diff --git a/updater_sample/src/com/example/android/systemupdatersample/util/FileDownloader.java b/updater_sample/src/com/example/android/systemupdatersample/util/FileDownloader.java
new file mode 100644
index 0000000..806f173
--- /dev/null
+++ b/updater_sample/src/com/example/android/systemupdatersample/util/FileDownloader.java
@@ -0,0 +1,93 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.android.systemupdatersample.util;
+
+import android.util.Log;
+
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.net.URL;
+import java.net.URLConnection;
+
+/**
+ * Downloads chunk of a file from given url using {@code offset} and {@code size},
+ * and saves to a given location.
+ *
+ * In real-life application this helper class should download from HTTP Server,
+ * but in this sample app it will only download from a local file.
+ */
+public final class FileDownloader {
+
+    private String mUrl;
+    private long mOffset;
+    private long mSize;
+    private File mOut;
+
+    public FileDownloader(String url, long offset, long size, File out)  {
+        this.mUrl = url;
+        this.mOffset = offset;
+        this.mSize = size;
+        this.mOut = out;
+    }
+
+    /**
+     * Downloads the file with given offset and size.
+     */
+    public void download() throws IOException {
+        Log.d("FileDownloader", "downloading " + mOut.getName()
+                + " from " + mUrl
+                + " to " + mOut.getAbsolutePath());
+
+        URL url = new URL(mUrl);
+        URLConnection connection = url.openConnection();
+        connection.connect();
+
+        // download the file
+        try (InputStream input = connection.getInputStream()) {
+            try (OutputStream output = new FileOutputStream(mOut)) {
+                long skipped = input.skip(mOffset);
+                if (skipped != mOffset) {
+                    throw new IOException("Can't download file "
+                            + mUrl
+                            + " with given offset "
+                            + mOffset);
+                }
+                byte[] data = new byte[4096];
+                long total = 0;
+                while (total < mSize) {
+                    int needToRead = (int) Math.min(4096, mSize - total);
+                    int count = input.read(data, 0, needToRead);
+                    if (count <= 0) {
+                        break;
+                    }
+                    output.write(data, 0, count);
+                    total += count;
+                }
+                if (total != mSize) {
+                    throw new IOException("Can't download file "
+                            + mUrl
+                            + " with given size "
+                            + mSize);
+                }
+            }
+        }
+    }
+
+}
diff --git a/updater_sample/tests/src/com/example/android/systemupdatersample/util/FileDownloaderTest.java b/updater_sample/tests/src/com/example/android/systemupdatersample/util/FileDownloaderTest.java
new file mode 100644
index 0000000..df47c8c
--- /dev/null
+++ b/updater_sample/tests/src/com/example/android/systemupdatersample/util/FileDownloaderTest.java
@@ -0,0 +1,80 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.android.systemupdatersample.util;
+
+import static junit.framework.Assert.assertEquals;
+
+import android.content.Context;
+import android.support.test.InstrumentationRegistry;
+import android.support.test.filters.SmallTest;
+import android.support.test.runner.AndroidJUnit4;
+
+import com.example.android.systemupdatersample.tests.R;
+
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.ExpectedException;
+import org.junit.runner.RunWith;
+
+import java.io.File;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+
+/**
+ * Tests for {@link FileDownloader}
+ */
+@RunWith(AndroidJUnit4.class)
+@SmallTest
+public class FileDownloaderTest {
+
+    @Rule
+    public final ExpectedException thrown = ExpectedException.none();
+
+    private Context mTestContext;
+    private Context mTargetContext;
+
+    @Before
+    public void setUp() {
+        mTestContext = InstrumentationRegistry.getContext();
+        mTargetContext = InstrumentationRegistry.getTargetContext();
+    }
+
+    @Test
+    public void download_downloadsChunkOfZip() throws Exception {
+        // Prepare the target file
+        File packageFile = Paths
+                .get(mTargetContext.getCacheDir().getAbsolutePath(), "ota.zip")
+                .toFile();
+        Files.delete(packageFile.toPath());
+        Files.copy(mTestContext.getResources().openRawResource(R.raw.ota_002_package),
+                packageFile.toPath());
+        String url = "file://" + packageFile.getAbsolutePath();
+        // prepare where to download
+        File outFile = Paths
+                .get(mTargetContext.getCacheDir().getAbsolutePath(), "care_map.txt")
+                .toFile();
+        Files.delete(outFile.toPath());
+        // download a chunk of ota.zip
+        FileDownloader downloader = new FileDownloader(url, 160, 8, outFile);
+        downloader.download();
+        String downloadedContent = String.join("\n", Files.readAllLines(outFile.toPath()));
+        // Look at tools/create_test_ota.py
+        assertEquals("CARE_MAP", downloadedContent);
+    }
+
+}