diff --git a/media/java/android/mtp/MtpDevice.java b/media/java/android/mtp/MtpDevice.java
index aa99248485c8a..d0ef37c77055c 100644
--- a/media/java/android/mtp/MtpDevice.java
+++ b/media/java/android/mtp/MtpDevice.java
@@ -358,9 +358,10 @@ public final class MtpDevice {
     /**
      * Returns object size in 64-bit integer.
      *
-     * The object size stored in MtpObjectInfo is unsigned 32-bit integer.
-     * The methods reads 64-bit object size from the object property so that it can fetch 4GB+
-     * object size correctly.
+     * Though MtpObjectInfo#getCompressedSize returns the object size in 32-bit unsigned integer,
+     * this method returns the object size in 64-bit integer from the object property. Thus it can
+     * fetch 4GB+ object size correctly. If the device does not support objectSize property, it
+     * throws IOException.
      * @hide
      */
     public long getObjectSizeLong(int handle, int format) throws IOException {
diff --git a/media/jni/android_mtp_MtpDevice.cpp b/media/jni/android_mtp_MtpDevice.cpp
index f7c6770054892..6e434b2756a58 100644
--- a/media/jni/android_mtp_MtpDevice.cpp
+++ b/media/jni/android_mtp_MtpDevice.cpp
@@ -701,6 +701,8 @@ static void android_mtp_MtpDevice_discard_event_request(JNIEnv *env, jobject thi
     device->discardEventRequest(seq);
 }
 
+// Returns object size in 64-bit integer. If the MTP device does not support the property, it
+// throws IOException.
 static jlong android_mtp_MtpDevice_get_object_size_long(
         JNIEnv *env, jobject thiz, jint handle, jint format) {
     MtpDevice* const device = get_device_from_object(env, thiz);