What is the difference between an Android activity and a fragment?

In Android development, activities and fragments are both fundamental components used to build user interfaces, but they serve different purposes and have distinct characteristics. Here's a detailed comparison:

1. Definition and Role:

• Activity:

  • An activity is a single, focused task that the user can perform. It acts as a container for UI components and is usually the entry point for an app. An app typically consists of multiple activities that interact with each other.
  • An activity represents a full-screen window, and it is responsible for managing the lifecycle of its UI components.

• Fragment:

  • A fragment is a modular section of an activity, which has its own lifecycle, receives its own input events, and can be added or removed while the activity is running. Fragments allow for more flexible and reusable UI components within an activity.
  • Fragments are used for creating dynamic and flexible UIs, especially on tablets and devices with larger screens.

2. Lifecycle:

• Activity Lifecycle:

  • Activities go through a well-defined lifecycle with states such as onCreate(), onStart(), onResume(), onPause(), onStop(), and onDestroy().
  • The activity lifecycle is more comprehensive because it controls the app's main flow and interactions with the system.

• Fragment Lifecycle:

  • Fragments have a lifecycle that is closely tied to the activity in which they are hosted, but with additional callbacks like onAttach(), onCreateView(), onActivityCreated(), and onDetach().
  • A fragment's lifecycle is managed by the activity, which means it cannot exist independently. However, it allows fragments to be created, attached, detached, and destroyed dynamically.

3. Independence and Reusability:

• Activity:

  • Activities are independent and serve as self-contained components. Each activity is typically associated with a single UI screen.
  • Reusing an activity is less common since they are usually tied to specific tasks or screens in an app.

• Fragment:

  • Fragments are designed to be reusable components. Multiple fragments can be combined to create a single activity’s UI, and the same fragment can be used in different activities or even in different parts of the same activity.
  • This reusability makes fragments ideal for creating flexible, multi-pane layouts and adapting to different screen sizes.

4. UI Composition:

• Activity:

  • An activity provides a single UI window. To create complex UIs with multiple sections or views, developers usually need to use multiple activities or complex layouts within a single activity.

• Fragment:

  • A fragment allows for more granular UI composition. You can use multiple fragments within an activity to create complex, dynamic UIs, such as side-by-side panes in a tablet app or swipeable tabs.
  • Fragments can be added, replaced, or removed at runtime, allowing for more dynamic UIs.

5. Communication:

• Activity:

  • Activities can communicate with each other by using intents. This method is typically used to start another activity and pass data between them.
  • Communication between activities is usually broader, focusing on passing data or triggering actions that affect the overall app flow.

• Fragment:

  • Fragments typically communicate with their hosting activity or other fragments through interfaces or the ViewModel (in a more modern approach). This allows for a tight coupling with the activity and other fragments, enabling more granular data sharing and event handling.

6. Flexibility and Use Cases:

• Activity:

  • Use an activity when you need a full-screen experience that represents a complete, independent task or screen within the app.

• Fragment:

  • Use fragments when you need a more modular, flexible UI that can adapt to different screen sizes or when you want to reuse components across multiple activities or areas within the same activity.

7. Example Scenarios:

• Activity:

  • An activity might represent the entire user registration screen, where each step of the registration process is handled within the same screen.

• Fragment:

  • A fragment might represent just one step in the registration process (like entering the user's name), and this fragment could be used within different activities or alongside other fragments on a larger screen.

In summary, while activities are the building blocks for an app's overall structure and navigation, fragments provide a way to create modular, reusable UI components within those activities, enabling more dynamic and adaptive interfaces.