[Java] Add support for enum default values in IDL serialization

lang/java/idl/src/main/java/org/apache/avro/idl/IdlUtils.java

@@ -285,7 +285,12 @@ private static void writeSchema(Schema schema, boolean insideProtocol, Writer wr
       } else {
         throw new AvroRuntimeException("Enum schema must have at least a symbol " + schema);
       }
-      writer.append(NEWLINE).append(indent).append("}").append(NEWLINE);
+      writer.append(NEWLINE).append(indent).append("}");
+      var enumDefault = schema.getEnumDefault();
+      if (enumDefault != null) {
+        writer.append(" = ").append(enumDefault).append(";");
+      }
+      writer.append(NEWLINE);
     } else /* (type == Schema.Type.FIXED) */ {
       writer.append(indent).append("fixed ").append(schemaName).append('(')
           .append(Integer.toString(schema.getFixedSize())).append(");").append(NEWLINE);

lang/java/idl/src/test/java/org/apache/avro/idl/IdlUtilsTest.java

@@ -33,11 +33,14 @@
 import org.apache.avro.Schema;
 import org.junit.jupiter.api.Test;
 
+import static java.util.Arrays.asList;
 import static java.util.Collections.emptyList;
 import static java.util.Collections.singletonList;
 import static java.util.Objects.requireNonNull;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 public class IdlUtilsTest {
   @Test
@@ -103,6 +106,23 @@ public void cannotWriteProtocolWithUnnamedTypes() {
         () -> IdlUtils.writeIdlProtocol(new StringWriter(), Schema.create(Schema.Type.STRING)));
   }
 
+  @Test
+  public void enumDefaultIsWrittenToIdl() throws IOException {
+    Schema withDefault = Schema.createEnum("Status", null, "naming", asList("ACTIVE", "INACTIVE"), "ACTIVE");
+    Schema withoutDefault = Schema.createEnum("Status", null, "naming", asList("ACTIVE", "INACTIVE"));
+
+    StringWriter withDefaultWriter = new StringWriter();
+    IdlUtils.writeIdlProtocol(withDefaultWriter, withDefault);
+
+    StringWriter withoutDefaultWriter = new StringWriter();
+    IdlUtils.writeIdlProtocol(withoutDefaultWriter, withoutDefault);
+
+    assertTrue(withDefaultWriter.toString().contains("} = ACTIVE;"),
+        "Enum with default should serialize default value");
+    assertFalse(withoutDefaultWriter.toString().contains("="),
+        "Enum without default should not contain '='");
+  }
+
   @Test
   public void cannotWriteEmptyEnums() {
     assertThrows(AvroRuntimeException.class,