diff --git a/app/build.gradle b/app/build.gradle
index af0597c1..9a9c0486 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -2,7 +2,11 @@ apply plugin: 'com.android.application'
 
 android {
     compileSdkVersion 23
-    buildToolsVersion "23.0.1"
+    buildToolsVersion "23.0.2"
+
+    dependencies {
+        compile 'com.android.support:support-annotations:23.1.1'
+    }
 
     sourceSets {
 	    main {
@@ -13,9 +17,9 @@ android {
     defaultConfig {
         applicationId "com.termux"
         minSdkVersion 21
-        targetSdkVersion 22
-        versionCode 20
-        versionName "0.20"
+        targetSdkVersion 23
+        versionCode 21
+        versionName "0.21"
     }
 
     buildTypes {
diff --git a/app/src/main/java/com/termux/filepicker/TermuxFilePickerProvider.java b/app/src/main/java/com/termux/filepicker/TermuxFilePickerProvider.java
index 0162c5e2..c10c0ae0 100644
--- a/app/src/main/java/com/termux/filepicker/TermuxFilePickerProvider.java
+++ b/app/src/main/java/com/termux/filepicker/TermuxFilePickerProvider.java
@@ -6,6 +6,7 @@ import android.content.ContentValues;
 import android.database.Cursor;
 import android.net.Uri;
 import android.os.ParcelFileDescriptor;
+import android.support.annotation.NonNull;
 
 import java.io.File;
 import java.io.FileNotFoundException;
@@ -18,32 +19,32 @@ public class TermuxFilePickerProvider extends ContentProvider {
     }
 
     @Override
-    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
+    public Cursor query(@NonNull Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
         return null;
     }
 
     @Override
-    public String getType(Uri uri) {
+    public String getType(@NonNull Uri uri) {
         return null;
     }
 
     @Override
-    public Uri insert(Uri uri, ContentValues values) {
+    public Uri insert(@NonNull Uri uri, ContentValues values) {
         return null;
     }
 
     @Override
-    public int delete(Uri uri, String selection, String[] selectionArgs) {
+    public int delete(@NonNull Uri uri, String selection, String[] selectionArgs) {
         return 0;
     }
 
     @Override
-    public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
+    public int update(@NonNull Uri uri, ContentValues values, String selection, String[] selectionArgs) {
         return 0;
     }
 
     @Override
-    public ParcelFileDescriptor openFile(Uri uri, String mode) throws FileNotFoundException {
+    public ParcelFileDescriptor openFile(@NonNull Uri uri, @NonNull String mode) throws FileNotFoundException {
         File file = new File(uri.getPath());
         return ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY);
     }