Kotlin and Android Fundamentals¶

Kotlin is the primary language for Android development. This entry covers Kotlin language basics, Android project structure, build.gradle configuration, Activities, Fragments, navigation, and UI components.

Key Facts¶

data class auto-generates equals(), hashCode(), toString(), copy()
companion object provides static-like members accessed via class name
lateinit var declares a non-nullable property initialized later (not in constructor)
@Volatile guarantees value is read from main memory, not CPU cache - critical for singletons
synchronized(this) prevents simultaneous access from multiple threads
Android uses XML layouts (res/layout/) while SwiftUI uses declarative code
SharedPreferences provides simple key-value persistent storage

Patterns¶

Project Structure¶

app/
  src/main/
    java/com.example.app/
      di/           - dependency injection (Dagger 2)
      database/     - Room entities, DAOs, database class
      rest/         - Retrofit API interfaces and data models
      fragments/    - Fragment classes
      activities/   - Activity classes
      viewmodel/    - ViewModel classes
    res/
      layout/       - XML layout files
      drawable/     - images and vector assets
      values/       - strings.xml, colors.xml, styles.xml
      menu/         - menu XML files
      navigation/   - Navigation graph XML
  build.gradle      - module-level dependencies
build.gradle        - project-level repository settings

build.gradle Key Dependencies¶

apply plugin: 'kotlin-kapt'

android {
    compileSdkVersion 34
    defaultConfig {
        applicationId "com.example.app"
        minSdkVersion 21
        targetSdkVersion 34
    }
}

dependencies {
    // Coroutines
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:1.7.0"
    implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.7.0"

    // Architecture components
    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:2.6.0"
    implementation "androidx.lifecycle:lifecycle-livedata-ktx:2.6.0"

    // Room
    implementation "androidx.room:room-runtime:2.5.0"
    kapt "androidx.room:room-compiler:2.5.0"

    // Retrofit + Gson
    implementation "com.squareup.retrofit2:retrofit:2.9.0"
    implementation "com.squareup.retrofit2:converter-gson:2.9.0"

    // Glide (image loading)
    implementation "com.github.bumptech.glide:glide:4.15.0"

    // Navigation
    implementation "androidx.navigation:navigation-fragment-ktx:2.6.0"
    implementation "androidx.navigation:navigation-ui-ktx:2.6.0"

    // RecyclerView
    implementation "androidx.recyclerview:recyclerview:1.3.0"
}

data class¶

data class GeckoCoin(
    val id: String,
    val symbol: String,
    val name: String,
    val image: String,
    val current_price: Float,
    val market_cap: Float,
    val market_cap_rank: Int
)

Fragment Lifecycle¶

class RecordFragment : Fragment() {
    private lateinit var viewModel: RecordViewModel

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_record, container, false)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        val application = requireNotNull(activity).application
        viewModel = ViewModelProviders.of(this,
            RecordViewModelFactory(application))
            .get(RecordViewModel::class.java)

        viewModel.elapsedTime.observe(viewLifecycleOwner, Observer { time ->
            timerText.text = time
        })
    }
}

// Start another Activity
val intent = Intent(this, SettingsActivity::class.java)
intent.putExtra("KEY", value)
startActivity(intent)

// Receive in destination
val value = intent.getStringExtra("KEY")

// Go back
finish()

<!-- nav_graph.xml -->
<navigation app:startDestination="@id/recordFragment">
    <fragment android:id="@+id/recordFragment"
              android:name=".record.RecordFragment" />
    <fragment android:id="@+id/listFragment"
              android:name=".list.ListFragment" />
</navigation>

// Navigate between fragments
findNavController().navigate(R.id.action_listFragment_to_playerFragment)

// With arguments
val bundle = bundleOf("itemPath" to filePath)
findNavController().navigate(R.id.action, bundle)

// Receive arguments
val itemPath = arguments?.getString("itemPath")

ConstraintLayout XML¶

<androidx.constraintlayout.widget.ConstraintLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/tvTitle"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />

    <Button
        android:id="@+id/btnAction"
        app:layout_constraintTop_toBottomOf="@id/tvTitle"
        app:layout_constraintEnd_toEndOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

RecyclerView Adapter¶

class CurrencyAdapter(
    private var items: List<GeckoCoin>,
    private val context: Context
) : RecyclerView.Adapter<CurrencyAdapter.ViewHolder>() {

    inner class ViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        val tvName: TextView = itemView.findViewById(R.id.tvName)
        val ivIcon: ImageView = itemView.findViewById(R.id.ivIcon)
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        val view = LayoutInflater.from(parent.context)
            .inflate(R.layout.recycler_view_item, parent, false)
        return ViewHolder(view)
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        val coin = items[position]
        holder.tvName.text = coin.name
        Glide.with(context).load(coin.image).into(holder.ivIcon)

        holder.itemView.setOnClickListener { /* handle click */ }
    }

    override fun getItemCount() = items.size

    fun updateItems(newItems: List<GeckoCoin>) {
        items = newItems
        notifyDataSetChanged()
    }
}

Setup in Fragment¶

val adapter = CurrencyAdapter(emptyList(), requireContext())
recyclerView.layoutManager = LinearLayoutManager(context)
recyclerView.adapter = adapter

viewModel.coins.observe(viewLifecycleOwner) { coins ->
    adapter.updateItems(coins)
}

AlertDialog¶

AlertDialog.Builder(activity)
    .setTitle(R.string.dialog_title)
    .setMessage(R.string.dialog_message)
    .setPositiveButton(R.string.yes) { dialog, _ -> dialog.cancel() }
    .setNegativeButton(R.string.no) { dialog, _ -> dialog.cancel() }
    .show()

SharedPreferences¶

val prefs = context.getSharedPreferences("app_prefs", Context.MODE_PRIVATE)
prefs.edit().putLong("TRIGGER_TIME", value).apply()  // write
val time = prefs.getLong("TRIGGER_TIME", 0L)          // read with default

Toast¶

Toast.makeText(context, "Message", Toast.LENGTH_SHORT).show()

String Resources¶

<!-- res/values/strings.xml -->
<resources>
    <string name="app_name">MyApp</string>
    <string name="recording_started">Recording started</string>
</resources>

getString(R.string.recording_started)   // in code
// android:text="@string/recording_started"  in XML

Glide (Image Loading)¶

Glide.with(context)
    .load(imageUrl)
    .placeholder(R.drawable.placeholder)
    .error(R.drawable.error_image)
    .into(imageView)

Gotchas¶

lateinit properties crash with UninitializedPropertyAccessException if accessed before initialization
notifyDataSetChanged() redraws all items - use DiffUtil for efficient partial updates in production
onActivityCreated() is deprecated - use onViewCreated() for modern Fragment lifecycle
Android XML layouts are verbose compared to SwiftUI declarative code
kapt plugin required for annotation processing (Room, Dagger) - without it, code generation fails
startActivityForResult() is deprecated - use Activity Result API in modern code