Fix Internal to Generic conversion of TIMESTAMP_NANO

Author: ayushtkn

core/src/main/java/org/apache/iceberg/data/GenericDataUtil.java

@@ -51,6 +51,12 @@ public static Object internalToGeneric(Type type, Object value) {
         } else {
           return DateTimeUtil.timestampFromMicros((Long) value);
         }
+      case TIMESTAMP_NANO:
+        if (((Types.TimestampNanoType) type).shouldAdjustToUTC()) {
+          return DateTimeUtil.timestamptzFromNanos((Long) value);
+        } else {
+          return DateTimeUtil.timestampFromNanos((Long) value);
+        }
       case FIXED:
         return ByteBuffers.toByteArray((ByteBuffer) value);
     }

core/src/test/java/org/apache/iceberg/data/DataTestBase.java

@@ -20,6 +20,7 @@
 
 import static org.apache.iceberg.types.Types.NestedField.optional;
 import static org.apache.iceberg.types.Types.NestedField.required;
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.assertj.core.api.Assumptions.assumeThat;
 
@@ -28,6 +29,8 @@
 import java.math.BigDecimal;
 import java.nio.ByteBuffer;
 import java.nio.file.Path;
+import java.time.LocalDateTime;
+import java.time.OffsetDateTime;
 import java.util.List;
 import java.util.Map;
 import java.util.UUID;
@@ -595,6 +598,7 @@ public void testPrimitiveTypeDefaultValues(Type.PrimitiveType type, Literal<?> d
   }
 
   @Test
+  @SuppressWarnings("checkstyle:AssertThatThrownByWithMessageCheck")
   public void testWriteNullValueForRequiredType() throws Exception {
     Schema schema =
         new Schema(
@@ -645,4 +649,18 @@ public void testRowLineage() throws Exception {
             record.copy(Map.of("id", 4L, "data", "d", "_row_id", 1_001L)),
             record.copy(Map.of("id", 5L, "data", "e"))));
   }
+
+  @Test
+  public void testTimestampNanoMaterializedAsJavaTime() {
+    assumeThat(supportsTimestampNanos()).as("timestamp_ns is not yet implemented").isTrue();
+
+    long epochNanos = DateTimeUtil.isoTimestampToNanos("2025-01-15T11:25:20.111456789");
+    assertThat(GenericDataUtil.internalToGeneric(Types.TimestampNanoType.withZone(), epochNanos))
+        .as("TIMESTAMP_NANO with zone must be materialized as OffsetDateTime")
+        .isInstanceOf(OffsetDateTime.class);
+
+    assertThat(GenericDataUtil.internalToGeneric(Types.TimestampNanoType.withoutZone(), epochNanos))
+        .as("TIMESTAMP_NANO must be materialized as LocalDateTime")
+        .isInstanceOf(LocalDateTime.class);
+  }
 }