Does Jetpack Compose automatically generate keys for LazyColumn items?

No, Jetpack Compose does not automatically generate stable, content-based keys for LazyColumn or LazyRow items. If you do not provide an explicit key, the framework defaults to using the item's index in the list as its implicit key.

The Impact of Index-Based Keying

Because the default key is the position (index), Compose identifies items by where they are, not what they are. This leads to specific behaviors when the underlying data changes:

• Reordering: If an item moves from index 0 to index 1, Compose treats the item at index 0 as having changed its content and the item at index 1 as having changed its content, rather than recognizing that a single item has moved.
• Filtering/Deletion: Removing an item shifts the indices of all subsequent items, triggering unnecessary recompositions for every item following the deleted one.
• State Loss: Any internal state (such as a rememberSaveable value or a TextField focus) tied to a specific index will remain at that index even if the data associated with that index changes.

Implementation of Stable Keys

To maintain item identity across reorders or filters, you must provide a unique, stable identifier from your data model via the key parameter in the items() function. This is the standard contract for all versions of Compose, including 1.4+.

val users = listOf(User(id = "a1", name = "Alice"), User(id = "b2", name = "Bob"))

LazyColumn {
    items(
        items = users,
        key = { user -> user.id } // Explicit stable key
    ) {
        UserRow(user)
    }
}

Verification Steps

You can verify the necessity of explicit keys with this test:

1. Create a LazyColumn where each item contains a TextField.
2. Enter text into the first item's field.
3. Trigger a list reorder (e.g., move the first item to the bottom).
4. Observation: Without explicit keys, the text will stay at the top of the list (index 0) while the data moves. With explicit keys, the text will move with the item.

Constraints and Assumptions

This behavior assumes you are using the standard androidx.compose.foundation.lazy package. Note that keys must be unique within the list; providing duplicate keys will result in a runtime exception. To provide a more tailored recommendation, please specify if your data source is a static list or a reactive stream (like Flow or LiveData).