diff --git a/app/src/main/java/org/fdroid/fdroid/net/Downloader.java b/app/src/main/java/org/fdroid/fdroid/net/Downloader.java
index 45ba5b884..f3ed8e207 100644
--- a/app/src/main/java/org/fdroid/fdroid/net/Downloader.java
+++ b/app/src/main/java/org/fdroid/fdroid/net/Downloader.java
@@ -1,7 +1,5 @@
 package org.fdroid.fdroid.net;
 
-import android.support.annotation.NonNull;
-
 import org.fdroid.fdroid.Utils;
 
 import java.io.File;
@@ -32,6 +30,11 @@ public abstract class Downloader {
     protected final URL sourceUrl;
     protected String cacheTag;
 
+    /**
+     * This is meant only to send progress to {@link DownloaderService}. This
+     * also keeps this class pure Java so that it can be tested on the JVM,
+     * without requiring an Android device or emulator.
+     */
     interface DownloaderProgressListener {
         void sendProgress(URL sourceUrl, int bytesRead, int totalBytes);
     }
@@ -213,12 +216,12 @@ public abstract class Downloader {
         }
 
         @Override
-        public int read(@NonNull byte[] buffer) throws IOException {
+        public int read(byte[] buffer) throws IOException {
             return toWrap.read(buffer);
         }
 
         @Override
-        public int read(@NonNull byte[] buffer, int byteOffset, int byteCount) throws IOException {
+        public int read(byte[] buffer, int byteOffset, int byteCount) throws IOException {
             return toWrap.read(buffer, byteOffset, byteCount);
         }