3.2 Code Standards (Important)

Null Safety:

// ❌ Avoid: Non-null assertion !! (may crash)
val name = user!!.name
// ✅ Recommended: Safe call + default value
val name = user?.name ?: "Unknown"
// ✅ Recommended: let handling
user?.let { processUser(it) }

Exception Handling:

// ❌ Avoid: Random try-catch in business layer swallowing exceptions
fun loadData() {
    try {
        val data = api.fetch()
    } catch (e: Exception) {
        // Swallowing exception, hard to debug
    }
}
// ✅ Recommended: Let exceptions propagate, handle at appropriate layer
suspend fun loadData(): Result {
    return try {
        Result.success(api.fetch())
    } catch (e: Exception) {
        Result.failure(e)  // Wrap and return, let caller decide handling
    }
}
// ✅ Recommended: Unified handling in ViewModel
viewModelScope.launch {
    runCatching { repository.loadData() }
        .onSuccess { _uiState.value = UiState.Success(it) }
        .onFailure { _uiState.value = UiState.Error(it.message) }
}

3.3 Threading & Coroutines (Critical)

Thread Selection Principles:

// In ViewModel
viewModelScope.launch {
    // Default Main thread, can update UI State
    _uiState.value = UiState.Loading
    
    // Switch to IO thread for network request
    val result = withContext(Dispatchers.IO) {
        repository.fetchData()
    }
    
    // Automatically returns to Main thread, update UI
    _uiState.value = UiState.Success(result)
}
// In Repository (suspend functions should be main-safe)
suspend fun fetchData(): Data = withContext(Dispatchers.IO) {
    api.getData()
}

Common Mistakes:

// ❌ Wrong: Updating UI on IO thread
viewModelScope.launch(Dispatchers.IO) {
    val data = api.fetch()
    _uiState.value = data  // Crash or warning!
}
// ❌ Wrong: Executing time-consuming operation on Main thread
viewModelScope.launch {
    val data = api.fetch()  // Blocking main thread! ANR
}
// ✅ Correct: Fetch on IO, update on Main
viewModelScope.launch {
    val data = withContext(Dispatchers.IO) { api.fetch() }
    _uiState.value = data
}

3.4 Visibility Rules

// Default is public, declare explicitly when needed
class UserRepository {           // public
    private val cache = mutableMapOf()  // Visible only within class
    internal fun clearCache() {} // Visible only within module
}
// data class properties are public by default, be careful when used across modules
data class User(
    val id: String,       // public
    val name: String
)

3.5 Common Syntax Pitfalls

// ❌ Wrong: Accessing uninitialized lateinit
class MyViewModel : ViewModel() {
    lateinit var data: String
    fun process() = data.length  // May crash
}
// ✅ Correct: Use nullable or default value
class MyViewModel : ViewModel() {
    var data: String? = null
    fun process() = data?.length ?: 0
}
// ❌ Wrong: Using return in lambda
list.forEach { item ->
    if (item.isEmpty()) return  // Returns from outer function!
}
// ✅ Correct: Use return@forEach
list.forEach { item ->
    if (item.isEmpty()) return@forEach
}

3.6 Server Response Data Class Fields Must Be Nullable

// ❌ Wrong: Fields declared as non-null (server may not return them)
data class UserResponse(
    val id: String = "",
    val name: String = "",
    val avatar: String = ""
)
// ✅ Correct: All fields declared as nullable
data class UserResponse(
    @SerializedName("id")
    val id: String? = null,
    @SerializedName("name")
    val name: String? = null,
    @SerializedName("avatar")
    val avatar: String? = null
)

3.7 Lifecycle Resource Management

// ❌ Wrong: Only adding Observer, not removing
class MyView : View {
    override fun onAttachedToWindow() {
        super.onAttachedToWindow()
        activity?.lifecycle?.addObserver(this)
    }
    // Memory leak!
}
// ✅ Correct: Paired add and remove
class MyView : View {
    override fun onAttachedToWindow() {
        super.onAttachedToWindow()
        activity?.lifecycle?.addObserver(this)
    }
    override fun onDetachedFromWindow() {
        activity?.lifecycle?.removeObserver(this)
        super.onDetachedFromWindow()
    }
}

3.8 Logging Level Usage

import android.util.Log
// Info: Key checkpoints in normal flow
Log.i(TAG, "loadData: started, userId = $userId")
// Warning: Abnormal but recoverable situations
Log.w(TAG, "loadData: cache miss, fallback to network")
// Error: Failure/error situations
Log.e(TAG, "loadData failed: ${error.message}")

4. Jetpack Compose Standards

4.1 @Composable Context Rules

// ❌ Wrong: Calling Composable from non-Composable function
fun showError(message: String) {
    Text(message)  // Compile error!
}
// ✅ Correct: Mark as @Composable
@Composable
fun ErrorMessage(message: String) {
    Text(message)
}
// ❌ Wrong: Using suspend outside LaunchedEffect
@Composable
fun MyScreen() {
    val data = fetchData()  // Error!
}
// ✅ Correct: Use LaunchedEffect
@Composable
fun MyScreen() {
    var data by remember { mutableStateOf(null) }
    LaunchedEffect(Unit) {
        data = fetchData()
    }
}

4.2 State Management

// Basic State
var count by remember { mutableStateOf(0) }
// Derived State (avoid redundant computation)
val isEven by remember { derivedStateOf { count % 2 == 0 } }
// Persist across recomposition (e.g., scroll position)
val scrollState = rememberScrollState()
// State in ViewModel
class MyViewModel : ViewModel() {
    private val _uiState = MutableStateFlow(UiState())
    val uiState: StateFlow = _uiState.asStateFlow()
}

4.3 Common Compose Mistakes

// ❌ Wrong: Creating objects in Composable (created on every recomposition)
@Composable
fun MyScreen() {
    val viewModel = MyViewModel()  // Wrong!
}
// ✅ Correct: Use viewModel() or remember
@Composable
fun MyScreen(viewModel: MyViewModel = viewModel()) {
    // ...
}

5. Resources & Icons

5.1 App Icon Requirements

Must provide multi-resolution icons:

5.2 Resource Naming Conventions

5.3 Avoid Android Reserved Names (Important)

Variable names, resource IDs, colors, icons, and XML elements must not use Android reserved words or system resource names. Using reserved names causes build errors or resource conflicts.

Common Reserved Names to Avoid:

#FFFFFF
#000000

#FFFFFF
#000000

// ❌ Wrong: Variable names conflict with system
val icon = R.drawable.my_icon
val background = Color.White
// ✅ Correct: Use descriptive names
val appIcon = R.drawable.my_icon
val screenBackground = Color.White

6. Build Error Diagnosis & Fixes

6.1 Common Error Quick Reference

6.2 Fix Best Practices

• Read the complete error message first: Locate file and line number
• Check recent changes: Problems usually in latest modifications
• Clean Build: ./gradlew clean assembleDebug
• Check dependency versions: Version conflicts are common causes
• Refresh dependencies if needed: Clear cache and rebuild

6.3 Debugging Commands

# Clean and build
./gradlew clean assembleDebug
# View dependency tree (investigate conflicts)
./gradlew :app:dependencies
# View detailed errors
./gradlew assembleDebug --stacktrace
# Refresh dependencies
./gradlew --refresh-dependencies

7. Material Design 3 Guidelines

Review Android UI files for compliance with Material Design 3 Guidelines and Android best practices.

Design Philosophy

M3 Core Principles

M3 Expressive (Latest)

The latest evolution adds emotion-driven UX through:

• Vibrant, dynamic colors
• Intuitive motion physics
• Adaptive components
• Flexible typography
• Contrasting shapes (35 new shape options)

App Style Selection

Critical Decision: Match visual style to app category and target audience.

Quick Reference: Key Specifications

Color Contrast Requirements

Touch Targets

8dp Grid System

Typography Scale (Summary)

Animation Duration

Component Dimensions

Anti-Patterns (Must Avoid)

UI Anti-Patterns

• More than 5 bottom navigation items
• Multiple FABs on same screen
• Touch targets smaller than 48dp
• Inconsistent spacing (non-8dp multiples)
• Missing dark theme support
• Text on colored backgrounds without contrast check

Performance Anti-Patterns

• Startup time > 2 seconds without progress indicator
• Frame rate < 60 FPS (> 16ms per frame)
• Crash rate > 1.09% (Google Play threshold)
• ANR rate > 0.47% (Google Play threshold)

Accessibility Anti-Patterns

• Missing contentDescription on interactive elements
• Element type in labels (e.g., "Save button" instead of "Save")
• Complex gestures in kids apps
• Text-only buttons for non-readers

Review Checklist

• [ ] 8dp spacing grid compliance
• [ ] 48dp minimum touch targets
• [ ] Proper typography scale usage
• [ ] Color contrast compliance (4.5:1+ for text)
• [ ] Dark theme support
• [ ] contentDescription on all interactive elements
• [ ] Startup < 2 seconds or shows progress
• [ ] Visual style matches app category

Design References

8. Testing

Note: Only add test dependencies when the user explicitly asks for testing.

A well-tested Android app uses layered testing: fast local unit tests for logic, instrumentation tests for UI and integration, and Gradle Managed Devices to run emulators reproducibly on any machine — including CI.

8.1 Test Dependencies

Before adding test dependencies, inspect the project's existing versions to avoid conflicts:

• Check gradle/libs.versions.toml — if present, add test deps using the project's version catalog style
• Check existing build.gradle.kts for already-pinned dependency versions
• Match version families using the table below

Version Alignment Rules:

dependencies {
    // --- Local unit tests (src/test/) ---
    testImplementation("junit:junit:")                          // 4.13.2+
    testImplementation("org.robolectric:robolectric:")          // 4.16.1+
    testImplementation("io.mockk:mockk:")                      // match Kotlin version
    testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:")  // match coroutines-core
    testImplementation("androidx.arch.core:core-testing:")      // InstantTaskExecutorRule for LiveData
    testImplementation("app.cash.turbine:turbine:")             // Flow/StateFlow testing
    // --- Instrumentation tests (src/androidTest/) ---
    androidTestImplementation("androidx.test.ext:junit:")
    androidTestImplementation("androidx.test:runner:")
    androidTestImplementation("androidx.test:rules:")
    androidTestImplementation("androidx.test.espresso:espresso-core:")
    androidTestImplementation("androidx.test.espresso:espresso-contrib:")   // RecyclerView, Drawer
    androidTestImplementation("androidx.test.espresso:espresso-intents:")   // Intent verification
    androidTestImplementation("androidx.test.espresso:espresso-idling-resource:")
    androidTestImplementation("androidx.test.uiautomator:uiautomator:")
    // --- Compose UI tests (only if project uses Compose) ---
    androidTestImplementation("androidx.compose.ui:ui-test-junit4")      // version from Compose BOM
    debugImplementation("androidx.compose.ui:ui-test-manifest")          // required for createComposeRule
}

Note: If the project uses a Compose BOM, ui-test-junit4 and ui-test-manifest don't need explicit versions — the BOM manages them.

Enable Robolectric resource support in the android block:

android {
    testOptions {
        unitTests.isIncludeAndroidResources = true  // required for Robolectric
    }
}

8.2 Testing by Layer

8.3 Testing Commands

# Local unit tests (fast, no emulator)
./gradlew test                          # all modules
./gradlew :app:testDebugUnitTest        # app module, debug variant
# Single test class
./gradlew :app:testDebugUnitTest --tests "com.example.myapp.CounterViewModelTest"
# Instrumentation tests (requires device or managed device)
./gradlew connectedDebugAndroidTest     # on connected device
./gradlew pixel6Api34DebugAndroidTest   # on managed device
# Both together
./gradlew test connectedDebugAndroidTest
# Test with coverage report (JaCoCo)
./gradlew testDebugUnitTest jacocoTestReport