Avoid allocating iterators when calling Message.Builder.addAllFoo(RandomAccess List)

I've tried to keep the hot part of the loop (not null) in the loop without
requiring an extra function call, and only extracted the cold part (null
handling) to avoid repetition.

PiperOrigin-RevId: 670760103

Author: mhansen

java/core/src/main/java/com/google/protobuf/AbstractMessageLite.java

@@ -16,6 +16,7 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
+import java.util.RandomAccess;
 
 /**
  * A partial implementation of the {@link MessageLite} interface which implements as many methods of
@@ -343,19 +344,36 @@ private static <T> void addAllCheckingNulls(Iterable<T> values, List<? super T>
         ((ArrayList<T>) list).ensureCapacity(list.size() + ((Collection<T>) values).size());
       }
       int begin = list.size();
-      for (T value : values) {
-        if (value == null) {
-          // encountered a null value so we must undo our modifications prior to throwing
-          String message = "Element at index " + (list.size() - begin) + " is null.";
-          for (int i = list.size() - 1; i >= begin; i--) {
-            list.remove(i);
+      if (values instanceof List && values instanceof RandomAccess) {
+        List<T> valuesList = (List<T>) values;
+        int n = valuesList.size();
+        // Optimisation: avoid allocating Iterator for RandomAccess lists.
+        for (int i = 0; i < n; i++) {
+          T value = valuesList.get(i);
+          if (value == null) {
+            resetListAndThrow(list, begin);
+          }
+          list.add(value);
+        }
+      } else {
+        for (T value : values) {
+          if (value == null) {
+            resetListAndThrow(list, begin);
           }
-          throw new NullPointerException(message);
+          list.add(value);
         }
-        list.add(value);
       }
     }
 
+    /** Remove elements after index begin from the List and throw NullPointerException. */
+    private static void resetListAndThrow(List<?> list, int begin) {
+      String message = "Element at index " + (list.size() - begin) + " is null.";
+      for (int i = list.size() - 1; i >= begin; i--) {
+        list.remove(i);
+      }
+      throw new NullPointerException(message);
+    }
+
     /** Construct an UninitializedMessageException reporting missing fields in the given message. */
     protected static UninitializedMessageException newUninitializedMessageException(
         MessageLite message) {