Walkthrough - Kotlin Tour: Intermediate Null Safety

In this lesson, we advanced our understanding of null safety in Kotlin, covering safe casts, collection filtering, and handling mandatory non-null values.

Changes Made

Implementation

Created IntermediateNullSafety.kt which implements:
- Safe Cast (as?): Demonstrating how to attempt a cast and receive null instead of a crash if it fails.
- Not-Null Assertion (!!): Showing the operator that forces a value to be non-null (and how it can crash).
- Collection Filtering: Using filterNotNull() and listOfNotNull() to manage lists with potential nulls cleanly.
- Early Returns: Using the Elvis operator ?: to validate preconditions and return early (e.g., val x = input ?: return).
- Exercises:
  - Notification preferences filtering.
  - Finding a single active subscription with singleOrNull.
  - Stock validation with early returns.
Updated App.java to execute these demonstrations.

Verification

Build: Successfully executed ./gradlew :app:assemble.
Logic: Verified that as? handles type mismatches safely and that listOfNotNull excludes missing map entries.

See the Step-by-Step Explanation for technical details.

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