The Complete Guide to Acing Google Play Services Interview Questions

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Toptal sourced essential questions that the best mobile app developers can answer. Driven from our community, we encourage experts to submit questions and offer feedback.

How can you store data in Android and iOS? Which databases and native solutions are available?

The native data storage solution for Android is SharedPreferences. This is intended to store small amounts of data, like user preferences or settings. The data are stored in an XML file.

There is a similar solution for iOS called UserDefaults (previously NSUserDefaults). The data are stored in a . plist file.

Both platforms can use SQLite and come with frameworks and libraries that let you work with the data (though you can also use the database by hand):

• iOS has Core Data, a framework to manage object graphs. It may save data in a binary store or just in memory, or it may use SQLite.
• The Room persistence library for Android makes it easier to manage SQLite databases and cuts down on the extra work that needs to be done when using databases.

The other possibility is Realm, a non-relational database system designed for mobile devices. It’s also available for hybrid solutions like Xamarin or React Native and is more efficient than SQLite. 2 .

What are internationalization and localization? Do these terms mean the same thing?

No, these terms are not synonyms, but they are related. They’re both relevant to making the app available (and user-friendly) in languages other than the default language.

Internationalization (“i18n”) is the process of adapting app code to other languages. For instance, an app might need to show data in a different format, prices, or numbers with the right decimal or thousands separators. It might also need to know if text is written from left to right or right to left. It also requires taking into account the user’s timezone.

But localization, or “l10n,” is the process of making the app’s content available in more than one language. This includes translating most of the app’s resources, like texts, s, and sounds. Different language resources are usually kept in different folders or files, and the OS picks the right one based on what the user has set. 3 .

What is the best way for a server to notify an app that some new data is available?

A mobile app could use push notifications to get information from a server without having to make a lot of requests. This is better than asking for new data all the time because it will use less network data and not drain the battery.

Android and iOS each have their own solution for that:

• Apple Push Notifications sends push notifications to iOS devices only.
• Google no longer uses Google Cloud Messaging (GCM), but instead uses Firebase Cloud Messaging to send push notifications to devices. This is said to be “multi-platform,” which means it works with iOS devices by using the Apple Push Notification service (APNs).

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How can you debug your app when it’s already released?

There are lot of tools that provide crash and usage reports from mobile apps.

One of the most popular (and free) is Firebase Crashlytics, which supports both iOS and Android. It gives a lot of useful details about app crashes, such as the full-stack trace, the operating system version, the device manufacturer, and the language used by the user.

Some of the other tools are Sentry (useful when you write an app in JavaScript, e. g. , for React Native), Appsee, Raygun, and Instabug. They differ by provided crash and usage details and prices. 5 .

How can private information like contacts or camera (or other sensor) access be kept safe in the world of iOS apps?

Access to a user’s data or sensors is restricted by a permissions system. So, if the app wants to use a feature like GPS, it needs to ask the user for permission and explain why it needs permission.

The user can agree, or reject the request. The developer has to plan for rejecting and make sure there is a way to still use some of the app’s features that don’t need the rejected sensor or data. Permissions can be given or taken away from the user’s settings as well as when the app is running, so the app must always check for it.

The permissions mechanism can change between OS versions, so the developer has to be aware of that. For example, on Android below version 6. 0 (Marshmallow), permissions were declared in AndroidManifest. xml, and users granted all of them during the app’s installation. 6 .

How are native apps and hybrid apps different from each other? What are the pros and cons of hybrid apps?

Native apps are written specifically for one platform, in the language used on said platform, i. e. Objective-C or Swift for iOS, and Java or Kotlin for Android. The best thing about these apps is that they can be made to work on the platform that the user is used to. This way, iOS users won’t be confused by how some Android apps behave or look, and vice versa.

Hybrid apps can work on multiple platforms. A hybrid app is really just a web page that is shown in a web view, so it is usually written in both JavaScript and HTML. Popular hybrid frameworks are Ionic, Cordova, and PhoneGap.

Hybrid apps have worse performance than native apps, though. Accessing device hardware like sensors can be hard at times, and code often needs to be written separately for each platform. This means that the developer has to write native code and code that is specific to each platform.

Also, it’s tough to make a single app that works well on both iOS and Android and looks good. Instead, making a hybrid app might be less expensive and take less time than making a separate app for each platform. This technique is especially suitable for small and simple apps.

There is also a third group of apps, developed using technologies like React Native, Xamarin, or Flutter. These are frameworks that try to connect the ease of writing multi-platform apps with native performance. They offer the possibility to write code in one language—JavaScript, C#, or Dart—and compile it into native code. Because the frameworks often use native implementations of controls, the hybrid apps aren’t quite as bad for users. 7 .

What are the main things you have to be cautious about when writing for mobile?

A mobile phone is more prone to network problems than a standalone computer. The user can move, going to some tunnel or area without WiFi or cellular network access. So the internet is not always available, or is not of the best quality. The phone may frequently switch between networks. When people use their cell phones, downloading a lot of heavy data will use up their data packets and cost them more. The developer may offer an offline mode and different ways to load data when using WiFi, LTE, 3G, or H; e g. , do not download movies or request lower-quality s.

Developers need to keep in mind that the device they are testing their app on is just one of thousands that people will use. They have to check how the app looks in smaller phones, bigger phones, tablets, etc. , and adapt the layout to them.

They also have to think about the different system versions that are out there (mostly on Android phones), choose which ones to support, and know what makes each one unique. Also, the app might have bugs on devices made by different companies, so it’s always a good idea to test it on a range of hardware or with a test-device farm.

Even though phones are getting bigger and bigger, we still need to be good at managing their memory. For developers, memory leaks and making long-running tasks run more smoothly are very important. The OS will kill apps that use too much memory.

As with memory, manufacturers give us more and more disk space. For some devices, it’s also possible to extend this using SD cards.

Despite that, the developer should try to keep the app as small as possible. Users will be less likely to try a small app like a calculator if they have to download 500 MB of it first.

Users may uninstall an app and look for a better one if it keeps unnecessary files and otherwise takes up too much space after it has been installed.

This is one of the weakest points of contemporary phones. Every action we take in an app affects the battery.

Users will certainly remove apps that use too much battery. The developer must take care when it comes to using different services like GPS. When developers use these features, they should only do so when they have to. To update data in the background, they should use optimized OS solutions. 8 .

Describe the process of publishing apps on Google Play and in Apple’s App Store. What do you need to publish an app to each store?.

Both Google Play and the App Store require s, an app description, and a unique identifier.

Google requires that you have a developer account to publish apps.

When an app is ready, it should be built into an APK file.

The APK must be signed to prove that it comes from your specific company. A keystore, which is a file with one or more private keys belonging to the company or developer, is used to sign the document. (If this file is lost, no updates can be done for the app. ).

Google has also added a new way to publish apps called Android App Bundles. This system manages signing and offers a smaller APK file size, but it’s not compatible with older Android versions.

After sending files, it may take a few hours until the app is available in the store.

Apple requires a developer account connected with the Apple Developer Program to publish apps here.

First, the app must have a profile created for it in iTunes Connect.

The finished app is built into an IPA file, which has to be signed by some certificates. Xcode uses the Distribution Certificate (DC) and the Provisioning Profile to help it all happen automatically. The Provisioning Profile links the DC to the app ID.

Once sent, the app is checked by a team at Apple. It may take several days until the app is approved (or rejected) for publishing. 9 .

Different screen sizes and resolutions can be hard to deal with. What new unit has been made to make positioning and sizing elements easier? What do you do with s?

Both Apple and Google have introduced their own unit to help developers deal with different screen resolutions. For Apple, they repurposed (or perhaps modernized?) the term “point” (pt) to no longer be 1/72nd of an inch, but to be 2x that on retina displays, or 3x that on larger retina displays—but the exact scaling is not always an integer multiple like that.

Google, instead, uses a relatively new term, the “density-independent pixel” (DIP or “dp”). To hide the real screen resolution and give programmers a unit that isn’t based on pixels is the same idea behind both.

Each device has a scale factor of how many pixels corresponds to one point (or density pixel). That means that on some very old devices, when an iOS developer says that an is 80 pt x 80 pt, it will actually be 80×80 pixels. g. , the iPhone 3G), 160×160 on the iPhone 6, and 240×240 on the iPhone X.

The same rule applies to Android phones, but there are more scale factors that can be used on this platform.

Artists also need to be able to work with a range of screen sizes because they need to make graphics for each density group. Sometimes it is good to prepare different layouts for phones and tablets, or even portrait and landscape orientation.

When possible, s in SVG format should be used. They can be scaled without losing quality and they take less space than the same in different sizes. Both Android and iOS support using SVG s.

Also, both platforms support layout constraints. Using this technique, elements can be placed in relation to other elements or screen edges. This gives a more responsive design on different screens and orientations. 10 .

What is app sandboxing and what is it for?

App sandboxing is a security mechanism that isolates apps at the kernel level. The fact that each app runs in its own sandbox means that it can only access its own data and files. No other app can read or write to them.

This is done to keep user data safe from any bad software that might be installed on the device, whether on purpose or by accident. Even so, apps can still share some data—but only the data they want to—using system-built tools (e.g. g. , ContentProvider on Android).

There is more to interviewing than tricky technical questions, so these are intended merely as a guide. Not every good candidate for the job will be able to answer all of them, and answering all of them doesn’t mean they are a good candidate. At the end of the day, hiring remains an art, a science — and a lot of work.

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