Android Java Skill Load with: base.md Project Structure project/ ├── app/ │ ├── src/ │ │ ├── main/ │ │ │ ├── java/com/example/app/ │ │ │ │ ├── data/ # Data layer │ │ │ │ │ ├── local/ # Room database, SharedPreferences │ │ │ │ │ ├── remote/ # Retrofit services, API clients │ │ │ │ │ └── repository/ # Repository implementations │ │ │ │ ├── di/ # Dependency injection (Hilt/Dagger) │ │ │ │ ├── domain/ # Business logic │ │ │ │ │ ├── model/ # Domain models │ │ │ │ │ ├── repository/ # Repository interfaces │ │ │ │ │ └── usecase/ # Use cases │ │ │ │ ├── ui/ # Presentation layer │ │ │ │ │ ├── feature/ # Feature screens │ │ │ │ │ │ ├── FeatureActivity.java │ │ │ │ │ │ ├── FeatureFragment.java │ │ │ │ │ │ └── FeatureViewModel.java │ │ │ │ │ └── common/ # Shared UI components │ │ │ │ └── App.java # Application class │ │ │ ├── res/ │ │ │ │ ├── layout/ │ │ │ │ ├── values/ │ │ │ │ └── drawable/ │ │ │ └── AndroidManifest.xml │ │ ├── test/ # Unit tests │ │ └── androidTest/ # Instrumentation tests │ └── build.gradle ├── build.gradle # Project-level build file ├── gradle.properties ├── settings.gradle └── CLAUDE.md Gradle Configuration App-level build.gradle plugins { id 'com.android.application' } android { namespace 'com.example.app' compileSdk 34 defaultConfig { applicationId "com.example.app" minSdk 24 targetSdk 34 versionCode 1 versionName "1.0" testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner" } buildTypes { release { minifyEnabled true proguardFiles getDefaultProguardFile ( 'proguard-android-optimize.txt' ) , 'proguard-rules.pro' } } compileOptions { sourceCompatibility JavaVersion . VERSION_17 targetCompatibility JavaVersion . VERSION_17 } buildFeatures { viewBinding true } } dependencies { // AndroidX implementation 'androidx.core:core:1.12.0' implementation 'androidx.appcompat:appcompat:1.6.1' implementation 'com.google.android.material:material:1.11.0' implementation 'androidx.constraintlayout:constraintlayout:2.1.4' // Lifecycle implementation 'androidx.lifecycle:lifecycle-viewmodel:2.7.0' implementation 'androidx.lifecycle:lifecycle-livedata:2.7.0' // Testing testImplementation 'junit:junit:4.13.2' testImplementation 'org.mockito:mockito-core:5.8.0' androidTestImplementation 'androidx.test.ext:junit:1.1.5' androidTestImplementation 'androidx.test.espresso:espresso-core:3.5.1' } Architecture Patterns MVVM with ViewModel // ViewModel - holds UI state, survives configuration changes public class UserViewModel extends ViewModel { private final UserRepository repository ; private final MutableLiveData < User

user

new MutableLiveData <

( ) ; private final MutableLiveData < Boolean

loading

new MutableLiveData <

( false ) ; private final MutableLiveData < String

error

new MutableLiveData <

( ) ; public UserViewModel ( UserRepository repository ) { this . repository = repository ; } public LiveData < User

getUser ( ) { return user ; } public LiveData < Boolean

isLoading ( ) { return loading ; } public LiveData < String

getError ( ) { return error ; } public void loadUser ( String userId ) { loading . setValue ( true ) ; repository . getUser ( userId , new Callback < User

( ) { @Override public void onSuccess ( User result ) { user . setValue ( result ) ; loading . setValue ( false ) ; } @Override public void onError ( String message ) { error . setValue ( message ) ; loading . setValue ( false ) ; } } ) ; } } Repository Pattern // Repository interface (domain layer) public interface UserRepository { void getUser ( String userId , Callback < User

callback ) ; void saveUser ( User user , Callback < Void

callback ) ; } // Repository implementation (data layer) public class UserRepositoryImpl implements UserRepository { private final UserApi api ; private final UserDao dao ; public UserRepositoryImpl ( UserApi api , UserDao dao ) { this . api = api ; this . dao = dao ; } @Override public void getUser ( String userId , Callback < User

callback ) { // Try cache first, then network User cached = dao . getUserById ( userId ) ; if ( cached != null ) { callback . onSuccess ( cached ) ; return ; } api . getUser ( userId ) . enqueue ( new retrofit2 . Callback < User

( ) { @Override public void onResponse ( Call < User

call , Response < User

response ) { if ( response . isSuccessful ( ) && response . body ( ) != null ) { dao . insert ( response . body ( ) ) ; callback . onSuccess ( response . body ( ) ) ; } else { callback . onError ( "Failed to load user" ) ; } } @Override public void onFailure ( Call < User

call , Throwable t ) { callback . onError ( t . getMessage ( ) ) ; } } ) ; } } Activity & Fragment Patterns Activity with ViewBinding public class MainActivity extends AppCompatActivity { private ActivityMainBinding binding ; private MainViewModel viewModel ; @Override protected void onCreate ( Bundle savedInstanceState ) { super . onCreate ( savedInstanceState ) ; binding = ActivityMainBinding . inflate ( getLayoutInflater ( ) ) ; setContentView ( binding . getRoot ( ) ) ; viewModel = new ViewModelProvider ( this ) . get ( MainViewModel . class ) ; setupObservers ( ) ; setupListeners ( ) ; } private void setupObservers ( ) { viewModel . getUser ( ) . observe ( this , user -> { binding . userName . setText ( user . getName ( ) ) ; } ) ; viewModel . isLoading ( ) . observe ( this , isLoading -> { binding . progressBar . setVisibility ( isLoading ? View . VISIBLE : View . GONE ) ; } ) ; } private void setupListeners ( ) { binding . refreshButton . setOnClickListener ( v -> { viewModel . loadUser ( getCurrentUserId ( ) ) ; } ) ; } @Override protected void onDestroy ( ) { super . onDestroy ( ) ; binding = null ; } } Fragment with ViewBinding public class UserFragment extends Fragment { private FragmentUserBinding binding ; private UserViewModel viewModel ; @Override public View onCreateView ( LayoutInflater inflater , ViewGroup container , Bundle savedInstanceState ) { binding = FragmentUserBinding . inflate ( inflater , container , false ) ; return binding . getRoot ( ) ; } @Override public void onViewCreated ( View view , Bundle savedInstanceState ) { super . onViewCreated ( view , savedInstanceState ) ; viewModel = new ViewModelProvider ( requireActivity ( ) ) . get ( UserViewModel . class ) ; setupObservers ( ) ; } private void setupObservers ( ) { viewModel . getUser ( ) . observe ( getViewLifecycleOwner ( ) , user -> { binding . userName . setText ( user . getName ( ) ) ; } ) ; } @Override public void onDestroyView ( ) { super . onDestroyView ( ) ; binding = null ; // Prevent memory leaks } } Testing Unit Tests with JUnit & Mockito @RunWith ( MockitoJUnitRunner . class ) public class UserViewModelTest { @Mock private UserRepository repository ; @Rule public InstantTaskExecutorRule instantTaskExecutorRule = new InstantTaskExecutorRule ( ) ; private UserViewModel viewModel ; @Before public void setup ( ) { viewModel = new UserViewModel ( repository ) ; } @Test public void loadUser_success_updatesUserLiveData ( ) { // Arrange User expectedUser = new User ( "1" , "John Doe" ) ; doAnswer ( invocation -> { Callback < User

callback

invocation . getArgument ( 1 ) ; callback . onSuccess ( expectedUser ) ; return null ; } ) . when ( repository ) . getUser ( eq ( "1" ) , any ( ) ) ; // Act viewModel . loadUser ( "1" ) ; // Assert assertEquals ( expectedUser , viewModel . getUser ( ) . getValue ( ) ) ; assertFalse ( viewModel . isLoading ( ) . getValue ( ) ) ; } @Test public void loadUser_error_updatesErrorLiveData ( ) { // Arrange doAnswer ( invocation -> { Callback < User

callback

invocation . getArgument ( 1 ) ; callback . onError ( "Network error" ) ; return null ; } ) . when ( repository ) . getUser ( eq ( "1" ) , any ( ) ) ; // Act viewModel . loadUser ( "1" ) ; // Assert assertEquals ( "Network error" , viewModel . getError ( ) . getValue ( ) ) ; assertFalse ( viewModel . isLoading ( ) . getValue ( ) ) ; } } Instrumentation Tests with Espresso @RunWith ( AndroidJUnit4 . class ) public class MainActivityTest { @Rule public ActivityScenarioRule < MainActivity

activityRule

new ActivityScenarioRule <

( MainActivity . class ) ; @Test public void userName_isDisplayed ( ) { onView ( withId ( R . id . userName ) ) . check ( matches ( isDisplayed ( ) ) ) ; } @Test public void refreshButton_click_triggersRefresh ( ) { onView ( withId ( R . id . refreshButton ) ) . perform ( click ( ) ) ; onView ( withId ( R . id . progressBar ) ) . check ( matches ( isDisplayed ( ) ) ) ; } @Test public void userList_scrollToItem_displaysCorrectly ( ) { onView ( withId ( R . id . userList ) ) . perform ( RecyclerViewActions . scrollToPosition ( 10 ) ) ; onView ( withText ( "User 10" ) ) . check ( matches ( isDisplayed ( ) ) ) ; } } GitHub Actions name : Android CI on : push : branches : [ main ] pull_request : branches : [ main ] jobs : build : runs-on : ubuntu - latest steps : - uses : actions/checkout@v4 - name : Set up JDK 17 uses : actions/setup - java@v4 with : java-version : '17' distribution : 'temurin' - name : Setup Gradle uses : gradle/actions/setup - gradle@v3 - name : Grant execute permission for gradlew run : chmod +x gradlew - name : Run Lint run : ./gradlew lint - name : Run Unit Tests run : ./gradlew testDebugUnitTest - name : Build Debug APK run : ./gradlew assembleDebug - name : Upload APK uses : actions/upload - artifact@v4 with : name : debug - apk path : app/build/outputs/apk/debug/app - debug.apk instrumentation-tests : runs-on : ubuntu - latest steps : - uses : actions/checkout@v4 - name : Set up JDK 17 uses : actions/setup - java@v4 with : java-version : '17' distribution : 'temurin' - name : Enable KVM run : | echo 'KERNEL=="kvm", GROUP="kvm", MODE="0666", OPTIONS+="static_node=kvm"' | sudo tee /etc/udev/rules.d/99-kvm4all.rules sudo udevadm control --reload-rules sudo udevadm trigger --name-match=kvm - name : Run Instrumentation Tests uses : reactivecircus/android - emulator - runner@v2 with : api-level : 29 script : ./gradlew connectedDebugAndroidTest Lint Configuration lint.xml

< lint

< issue id = " HardcodedText " severity = " error " /> < issue id = " MissingTranslation " severity = " error " /> < issue id = " UnusedResources " severity = " warning " />

< issue id = " StaticFieldLeak " severity = " error " />

< issue id = " HardcodedDebugMode " severity = " error " /> < issue id = " AllowBackup " severity = " warning " />

< issue id = " ViewHolder " severity = " error " /> < issue id = " Overdraw " severity = " warning " />

< issue id = " InvalidPackage "

< ignore path = " /test/ " /> < ignore path = " /androidTest/ " /> </ issue

</ lint

build.gradle lint options android { lint { abortOnError true warningsAsErrors false checkReleaseBuilds true xmlReport true htmlReport true } } Common Patterns Null-Safe Callbacks // Define callback interface public interface Callback < T

{ void onSuccess ( T result ) ; void onError ( String message ) ; } // Use with null checks public void fetchData ( Callback < Data

callback ) { if ( callback == null ) return ; try { Data result = performFetch ( ) ; callback . onSuccess ( result ) ; } catch ( Exception e ) { callback . onError ( e . getMessage ( ) ) ; } } Safe Context Usage // Use application context for long-lived objects public class DataManager { private final Context appContext ; public DataManager ( Context context ) { // Always use application context to prevent Activity leaks this . appContext = context . getApplicationContext ( ) ; } } // Check for null context in callbacks private void updateUI ( ) { Context context = getContext ( ) ; if ( context == null || ! isAdded ( ) ) return ; // Safe to use context } Thread-Safe Singleton public class ApiClient { private static volatile ApiClient instance ; private final Retrofit retrofit ; private ApiClient ( ) { retrofit = new Retrofit . Builder ( ) . baseUrl ( BASE_URL ) . addConverterFactory ( GsonConverterFactory . create ( ) ) . build ( ) ; } public static ApiClient getInstance ( ) { if ( instance == null ) { synchronized ( ApiClient . class ) { if ( instance == null ) { instance = new ApiClient ( ) ; } } } return instance ; } } Android Anti-Patterns ❌ Context leaks - Never hold Activity/Fragment references in static fields or singletons ❌ Memory leaks in callbacks - Always use WeakReference or clear callbacks in onDestroy ❌ UI updates on background thread - Always post to main thread for UI changes ❌ Hardcoded strings - Use string resources for all user-visible text ❌ God Activities - Keep Activities under 200 lines, extract logic to ViewModels ❌ NetworkOnMainThreadException - Never perform network calls on main thread ❌ Ignoring lifecycle - Always respect Activity/Fragment lifecycle states ❌ Blocking the main thread - Keep main thread operations under 16ms ❌ Not handling configuration changes - Use ViewModel to survive rotation ❌ Hardcoded dimensions - Use dp/sp units and dimension resources ❌ Deep view hierarchies - Keep layout depth under 10 levels, use ConstraintLayout ❌ Not closing resources - Always close Cursor, InputStream, database connections