What is remember in Compose and how do you use mutableStateOf?

WHAT THIS TESTS: This question checks whether you understand the Compose composition lifecycle and the distinction between ephemeral local variables and state that survives recomposition. Interviewers want to know if you can explain why plain vals inside a composable are recreated on every recomposition, and how remember provides a slot in the composition tree to persist a value across recompositions without leaking it across the entire app like a ViewModel might.

A GOOD ANSWER COVERS: First, define remember as a composable function that stores a single object in the composition and returns it on subsequent recompositions. Second, explain that remember alone does not make state observable; you pair it with mutableStateOf to create a state holder that both survives recomposition and triggers recomposition when the value changes. Third, mention that the remembered value is scoped to the composable's position in the tree and is forgotten when the composable leaves the composition. Fourth, provide a concise code snippet showing the pattern val count by remember { mutableStateOf(0) } and a button that increments it.

COMMON WRONG ANSWERS: A major red flag is claiming that mutableStateOf alone is sufficient; without remember, the state object is recreated every recomposition and all user changes are lost. Another mistake is confusing remember with rememberSaveable; candidates should not claim remember survives configuration changes or process death. A third error is suggesting global variables or ViewModel usage for purely local UI state like a text field's transient input or a dropdown's expanded flag, which shows poor scoping judgment.

LIKELY FOLLOW-UPS: The interviewer may ask when to use rememberSaveable instead of remember, which tests whether you know that rememberSaveable persists across configuration changes using the saved instance state mechanism. They might ask about the performance implications of remember versus derivedStateOf, or how to hoist state from a composable to a parent or ViewModel. You should also be ready to explain why remember accepts a key or keys parameter and what happens when those keys change.

ONE CONCRETE EXAMPLE: A clean example is a simple counter inside a greeting card. Inside a composable function named CounterCard, you write var count by remember { mutableStateOf(0) }. Then you display the count in a Text composable and provide a Button whose onClick lambda increments count. Because remember caches the MutableState instance, clicking the button updates the observed value, Compose invalidates the relevant scope, and the UI recomposes to show the new number. If you removed remember and only wrote var count = mutableStateOf(0), the count would reset to zero on every recomposition because the variable would be reinitialized each time the function ran.

Source: developer.android.com