Migrate project from Java to Kotlin

[yehiahd]

Summary

This pull request migrates the entire FastSave library and example app from Java to Kotlin, modernizing the codebase and improving maintainability.

Key Changes

Build Configuration

• Added Kotlin Gradle plugin to root build.gradle with version 1.9.22
• Applied kotlin-android and kotlin-kapt plugins to app module
• Applied kotlin-android plugin to fastsave library module
• Updated annotation processor configuration to use kapt instead of annotationProcessor for ButterKnife
• Added Kotlin stdlib dependency to both app and library modules

Core Library Migration

• FastSave.java → FastSave.kt: Converted singleton pattern to use Kotlin's companion object, improved null safety with non-null assertions, simplified SharedPreferences operations with method chaining
• FastException.java → FastException.kt: Simplified to single-line data class-style exception
• Updated all generic methods to use Kotlin's type parameter syntax

Example App Migration

• MainActivity.java → MainActivity.kt: Converted to Kotlin with lateinit properties for ButterKnife bindings, replaced switch statement with when expression
• MyApplication.java → MyApplication.kt: Simplified Application subclass
• Person.java → Person.kt: Converted to Kotlin with property backing fields and private setters

Test Files Migration

• Converted all unit and instrumented tests from Java to Kotlin in both app and library modules
• Updated test syntax to use Kotlin idioms (e.g., fun instead of @Test public void)

Notable Implementation Details

• Maintained backward compatibility with existing Java code through @JvmStatic annotations on companion object methods
• Used non-null assertion operators (!!) where SharedPreferences initialization is guaranteed
• Leveraged Kotlin's string interpolation for improved readability in logging and error messages
• Simplified null-safety handling with Kotlin's nullable types and safe call operators

https://claude.ai/code/session_012JWUEtDERhSssz4PP5uA7v