
+
+        when:
+        def result = fetcher.get(environment)
+
+        then:
+        result == "testKey"
+
+        where:
+        fetcher                                  | _
+        new PropertyDataFetcher("key")           | _
+        SingletonPropertyDataFetcher.singleton() | _
+    }
+
+    def "fetch method from public interface through package-private interface chain"() {
+        given:
+        // PackagePrivateChainImpl (package-private) implements PackagePrivateMiddleInterface (package-private)
+        // which extends PublicBaseInterface (public) — defines getBaseValue()
+        // The recursive interface search must traverse through the package-private middle interface
+        PropertyDataFetcherHelper.setUseLambdaFactory(false)
+        PropertyDataFetcher.clearReflectionCache()
+
+        def obj = InterfaceInheritanceHolder.createChainImpl()
+        def environment = env("baseValue", obj)
+
+        when:
+        def result = new PropertyDataFetcher("baseValue").get(environment)
+
+        then:
+        result == "baseValue"
+    }
+
+    def "fetch method through diamond interface inheritance"() {
+        given:
+        // DiamondImpl (package-private) implements both PackagePrivateBranchA and PackagePrivateBranchB
+        // Both are package-private interfaces extending PublicBaseInterface (public) — defines getBaseValue()
+        // The search must find getBaseValue() through either branch
+        PropertyDataFetcherHelper.setUseLambdaFactory(false)
+        PropertyDataFetcher.clearReflectionCache()
+
+        def obj = InterfaceInheritanceHolder.createDiamondImpl()
+
+        expect:
+        new PropertyDataFetcher(property).get(env(property, obj)) == expected
+
+        where:
+        property    | expected
+        "baseValue" | "diamondBaseValue"
+    }
+
+    def "fetch via public interface method with DataFetchingEnvironment parameter on non-public class"() {
+        given:
+        // PackagePrivateDfeImpl implements PublicDfeInterface which declares getDfeValue(DataFetchingEnvironment)
+        // This exercises the dfeInUse path in findMethodOnPublicInterfaces (lines 262-267)
+        PropertyDataFetcherHelper.setUseLambdaFactory(false)
+        PropertyDataFetcher.clearReflectionCache()
+
+        def obj = InterfaceInheritanceHolder.createDfeImpl()
+        def environment = env("dfeValue", obj)
+
+        when:
+        def result = new PropertyDataFetcher("dfeValue").get(environment)
+
+        then:
+        result == "dfeValue"
+    }
+
+    def "fetch via interface search hits NoSuchMethodException and continues to next interface"() {
+        given:
+        // PackagePrivateMultiInterfaceImpl implements PublicInterfaceWithoutTarget (no getBaseValue)
+        // and PublicBaseInterface (has getBaseValue). The search must hit NoSuchMethodException
+        // on the first interface and continue to find it on the second.
+        PropertyDataFetcherHelper.setUseLambdaFactory(false)
+        PropertyDataFetcher.clearReflectionCache()
+
+        def obj = InterfaceInheritanceHolder.createMultiInterfaceImpl()
+        def environment = env("baseValue", obj)
+
+        when:
+        def result = new PropertyDataFetcher("baseValue").get(environment)
+
+        then:
+        result == "foundViaSecondInterface"
+    }
+
     def "Can access private property from base class that starts with i in Turkish"() {
         // see https://github.com/graphql-java/graphql-java/issues/3385
         given:

diff --git a/src/test/groovy/graphql/schema/somepackage/InterfaceInheritanceHolder.java b/src/test/groovy/graphql/schema/somepackage/InterfaceInheritanceHolder.java
@@ -0,0 +1,138 @@
+package graphql.schema.somepackage;
+
+import graphql.schema.DataFetchingEnvironment;
+
+/**
+ * Test fixtures for interface-extends-interface method resolution.
+ * <p>
+ * Tests the recursive interface search in findMethodOnPublicInterfaces:
+ * a package-private class implements a package-private interface that extends
+ * a public interface. The method is only declared on the public grandparent,
+ * so the algorithm must recursively traverse through the package-private
+ * interface to find it.
+ * <p>
+ * Linear chain:
+ * <pre>
+ *   PublicBaseInterface (public)              — defines getBaseValue()
+ *        |
+ *   PackagePrivateMiddleInterface            — extends PublicBaseInterface (adds nothing new)
+ *        |
+ *   PackagePrivateChainImpl (package-private) — implements PackagePrivateMiddleInterface
+ * </pre>
+ * <p>
+ * Diamond pattern:
+ * <pre>
+ *        PublicBaseInterface (public)          — defines getBaseValue()
+ *             /          \
+ *  PackagePrivateBranchA   PackagePrivateBranchB  — each adds own method
+ *             \          /
+ *        DiamondImpl (package-private)
+ * </pre>
+ */
+public class InterfaceInheritanceHolder {
+
+    // --- Linear interface chain ---
+
+    public interface PublicBaseInterface {
+        String getBaseValue();
+    }
+
+    // Package-private interface extending a public interface — adds no new methods
+    interface PackagePrivateMiddleInterface extends PublicBaseInterface {
+    }
+
+    // Package-private class: only implements the package-private interface.
+    // getBaseValue() is declared on PublicBaseInterface — the recursive search
+    // must traverse PackagePrivateMiddleInterface -> PublicBaseInterface to find it.
+    static class PackagePrivateChainImpl implements PackagePrivateMiddleInterface {
+        @Override
+        public String getBaseValue() {
+            return "baseValue";
+        }
+    }
+
+    // --- Diamond pattern ---
+
+    // Two package-private interfaces both extending PublicBaseInterface
+    interface PackagePrivateBranchA extends PublicBaseInterface {
+        String getBranchAValue();
+    }
+
+    interface PackagePrivateBranchB extends PublicBaseInterface {
+        String getBranchBValue();
+    }
+
+    // Package-private class implementing both branches (diamond).
+    // getBaseValue() is only on PublicBaseInterface — must be found through either branch.
+    static class DiamondImpl implements PackagePrivateBranchA, PackagePrivateBranchB {
+        @Override
+        public String getBaseValue() {
+            return "diamondBaseValue";
+        }
+
+        @Override
+        public String getBranchAValue() {
+            return "branchAValue";
+        }
+
+        @Override
+        public String getBranchBValue() {
+            return "branchBValue";
+        }
+    }
+
+    // --- DFE interface: public interface with a method accepting DataFetchingEnvironment ---
+
+    public interface PublicDfeInterface {
+        String getDfeValue(DataFetchingEnvironment dfe);
+    }
+
+    // Package-private class implementing the public DFE interface.
+    // Exercises the dfeInUse path in findMethodOnPublicInterfaces.
+    static class PackagePrivateDfeImpl implements PublicDfeInterface {
+        @Override
+        public String getDfeValue(DataFetchingEnvironment dfe) {
+            return "dfeValue";
+        }
+    }
+
+    // --- Interface with multiple methods: one exists, one doesn't ---
+    // Used to exercise the NoSuchMethodException catch path in findMethodOnPublicInterfaces.
+
+    public interface PublicInterfaceWithoutTarget {
+        String getUnrelatedValue();
+    }
+
+    // Package-private class implementing an interface that does NOT have the fetched property.
+    // Also implements PublicBaseInterface which DOES have it.
+    // The search hits NoSuchMethodException on PublicInterfaceWithoutTarget, then finds it on PublicBaseInterface.
+    static class PackagePrivateMultiInterfaceImpl implements PublicInterfaceWithoutTarget, PublicBaseInterface {
+        @Override
+        public String getUnrelatedValue() {
+            return "unrelated";
+        }
+
+        @Override
+        public String getBaseValue() {
+            return "foundViaSecondInterface";
+        }
+    }
+
+    // --- Factory methods (public entry points for tests) ---
+
+    public static Object createChainImpl() {
+        return new PackagePrivateChainImpl();
+    }
+
+    public static Object createDiamondImpl() {
+        return new DiamondImpl();
+    }
+
+    public static Object createDfeImpl() {
+        return new PackagePrivateDfeImpl();
+    }
+
+    public static Object createMultiInterfaceImpl() {
+        return new PackagePrivateMultiInterfaceImpl();
+    }
+}