Android Java Skill

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---

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<

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