address comments

java/CMakeLists.txt

@@ -123,6 +123,7 @@ set(NATIVE_JAVA_CLASSES
         org.rocksdb.SstFileWriter
         org.rocksdb.Statistics
         org.rocksdb.StringAppendOperator
+        org.rocksdb.StringAppendOperatorWithVariableDelimitor
         org.rocksdb.TableFormatConfig
         org.rocksdb.Transaction
         org.rocksdb.TransactionDB
@@ -260,6 +261,7 @@ add_jar(
   src/main/java/org/rocksdb/StatsLevel.java
   src/main/java/org/rocksdb/Status.java
   src/main/java/org/rocksdb/StringAppendOperator.java
+  src/main/java/org/rocksdb/StringAppendOperatorWithVariableDelimitor.java
   src/main/java/org/rocksdb/TableFormatConfig.java
   src/main/java/org/rocksdb/TickerType.java
   src/main/java/org/rocksdb/TransactionalDB.java

java/Makefile

@@ -60,6 +60,7 @@ NATIVE_JAVA_CLASSES = org.rocksdb.AbstractCompactionFilter\
 	org.rocksdb.VectorMemTableConfig\
 	org.rocksdb.Snapshot\
 	org.rocksdb.StringAppendOperator\
+	org.rocksdb.StringAppendOperatorWithVariableDelimitor\
 	org.rocksdb.WriteBatch\
 	org.rocksdb.WriteBatch.Handler\
 	org.rocksdb.WriteOptions\

java/rocksjni/merge_operator.cc

@@ -13,6 +13,7 @@
 #include <string>
 
 #include "include/org_rocksdb_StringAppendOperator.h"
+#include "include/org_rocksdb_StringAppendOperatorWithVariableDelimitor.h"
 #include "rocksdb/db.h"
 #include "rocksdb/memtablerep.h"
 #include "rocksdb/merge_operator.h"
@@ -37,10 +38,23 @@ jlong Java_org_rocksdb_StringAppendOperator_newSharedStringAppendOperator(
 
 /*
  * Class:     org_rocksdb_StringAppendOperator
+ * Method:    disposeInternal
+ * Signature: (J)V
+ */
+void Java_org_rocksdb_StringAppendOperator_disposeInternal(JNIEnv* /*env*/,
+                                                           jobject /*jobj*/,
+                                                           jlong jhandle) {
+  auto* sptr_string_append_op =
+      reinterpret_cast<std::shared_ptr<rocksdb::MergeOperator>*>(jhandle);
+  delete sptr_string_append_op;  // delete std::shared_ptr
+}
+
+/*
+ * Class:     org_rocksdb_StringAppendOperatorWithVariableDelimitor
  * Method:    newSharedStringAppendTESTOperator
  * Signature: ([B)J
  */
-jlong Java_org_rocksdb_StringAppendOperator_newSharedStringAppendTESTOperator(
+jlong Java_org_rocksdb_StringAppendOperatorWithVariableDelimitor_newSharedStringAppendTESTOperator(
         JNIEnv* env, jclass /*jclazz*/, jbyteArray jdelim) {
   jboolean has_exception = JNI_FALSE;
   std::string delim = rocksdb::JniUtil::byteString<std::string>(
@@ -58,14 +72,14 @@ jlong Java_org_rocksdb_StringAppendOperator_newSharedStringAppendTESTOperator(
 }
 
 /*
- * Class:     org_rocksdb_StringAppendOperator
+ * Class:     org_rocksdb_StringAppendOperatorWithVariableDelimitor
  * Method:    disposeInternal
  * Signature: (J)V
  */
-void Java_org_rocksdb_StringAppendOperator_disposeInternal(JNIEnv* /*env*/,
-                                                           jobject /*jobj*/,
-                                                           jlong jhandle) {
-  auto* sptr_string_append_op =
-      reinterpret_cast<std::shared_ptr<rocksdb::MergeOperator>*>(jhandle);
-  delete sptr_string_append_op;  // delete std::shared_ptr
+void Java_org_rocksdb_StringAppendOperatorWithVariableDelimitor_disposeInternal(JNIEnv* /*env*/,
+                                                                                jobject /*jobj*/,
+                                                                                jlong jhandle) {
+  auto* sptr_string_append_test_op =
+          reinterpret_cast<std::shared_ptr<rocksdb::MergeOperator>*>(jhandle);
+  delete sptr_string_append_test_op;  // delete std::shared_ptr
 }

java/src/main/java/org/rocksdb/StringAppendOperator.java

@@ -5,8 +5,6 @@
 
 package org.rocksdb;
 
-import java.nio.charset.Charset;
-
 /**
  * StringAppendOperator is a merge operator that concatenates
  * two strings.
@@ -20,19 +18,6 @@ public StringAppendOperator(char delim) {
         super(newSharedStringAppendOperator(delim));
     }
 
-    public StringAppendOperator(byte[] delim) {
-        super(newSharedStringAppendTESTOperator(delim));
-    }
-
-    public StringAppendOperator(String delim) {
-        this(delim.getBytes());
-    }
-
-    public StringAppendOperator(String delim, Charset charset) {
-        this(delim.getBytes(charset));
-    }
-
     private native static long newSharedStringAppendOperator(final char delim);
-    private native static long newSharedStringAppendTESTOperator(final byte[] delim);
     @Override protected final native void disposeInternal(final long handle);
 }

java/src/main/java/org/rocksdb/StringAppendOperatorWithVariableDelimitor.java

@@ -0,0 +1,49 @@
+/*
+ * 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
+ *
+ *     http://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.rocksdb;
+
+import java.nio.charset.Charset;
+
+/**
+ * Merge operator that concatenates two strings with a delimiter of variable length, string or byte array.
+ */
+public class StringAppendOperatorWithVariableDelimitor extends MergeOperator {
+    public StringAppendOperatorWithVariableDelimitor() {
+        this(',');
+    }
+
+    public StringAppendOperatorWithVariableDelimitor(char delim) {
+        this(Character.toString(delim));
+    }
+
+    public StringAppendOperatorWithVariableDelimitor(byte[] delim) {
+        super(newSharedStringAppendTESTOperator(delim));
+    }
+
+    public StringAppendOperatorWithVariableDelimitor(String delim) {
+        this(delim.getBytes());
+    }
+
+    public StringAppendOperatorWithVariableDelimitor(String delim, Charset charset) {
+        this(delim.getBytes(charset));
+    }
+
+    private native static long newSharedStringAppendTESTOperator(final byte[] delim);
+    @Override protected final native void disposeInternal(final long handle);
+}

java/src/test/java/org/rocksdb/MergeTest.java

@@ -115,7 +115,7 @@ public void stringDelimiter() throws RocksDBException {
   }
 
   private void stringDelimiter(String delim) throws RocksDBException {
-    try (final StringAppendOperator stringAppendOperator = new StringAppendOperator(delim.getBytes());
+    try (final MergeOperator stringAppendOperator = new StringAppendOperatorWithVariableDelimitor(delim.getBytes());
          final Options opt = new Options()
                  .setCreateIfMissing(true)
                  .setMergeOperator(stringAppendOperator);