You can make variables nullable by adding ? to the end of the variable. By default, variables are non-null references and cannot be set to null. If you haven’t implemented exception handling, the app questions the nature of reality, and then crashes.

When the system expects a reference to be valid but instead it’s null, it throws a NullPointerException, NPE for short. Because they are references, you can set the variables to null. In Java, all variables except primitive variables actually store references to memory addresses.

If you’re a Java developer, you’ll already be familiar with the misery of NullPointerExceptions. This app crashes when you tap a file due to a null variable. For Mac you’ll need Android File Transfer. On Linux and Windows you can access the device as a regular drive. If you want to move test files to your physical device, set your device to file transfer mode: Yes, it’s a list of various black holes in the universe. But first, if you don’t have test files to delete, you can copy the sample text file included in the root folder of the materials, BlackHoles.txt, to your device. Once you select and tap the file, you’ll encounter this error: Start using it by browsing through the various folders and tapping on a file. When you open the app for the first time you will be prompted for File access permission, simply choose Allow to grant file access for the app: Open and run the starter project in Android Studio 3.3.0 or higher to see what you’ll be working with. Getting Startedĭownload and unzip the materials for this tutorial using the Download Materials button at the top or bottom of this page. If Android development is new to you, first read through the Beginning Android Development and Kotlin for Android tutorials. Note: This tutorial assumes that you’re already familiar with the basics of Android development and Android Studio.

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