Android Kotlin Tutorial

// Kotlin Basics
fun main() {
    // Variables
    val appName = "My Android App"  // Immutable
    var version = 1.0  // Mutable
    
    println("Welcome to $appName v$version")
    
    // Functions
    fun greet(name: String): String {
        return "Hello, $name!"
    }
    
    println(greet("Developer"))
    
    // Lists
    val frameworks = listOf("Jetpack Compose", "Room", "Retrofit")
    frameworks.forEach { println(it) }
    
    // Null safety
    var nullableString: String? = null
    println(nullableString?.length ?: "String is null")
}

// MainActivity.kt
package com.example.myapp

import android.os.Bundle
import android.widget.Button
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity

class MainActivity : AppCompatActivity() {
    private lateinit var textView: TextView
    private lateinit var button: Button
    
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        
        textView = findViewById(R.id.textView)
        button = findViewById(R.id.button)
        
        button.setOnClickListener {
            textView.text = "Button Clicked!"
        }
    }
}

<!-- activity_main.xml -->
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="16dp"
    android:gravity="center">
    
    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello Android!"
        android:textSize="24sp"
        android:textStyle="bold"/>
    
    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Click Me"
        android:layout_marginTop="20dp"/>
        
</LinearLayout>

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.*
import androidx.compose.material3.*
import androidx.compose.runtime.*
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.unit.dp

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MyApp()
        }
    }
}

@Composable
fun MyApp() {
    var count by remember { mutableStateOf(0) }
    
    MaterialTheme {
        Surface(
            modifier = Modifier.fillMaxSize(),
            color = MaterialTheme.colorScheme.background
        ) {
            Column(
                modifier = Modifier
                    .fillMaxSize()
                    .padding(16.dp),
                horizontalAlignment = Alignment.CenterHorizontally,
                verticalArrangement = Arrangement.Center
            ) {
                Text(
                    text = "Count: $count",
                    style = MaterialTheme.typography.headlineLarge
                )
                Spacer(modifier = Modifier.height(16.dp))
                Button(onClick = { count++ }) {
                    Text("Increment")
                }
            }
        }
    }
}

// Item data class
data class Item(val id: Int, val title: String, val description: String)

// Adapter
class ItemAdapter(private val items: List<Item>) : 
    RecyclerView.Adapter<ItemAdapter.ViewHolder>() {
    
    class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
        val titleText: TextView = view.findViewById(R.id.titleText)
        val descText: TextView = view.findViewById(R.id.descText)
    }
    
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        val view = LayoutInflater.from(parent.context)
            .inflate(R.layout.item_layout, parent, false)
        return ViewHolder(view)
    }
    
    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        val item = items[position]
        holder.titleText.text = item.title
        holder.descText.text = item.description
    }
    
    override fun getItemCount() = items.size
}

// In Activity
val items = listOf(
    Item(1, "Kotlin", "Modern Android language"),
    Item(2, "Jetpack", "Android libraries"),
    Item(3, "Compose", "Declarative UI")
)

val recyclerView = findViewById<RecyclerView>(R.id.recyclerView)
recyclerView.layoutManager = LinearLayoutManager(this)
recyclerView.adapter = ItemAdapter(items)

// Add to build.gradle
dependencies {
    implementation "androidx.navigation:navigation-fragment-ktx:2.7.0"
    implementation "androidx.navigation:navigation-ui-ktx:2.7.0"
}

// In Fragment
import androidx.navigation.fragment.findNavController

class HomeFragment : Fragment() {
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        
        val button = view.findViewById<Button>(R.id.navigateButton)
        button.setOnClickListener {
            findNavController().navigate(R.id.action_home_to_details)
        }
    }
}

// Add to build.gradle
dependencies {
    implementation 'com.squareup.retrofit2:retrofit:2.9.0'
    implementation 'com.squareup.retrofit2:converter-gson:2.9.0'
}

// Data model
data class Post(
    val id: Int,
    val title: String,
    val body: String
)

// API interface
interface ApiService {
    @GET("posts")
    suspend fun getPosts(): List<Post>
}

// Retrofit instance
object RetrofitClient {
    private const val BASE_URL = "https://jsonplaceholder.typicode.com/"
    
    val apiService: ApiService by lazy {
        Retrofit.Builder()
            .baseUrl(BASE_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .build()
            .create(ApiService::class.java)
    }
}

// In ViewModel or Activity
lifecycleScope.launch {
    try {
        val posts = RetrofitClient.apiService.getPosts()
        // Update UI with posts
    } catch (e: Exception) {
        // Handle error
    }
}