AVRO-XXXX: FastReaderBuilder throws ClassCastException for schemas with java-class attribute

When using GenericDatumReader with schemas containing "java-class"
attributes on string fields (e.g., java.math.BigDecimal),
FastReaderBuilder throws:

    ClassCastException: Utf8 cannot be cast to String

The bug is in getTransformingStringReader() which casts the result of
stringReader.read() directly to String, but GenericData returns Utf8.

This commit adds a failing test to reproduce the issue.

Author: kevinburkesegment

lang/java/avro/src/test/java/org/apache/avro/io/FastReaderBuilderJavaClassTest.java

@@ -0,0 +1,116 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.avro.io;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+
+import org.apache.avro.Schema;
+import org.apache.avro.generic.GenericData;
+import org.apache.avro.generic.GenericDatumReader;
+import org.apache.avro.generic.GenericDatumWriter;
+import org.apache.avro.generic.GenericRecord;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests for FastReaderBuilder behavior with schemas containing "java-class"
+ * attributes.
+ */
+public class FastReaderBuilderJavaClassTest {
+
+  /**
+   * Tests that GenericDatumReader can deserialize records with string fields that
+   * have a "java-class" attribute (e.g., BigDecimal).
+   *
+   * This test reproduces a bug where
+   * FastReaderBuilder.getTransformingStringReader() casts the result of
+   * stringReader.read() directly to String, but in GenericData mode the reader
+   * returns Utf8, causing a ClassCastException.
+   */
+  @Test
+  void genericDatumReaderWithJavaClassAttribute() throws IOException {
+    // Schema with a string field that has "java-class": "java.math.BigDecimal"
+    // This is a common pattern for representing decimal values as strings
+    String schemaJson = "{\n" + "  \"type\": \"record\",\n" + "  \"name\": \"TestRecord\",\n" + "  \"fields\": [\n"
+        + "    {\"name\": \"id\", \"type\": \"string\"},\n" + "    {\"name\": \"price\", \"type\": [\"null\", {\n"
+        + "      \"type\": \"string\",\n" + "      \"java-class\": \"java.math.BigDecimal\"\n" + "    }]}\n" + "  ]\n"
+        + "}";
+
+    Schema schema = new Schema.Parser().parse(schemaJson);
+
+    GenericRecord record = new GenericData.Record(schema);
+    record.put("id", "123");
+    record.put("price", "-0.0002");
+
+    ByteArrayOutputStream out = new ByteArrayOutputStream();
+    GenericDatumWriter<GenericRecord> writer = new GenericDatumWriter<>(schema);
+    BinaryEncoder encoder = EncoderFactory.get().binaryEncoder(out, null);
+    writer.write(record, encoder);
+    encoder.flush();
+
+    byte[] serialized = out.toByteArray();
+
+    // Deserialize using GenericDatumReader (which uses FastReaderBuilder by
+    // default)
+    GenericDatumReader<GenericRecord> reader = new GenericDatumReader<>(schema);
+    BinaryDecoder decoder = DecoderFactory.get().binaryDecoder(serialized, null);
+
+    // AVRO-XXXX this should not throw ClassCastException: Utf8 cannot be cast to
+    // String
+    GenericRecord result = reader.read(null, decoder);
+
+    assertNotNull(result);
+    assertEquals("123", result.get("id").toString());
+    assertEquals("-0.0002", result.get("price").toString());
+  }
+
+  /**
+   * Tests that GenericDatumReader can deserialize records with a direct string
+   * field (not in a union) that has a "java-class" attribute.
+   */
+  @Test
+  void genericDatumReaderWithDirectJavaClassString() throws IOException {
+    String schemaJson = "{\n" + "  \"type\": \"record\",\n" + "  \"name\": \"TestRecord\",\n" + "  \"fields\": [\n"
+        + "    {\"name\": \"amount\", \"type\": {\n" + "      \"type\": \"string\",\n"
+        + "      \"java-class\": \"java.math.BigDecimal\"\n" + "    }}\n" + "  ]\n" + "}";
+
+    Schema schema = new Schema.Parser().parse(schemaJson);
+
+    GenericRecord record = new GenericData.Record(schema);
+    record.put("amount", "123.45");
+
+    ByteArrayOutputStream out = new ByteArrayOutputStream();
+    GenericDatumWriter<GenericRecord> writer = new GenericDatumWriter<>(schema);
+    BinaryEncoder encoder = EncoderFactory.get().binaryEncoder(out, null);
+    writer.write(record, encoder);
+    encoder.flush();
+
+    byte[] serialized = out.toByteArray();
+
+    GenericDatumReader<GenericRecord> reader = new GenericDatumReader<>(schema);
+    BinaryDecoder decoder = DecoderFactory.get().binaryDecoder(serialized, null);
+
+    GenericRecord result = reader.read(null, decoder);
+
+    assertNotNull(result);
+    assertEquals("123.45", result.get("amount").toString());
+  }
+}