Added java comments, java docs.

Signed-off-by: Dooyong Kim <[email protected]>

src/main/java/org/opensearch/knn/index/codec/util/KNNCodecUtil.java

@@ -90,6 +90,14 @@ public static List<String> getEngineFiles(String extension, String fieldName, Se
         return engineFiles;
     }
 
+    /**
+     * Get engine file name from given field and segment info.
+     * Ex: _0_165_my_field.faiss
+     *
+     * @param field : Field info that might have a vector index file. Not always it has it.
+     * @param segmentInfo : Segment where we are collecting an engine file list.
+     * @return : Found vector engine names, if not found, returns null.
+     */
     public static String getEngineFileFromFieldInfo(FieldInfo field, SegmentInfo segmentInfo) {
         if (!field.attributes().containsKey(KNN_FIELD)) {
             return null;

src/main/java/org/opensearch/knn/index/codec/util/NativeMemoryCacheKeyHelper.java

@@ -26,6 +26,14 @@ public static String constructCacheKey(final String vectorIndexFileName, final S
         return cacheKey;
     }
 
+    /**
+     * From cacheKey, we extract a vector file name.
+     * Note that expected format of cacheKey consists of two part with '@' as a delimiter.
+     * First part would be the vector file name, the second one is the segment id.
+     *
+     * @param cacheKey : Cache key for {@link org.opensearch.knn.index.memory.NativeMemoryCacheManager}
+     * @return : Vector file name, if the given cacheKey was invalid format, returns null.
+     */
     public static String extractVectorIndexFileName(final String cacheKey) {
         final int indexOfDelimiter = cacheKey.indexOf('@');
         if (indexOfDelimiter != -1) {

src/main/java/org/opensearch/knn/index/memory/NativeMemoryAllocation.java

@@ -129,7 +129,7 @@ class IndexAllocation implements NativeMemoryAllocation {
          * @param memoryAddress Pointer in memory to the index
          * @param sizeKb Size this index consumes in kilobytes
          * @param knnEngine KNNEngine associated with the index allocation
-         * @param vectorFileName File path to index
+         * @param vectorFileName Vector file name. Ex: _0_165_my_field.faiss
          * @param openSearchIndexName Name of OpenSearch index this index is associated with
          */
         IndexAllocation(
@@ -150,7 +150,7 @@ class IndexAllocation implements NativeMemoryAllocation {
          * @param memoryAddress Pointer in memory to the index
          * @param sizeKb Size this index consumes in kilobytes
          * @param knnEngine KNNEngine associated with the index allocation
-         * @param vectorFileName Vector file name
+         * @param vectorFileName Vector file name. Ex: _0_165_my_field.faiss
          * @param openSearchIndexName Name of OpenSearch index this index is associated with
          * @param sharedIndexState Shared index state. If not shared state present, pass null.
          */