RecyclerView (Lists)

// Subject.kt
data class Subject(val name: String, val teacher: String, val emoji: String, val chapters: Int)

val subjects = listOf(
    Subject("Maths", "Mr. Gupta", "📐", 14),
    Subject("Physics", "Ms. Khan", "🔭", 12),
    Subject("Chemistry", "Mr. Rao", "⚗️", 10),
    Subject("Biology", "Dr. Mehta", "🧬", 16),
    Subject("English", "Ms. Thomas", "📖", 9),
    Subject("Computer", "Mr. Singh", "💻", 11)
)

<?xml version="1.0" encoding="utf-8"?>
<!-- res/layout/item_subject_row.xml – ONE row -->
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:gravity="center_vertical"
    android:orientation="horizontal"
    android:padding="16dp">

    <TextView
        android:id="@+id/tvEmoji"
        android:layout_width="48dp"
        android:layout_height="48dp"
        android:gravity="center"
        android:textSize="26sp" />

    <LinearLayout
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="12dp"
        android:layout_weight="1"
        android:orientation="vertical">

        <TextView
            android:id="@+id/tvName"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="17sp"
            android:textStyle="bold" />

        <TextView
            android:id="@+id/tvTeacher"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" />
    </LinearLayout>

    <TextView
        android:id="@+id/tvChapters"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</LinearLayout>

// SubjectAdapter.kt
class SubjectAdapter(private val items: List<Subject>) :
    RecyclerView.Adapter<SubjectAdapter.SubjectViewHolder>() {

    // ViewHolder = holds the views of ONE row (so we don't look them up again)
    class SubjectViewHolder(val binding: ItemSubjectRowBinding) : RecyclerView.ViewHolder(binding.root)

    // 1. called only a few times: create a new row view
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): SubjectViewHolder {
        val binding = ItemSubjectRowBinding.inflate(LayoutInflater.from(parent.context), parent, false)
        return SubjectViewHolder(binding)
    }

    // 2. called many times: put the data of position X into a row
    override fun onBindViewHolder(holder: SubjectViewHolder, position: Int) {
        val s = items[position]
        holder.binding.tvEmoji.text = s.emoji
        holder.binding.tvName.text = s.name
        holder.binding.tvTeacher.text = s.teacher
        holder.binding.tvChapters.text = "${s.chapters} ch."
    }

    // 3. how many rows?
    override fun getItemCount() = items.size
}

<!-- activity_main.xml -->
<androidx.recyclerview.widget.RecyclerView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/recyclerView"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:listitem="@layout/item_subject_row" />

class MainActivity : AppCompatActivity() {
    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        binding.recyclerView.layoutManager = LinearLayoutManager(this)
        binding.recyclerView.adapter = SubjectAdapter(subjects)
        binding.recyclerView.addItemDecoration(
            DividerItemDecoration(this, DividerItemDecoration.VERTICAL)
        )
    }
}

class SubjectAdapter(private val items: MutableList<Subject>) : RecyclerView.Adapter<...>() {
    fun add(subject: Subject) {
        items.add(0, subject)
        notifyItemInserted(0)            // animate just the new row
    }
    fun removeAt(position: Int) {
        items.removeAt(position)
        notifyItemRemoved(position)
    }
}
// notifyDataSetChanged() redraws everything – avoid it; use ListAdapter + DiffUtil (next lesson)