Use size hint for HashMap in multiGet. Similar to https://github.com/facebook/rocksdb/pull/1344 (#1367)

java/src/main/java/org/rocksdb/RocksDB.java

@@ -824,7 +824,8 @@ public class RocksDB extends RocksObject {
     final byte[][] values = multiGet(nativeHandle_, keysArray, keyOffsets,
         keyLengths);
 
-    final Map<byte[], byte[]> keyValueMap = new HashMap<>();
+    final Map<byte[], byte[]> keyValueMap =
+        new HashMap<>(computeCapacityHint(values.length));
     for(int i = 0; i < values.length; i++) {
       if(values[i] == null) {
         continue;
@@ -836,6 +837,12 @@ public class RocksDB extends RocksObject {
     return keyValueMap;
   }
 
+  private static int computeCapacityHint(final int estimatedNumberOfItems) {
+    // Default load factor for HashMap is 0.75, so N * 1.5 will be at the load
+    // limit. We add +1 for a buffer.
+    return (int)Math.ceil(estimatedNumberOfItems * 1.5 + 1.0);
+  }
+
   /**
    * Returns a map of keys for which values were found in DB.
    * <p>
@@ -880,7 +887,8 @@ public class RocksDB extends RocksObject {
     final byte[][] values = multiGet(nativeHandle_, keysArray, keyOffsets,
         keyLengths, cfHandles);
 
-    final Map<byte[], byte[]> keyValueMap = new HashMap<>();
+    final Map<byte[], byte[]> keyValueMap =
+        new HashMap<>(computeCapacityHint(values.length));
     for(int i = 0; i < values.length; i++) {
       if (values[i] == null) {
         continue;
@@ -915,7 +923,8 @@ public class RocksDB extends RocksObject {
     final byte[][] values = multiGet(nativeHandle_, opt.nativeHandle_,
         keysArray, keyOffsets, keyLengths);
 
-    final Map<byte[], byte[]> keyValueMap = new HashMap<>();
+    final Map<byte[], byte[]> keyValueMap =
+        new HashMap<>(computeCapacityHint(values.length));
     for(int i = 0; i < values.length; i++) {
       if(values[i] == null) {
         continue;
@@ -971,7 +980,8 @@ public class RocksDB extends RocksObject {
     final byte[][] values = multiGet(nativeHandle_, opt.nativeHandle_,
         keysArray, keyOffsets, keyLengths, cfHandles);
 
-    final Map<byte[], byte[]> keyValueMap = new HashMap<>();
+    final Map<byte[], byte[]> keyValueMap
+        = new HashMap<>(computeCapacityHint(values.length));
     for(int i = 0; i < values.length; i++) {
       if(values[i] == null) {
         continue;