HeapCache shutdown method

here-naksha-lib-heapcache/src/main/java/com/here/naksha/lib/heapcache/HeapCache.java

@@ -19,14 +19,19 @@
 package com.here.naksha.lib.heapcache;
 
 import com.here.naksha.lib.core.NakshaContext;
+import com.here.naksha.lib.core.lambdas.Fe1;
 import com.here.naksha.lib.core.lambdas.Pe1;
 import com.here.naksha.lib.core.models.TxSignalSet;
 import com.here.naksha.lib.core.models.geojson.implementation.XyzFeature;
 import com.here.naksha.lib.core.storage.*;
 import com.here.naksha.lib.core.util.fib.FibSet;
+
 import java.lang.ref.WeakReference;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
@@ -101,7 +106,8 @@ public void removeCacheEntry(String key) {
   protected final @NotNull FibSet<String, CacheEntry> cache = new FibSet<>(CacheEntry::new);
 
   @Override
-  public void init() {}
+  public void init() {
+  }
 
   @Override
   public void maintain(@NotNull List<CollectionInfo> collectionInfoList) {
@@ -128,39 +134,45 @@ public void maintain(@NotNull List<CollectionInfo> collectionInfoList) {
   }
 
   @Override
-  public void addListener(@NotNull Pe1<@NotNull TxSignalSet> listener) {}
+  public void addListener(@NotNull Pe1<@NotNull TxSignalSet> listener) {
+  }
 
   @Override
   public boolean removeListener(@NotNull Pe1<@NotNull TxSignalSet> listener) {
     return false;
   }
 
   @Override
-  public void close() {}
+  public void close() {
+  }
 
   /**
    * Initializes the storage, create the transaction table, install needed scripts and extensions.
    */
   @Override
-  public void initStorage() {}
+  public void initStorage() {
+  }
 
   /**
    * Starts the maintainer thread that will take about history garbage collection, sequencing and other background jobs.
    */
   @Override
-  public void startMaintainer() {}
+  public void startMaintainer() {
+  }
 
   /**
    * Blocking call to perform maintenance tasks right now. One-time maintenance.
    */
   @Override
-  public void maintainNow() {}
+  public void maintainNow() {
+  }
 
   /**
    * Stops the maintainer thread.
    */
   @Override
-  public void stopMaintainer() {}
+  public void stopMaintainer() {
+  }
 
   /**
    * Open a new write-session, optionally to a master-node (when being in a multi-writer cluster).
@@ -185,4 +197,17 @@ public void stopMaintainer() {}
   public @NotNull IReadSession newReadSession(@Nullable NakshaContext context, boolean useMaster) {
     return IStorage.super.newReadSession(context, useMaster);
   }
+
+  /**
+   * Shutdown the storage instance asynchronously. This method returns asynchronously whatever the given {@code onShutdown} handler returns.
+   * If no shutdown handler given, then {@code null} is returned.
+   *
+   * @param onShutdown The (optional) method to call when the shutdown is done.
+   * @return The future when the shutdown will be done.
+   */
+  @Override
+  public @NotNull <T> Future<T> shutdown(@Nullable Fe1<T, IStorage> onShutdown) {
+    // actually the method is never called in HeapCache, but I don't want to return null in @NotNull tagged method.
+    return Executors.newSingleThreadExecutor().submit(() -> null);
+  }
 }