Add nullable annotations

Author: svenjacobs

firebase-functions/src/androidTest/java/com/google/firebase/functions/FirebaseFunctionsTest.java

@@ -15,17 +15,17 @@
 package com.google.firebase.functions;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertThrows;
 
 import androidx.test.platform.app.InstrumentationRegistry;
 import androidx.test.runner.AndroidJUnit4;
 import com.google.firebase.FirebaseApp;
 import com.google.firebase.FirebaseOptions;
 import java.net.URL;
+import okhttp3.OkHttpClient;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 
-import okhttp3.OkHttpClient;
-
 @RunWith(AndroidJUnit4.class)
 public class FirebaseFunctionsTest {
 
@@ -114,6 +114,15 @@ public void testSetOkHttpClient() {
     assertEquals(client, functions.getOkHttpClient());
   }
 
+  @Test
+  public void testSetOkHttpClient_throwNPE() {
+    FirebaseApp app = getApp("testSetOkHttpClient_throwNPE");
+    FirebaseFunctions functions = FirebaseFunctions.getInstance(app);
+
+    //noinspection DataFlowIssue
+    assertThrows(NullPointerException.class, () -> functions.setOkHttpClient(null));
+  }
+
   private FirebaseApp getApp(String name) {
     return FirebaseApp.initializeApp(
         InstrumentationRegistry.getInstrumentation().getTargetContext(),

firebase-functions/src/main/java/com/google/firebase/functions/FirebaseFunctions.java

@@ -282,10 +282,11 @@ public void useEmulator(@NonNull String host, int port) {
    *
    * @param client The OkHttpClient instance
    */
-  public void setOkHttpClient(OkHttpClient client) {
-    this.client = client;
+  public void setOkHttpClient(@NonNull OkHttpClient client) {
+    this.client = Preconditions.checkNotNull(client);
   }
 
+  @NonNull
   OkHttpClient getOkHttpClient() {
     return this.client;
   }