14 Data Storage Basics

Why Data Storage Matters in Android Apps

Every useful app needs to remember something. It might be a username, a setting, a list of notes, or downloaded content. Data storage is how your app keeps information between launches so that users do not lose everything when they close or restart the app.

In Android, you will work with different storage options depending on what kind of data you have, how big it is, and how private it must be. In this chapter, you will get a big picture of those options, where they live, and when you might choose one over another. The specific storage techniques listed in the outline, such as SharedPreferences and file handling, will have their own dedicated chapters later, so here you focus on understanding the landscape.

Android storage is split into options that are private to your app and options that are more shared with the rest of the device. On top of that, there are different formats and tools such as plain files, key value pairs, and full databases. Learning to pick the right option early helps you design apps that are safe, fast, and easy to maintain.

Types of Data You Might Store

Before thinking about the how, it is useful to think about the what. Different kinds of data are better suited to different storage mechanisms.

You will commonly need to store small user preferences, temporary caches, larger structured data, and sometimes media files. Small settings such as theme choice or whether notifications are enabled are good examples of key value data. A list of notes or tasks is more like structured data that you might keep in a database. Images, PDFs, or audio clips are larger binary files that usually belong in file storage.

You also need to decide how long the data should live. Some data should survive app restarts but can be deleted if storage is low, such as a downloaded thumbnail cache. Other data must be kept until the user explicitly deletes it, such as saved documents or important records. Finally, some data is sensitive, such as access tokens or private user information, and must be stored only in places that are not world readable.

Thinking in terms of type, size, lifetime, and sensitivity of the data will guide your storage choice.

Internal vs External Storage Basics

Android distinguishes between internal and external storage. This distinction is about visibility and access patterns, not just physical hardware.

Internal storage belongs to your app and is private by default. Files stored here are not visible to other apps, and when the user uninstalls your app, the system deletes this data. Internal storage is where you usually put data that is only meaningful to your app or that must stay private. You will see later how to write and read files in this internal space using file handling APIs.

External storage is a broader area that is accessible to more apps, and in some cases to the user through file managers or a computer connection. Historically this often meant an SD card, but now it usually refers to a shared storage region on the device. Modern Android versions use a system called scoped storage to limit how apps see external storage, but the basic idea is the same. External storage is useful for user facing files such as photos or documents that you expect users might want to access outside your app.

Remember that internal storage is best for privacy and encapsulation, and external storage is better when the data belongs to the user rather than just to your app.

Persistent vs In Memory Data

Not all data needs to be saved permanently. Your app can hold information in memory while it is running, for example in variables, collections, or objects stored in a ViewModel. This in memory data disappears when the app is killed or the process is recreated. In memory storage is fast and simple but not persistent.

Persistent storage means that the data remains available after the app closes, the device restarts, or the process is killed. All the options discussed in this chapter, such as preferences, files, and databases, are persistent by design.

You should only persist data that has value beyond the current session. Persisting everything can waste space and complicate your data model. A good pattern is to keep transient calculations, temporary states, and UI details only in memory, and persist settings, user content, and important server data that would be costly or impossible to recreate.

Overview of Common Android Storage Options

On Android you will typically rely on three fundamental storage options that this section introduces and that later chapters will explore in detail.

The first is preferences. SharedPreferences provides a way to store small sets of key value pairs such as isFirstLaunch = true or username = "Alex". This is ideal for simple configuration, flags, and user preferences. It is not designed for large data, complex structures, or big lists.

The second is file storage. You can create and manage files in your app's internal storage area or in specific external storage directories. Files can be text, JSON, images, or any other binary data. File storage is flexible and works well for large or media rich content, but you must do more work to structure and organize the data yourself.

The third is databases. SQLite is the built in database engine on Android, and Room is a higher level library that simplifies working with SQLite. Databases are good for structured and relational data such as lists of items with properties, relationships between them, and queries that filter or sort the data efficiently. For example, a notes app with tags, search, and sorting is a natural fit for a database.

Each of these tools has strengths and weaknesses. Preferences are simple but limited, files are flexible but unstructured, and databases are powerful but require more design.

Choosing the Right Storage Strategy

When you design a feature, you should map its data requirements to the appropriate storage option. A simple rule of thumb is to use preferences for configuration, files for content, and databases for records.

You can refine this by asking a few guiding questions. How much data do you need to store? If it is just a handful of small values, preferences are usually enough. If it is hundreds or thousands of items, or if you need to search and filter, a database becomes more suitable. What shape is the data? If it is hierarchical, contains lists and nested objects, you might favor JSON files or a database instead of many independent key value entries.

You should also consider how the user interacts with the data. If users expect to back up or share files directly, storing them in clear files is beneficial. If the user only sees the data inside your app and expects fast searches and synchronisation, a database is often best.

Finally, consider evolution over time. If your data model is likely to change, such as adding fields or new related entities, a database with migrations can handle that more gracefully than ad hoc file formats or scattered preference keys.

Security and Privacy Considerations

Data storage is closely related to security and privacy. On a shared device, other apps or users must not be able to read sensitive information that your app stores without permission.

Internal storage is automatically private to your app, which makes it a safer default for sensitive data such as tokens or user specific configuration. However, even internal storage is not a substitute for proper encryption when you handle very sensitive information, especially if the device might be rooted or physically accessed.

External storage should be treated as semi public. On many devices, users and some apps with suitable permissions can access files stored there. You should avoid storing secrets or confidential data on external storage. If you must, you should encrypt the data before writing it.

You also need to respect user expectations and legal requirements. Do not store more information than you need, and provide ways for users to clear their data. When users log out, clear or invalidate their credentials. When users uninstall your app, internal storage is removed automatically, but data you have written to shared locations might remain unless you manage it carefully.

Platform Constraints and Permissions

Access to storage on Android is controlled through the permission system. For internal storage, your app does not usually need special runtime permissions. You can read and write your own internal files freely.

External storage has stricter rules. On modern Android versions, apps usually interact with external storage through scoped storage and specific APIs that limit broad file system access. In some cases, you may need storage related permissions to read or write certain shared locations or user facing collections.

Networking and permission details are covered in separate chapters, but it is important to understand that just because an API exists to write a file, that does not mean it will succeed without user consent or correct configuration. You should always check current platform guidelines and test your storage code on recent Android versions.

Data Lifecycles and Cleanup

Stored data must be managed over its entire lifetime. If you only add and never remove, your app can grow in size, affect device performance, and frustrate users.

Think about when data should be created, when it should be updated, and when it should be deleted. Temporary caches on disk can speed up your app, but you should clear them when they grow too large or when the data becomes outdated. Android provides cache directories specifically for this purpose which the system may clear when storage is low.

User generated content deserves more careful handling. Do not delete important user data without clear intent and confirmation. Provide options to export or back up information where reasonable.

You should also plan for data migrations. When you update your app and change how data is stored, you need a way to convert older data formats to the new structure. Room provides a formal migration mechanism for databases, and for files or preferences you must implement your own version checks and transformations.

How This Chapter Connects to Later Topics

This chapter gives you the conceptual map of data storage on Android. The next chapters in this section will take you deeper into the specific tools.

You will learn how to use SharedPreferences for small key value data, how to work with internal storage and files for more flexible content, and how to apply file handling techniques safely and correctly. Later in the course, you will also explore databases using Room for more complex structured data.

As you progress, refer back to the ideas in this chapter. Before you write code to store anything, ask what type of data you have, how long it should live, how secure it must be, and which users or apps should be able to see it. With these questions, you will be ready to choose the right storage option in any Android project.