How to Properly Use a Final Variable in Android That Requires Initialization Later

How can I declare a final variable in Android and ensure it is initialized later in the lifecycle?

private final long mId;

In Android development, it's common to declare final variables that need to be initialized later, particularly in lifecycle methods like onCreate(). However, the final modifier restricts variables from being reassigned, meaning they must be initialized at the time of declaration or in a constructor. This guidance explains how you can manage this effectively without breaking the immutability requirements of final variables.

public class MyActivity extends Activity {
    private final long mId;

    public MyActivity(long id) {
        this.mId = id;
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // Additional setup can happen here
    }

    public static MyActivity newInstance(long id) {
        return new MyActivity(id);
    }
}

Causes

• Final variables must be assigned once at the time of declaration or within the constructor.
• onCreate() is called after the instance is created, preventing direct final variable assignment.

Solutions

• Use a constructor to initialize the final variable when creating your Activity instance, allowing for safe assignment that adheres to the rules of final modifiers.
• If the choice of initializing in onCreate() is necessary, consider using a non-final variable and applying conventions or documentation to signal its intended immutability.

Mistake: Trying to assign a final variable in onCreate() which leads to a compilation error.

Solution: Initialize the final variable in the constructor or consider patterns that signal intended usage.

Mistake: Forgetting to provide a constructor or factory method to create Activity instances with the required parameters.

Solution: Implement a static factory method to properly assign final values while ensuring immutability.