Infer database URL from Project ID

firebase-database/CHANGELOG.md

@@ -1,5 +1,6 @@
 # Unreleased
-
+- [changed] The SDK can now infer a default database URL if none is provided in
+  the config.
 - [changed] Added internal HTTP header to the WebChannel connection.
 - [feature] Realtime Database now supports connecting to a local emulator via
  `FirebaseDatabase#useEmulator()`

firebase-database/src/androidTest/java/com/google/firebase/database/FirebaseDatabaseTest.java

@@ -119,6 +119,21 @@ public void getInstanceForAppWithHttpsUrl() {
     assertEquals("https://tests.fblocal.com:9000", db.getReference().toString());
   }
 
+  @Test
+  public void canInferDatabaseUrlFromProjectId() {
+    FirebaseApp app =
+        FirebaseApp.initializeApp(
+            InstrumentationRegistry.getInstrumentation().getTargetContext(),
+            new FirebaseOptions.Builder()
+                .setApplicationId("appid")
+                .setApiKey("apikey")
+                .setProjectId("abc123")
+                .build(),
+            "canInferDatabaseUrlFromProjectId");
+    FirebaseDatabase db = FirebaseDatabase.getInstance(app);
+    assertEquals("https://abc123-default-rtdb.firebaseio.com", db.getReference().toString());
+  }
+
   @Test
   public void getDifferentInstanceForAppWithUrl() {
     FirebaseApp app =

firebase-database/src/main/java/com/google/firebase/database/FirebaseDatabase.java

@@ -83,7 +83,16 @@ public static FirebaseDatabase getInstance(@NonNull String url) {
    */
   @NonNull
   public static FirebaseDatabase getInstance(@NonNull FirebaseApp app) {
-    return getInstance(app, app.getOptions().getDatabaseUrl());
+    String databaseUrl = app.getOptions().getDatabaseUrl();
+    if (databaseUrl == null) {
+      if (app.getOptions().getProjectId() == null) {
+        throw new DatabaseException(
+            "Failed to get FirebaseDatabase instance: Can't determine Firebase Database URL. "
+                + "Be sure to include a Project ID in your configuration.");
+      }
+      databaseUrl = "https://" + app.getOptions().getProjectId() + "-default-rtdb.firebaseio.com";
+    }
+    return getInstance(app, databaseUrl);
   }
 
   /**