From 4b12aa52f8aaf5b873bda0e2b615ce9dce05fc77 Mon Sep 17 00:00:00 2001
From: Sean Arms <67096+lesserwhirls@users.noreply.github.com>
Date: Wed, 10 Jun 2026 16:31:10 -0600
Subject: [PATCH] Fix NcML DataType conversion in old NetcdfDataset API

When using NcML to redefine coordinate variable type, the old
NetcdfDataset API was falling back to the original variable datatype.
This behavior seems to be OK in the old API datapath as long as the old
and new data types are consistently numeric (or not), as enhancements
will "do the rigth thing", but otherwise not (e.g. int to String
conversion using NcML). Fixes Unidata/netcdf-java#1540.
---
 .../java/ucar/nc2/dataset/VariableDS.java     |   9 +++-
 .../test/data/ncml/coords/nc/string_coord.nc  | Bin 0 -> 6152 bytes
 .../test/data/ncml/coords/string_coord.ncml   |   8 ++++
 .../dataset/TestCoordinateAxisDataType.java   |  45 ++++++++++++++++++
 4 files changed, 60 insertions(+), 2 deletions(-)
 create mode 100644 cdm/core/src/test/data/ncml/coords/nc/string_coord.nc
 create mode 100644 cdm/core/src/test/data/ncml/coords/string_coord.ncml
 create mode 100644 cdm/core/src/test/java/ucar/nc2/dataset/TestCoordinateAxisDataType.java

diff --git a/cdm/core/src/main/java/ucar/nc2/dataset/VariableDS.java b/cdm/core/src/main/java/ucar/nc2/dataset/VariableDS.java
index a57ece8994..dc82c774f5 100644
--- a/cdm/core/src/main/java/ucar/nc2/dataset/VariableDS.java
+++ b/cdm/core/src/main/java/ucar/nc2/dataset/VariableDS.java
@@ -1,7 +1,8 @@
 /*
- * Copyright (c) 1998-2025 John Caron and University Corporation for Atmospheric Research/Unidata
+ * Copyright (c) 1998-2026 John Caron and University Corporation for Atmospheric Research/Unidata
  * See LICENSE for license information.
  */
+
 package ucar.nc2.dataset;
 
 import com.google.common.collect.ImmutableList;
@@ -239,7 +240,11 @@ public void enhance(Set<Enhance> enhancements) {
     // So, we need to reset to default before we process this new set.
     // LOOK this seems bogus
     if (orgDataType != null) {
-      setDataType(orgDataType);
+      // only reset if original DataType and current DataType are compatible (both
+      // are numeric or both are not numeric).
+      if (orgDataType.isNumeric() == dataType.isNumeric()) {
+        setDataType(orgDataType);
+      }
     }
 
     createEnhancements();
diff --git a/cdm/core/src/test/data/ncml/coords/nc/string_coord.nc b/cdm/core/src/test/data/ncml/coords/nc/string_coord.nc
new file mode 100644
index 0000000000000000000000000000000000000000..56d8e8d2991a556f681874857e0c39eaf0b97a2b
GIT binary patch
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literal 0
HcmV?d00001

diff --git a/cdm/core/src/test/data/ncml/coords/string_coord.ncml b/cdm/core/src/test/data/ncml/coords/string_coord.ncml
new file mode 100644
index 0000000000..9d9662b335
--- /dev/null
+++ b/cdm/core/src/test/data/ncml/coords/string_coord.ncml
@@ -0,0 +1,8 @@
+<netcdf xmlns="https://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2"
+  location="./nc/string_coord.nc">
+  <!-- https://github.com/Unidata/netcdf-java/issues/1540 -->
+  <!-- CoordinateAxis1D should be of type String in both old and new API  -->
+  <variable name="var" type="String" shape="dim">
+    <values>0 1</values>
+  </variable>
+</netcdf>
\ No newline at end of file
diff --git a/cdm/core/src/test/java/ucar/nc2/dataset/TestCoordinateAxisDataType.java b/cdm/core/src/test/java/ucar/nc2/dataset/TestCoordinateAxisDataType.java
new file mode 100644
index 0000000000..d78a07f630
--- /dev/null
+++ b/cdm/core/src/test/java/ucar/nc2/dataset/TestCoordinateAxisDataType.java
@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2026 University Corporation for Atmospheric Research/Unidata
+ * See LICENSE for license information.
+ */
+
+package ucar.nc2.dataset;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import java.io.IOException;
+
+import org.junit.Test;
+import ucar.ma2.DataType;
+import ucar.nc2.Variable;
+import ucar.unidata.util.test.TestDir;
+
+public class TestCoordinateAxisDataType {
+
+  String testFile = TestDir.cdmLocalTestDataDir + "/ncml/coords/string_coord.ncml";
+
+  @Test
+  public void testDataTypeChangeOldApi() throws IOException {
+    try (NetcdfDataset ncdOld = NetcdfDataset.openDataset(testFile)) {
+      checkDataType(ncdOld);
+    }
+  }
+
+  @Test
+  public void testDataTypeChangeNewApi() throws IOException {
+    String testFile = TestDir.cdmLocalTestDataDir + "/ncml/coords/string_coord.ncml";
+    try (NetcdfDataset ncdNew = NetcdfDatasets.openDataset(testFile)) {
+      checkDataType(ncdNew);
+    }
+  }
+
+  private void checkDataType(NetcdfDataset ncd) {
+    Variable var = ncd.findVariable("var");
+    assertThat(var != null).isTrue();
+    CoordinateAxis1D axis = (CoordinateAxis1D) var;
+    assertThat(axis.getOriginalDataType()).isEqualTo(DataType.INT);
+    assertThat(axis.getDataType()).isEqualTo(DataType.STRING);
+    assertThat(axis.isNumeric()).isFalse();
+  }
+
+}