Android Academy · Lesson

Error Handling & UX

Model UI states with sealed classes (Loading/Success/Error). Show ProgressBar, Snackbar with Retry actions, and inline TextInputLayout validation errors.

Lesson 3 of 611 steps

Error Handling & UX is a free Android Academy lesson on CoddyKit. This is lesson 3 of 6. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Android Academy learning path, and your progress syncs across the web and the CoddyKit app. The Android Academy course includes 6 lessons in total.

Why Error Handling Matters

Apps that crash or freeze without explanation feel broken. Users don't know what happened or what to do next.

Good error handling means:

The app stays stable
Users get a clear message
They can retry or take action

This is the difference between a 2-star and a 5-star app review.

Modeling UI State

A clean pattern: model all possible UI states with a sealed class:

Loading — request is in progress
Success(data) — data is available
Error(message) — something went wrong

The ViewModel emits one of these states; the UI renders it.

Sealed Class UiState

Define a sealed class for all possible states:

sealed class UiState<out T> {
    object Loading : UiState<Nothing>()
    data class Success<T>(val data: T) : UiState<T>()
    data class Error(val message: String) : UiState<Nothing>()
}

// In ViewModel:
private val _state = MutableLiveData<UiState<List<Post>>>()
val state: LiveData<UiState<List<Post>>> = _state

fun loadPosts() {
    _state.value = UiState.Loading
    viewModelScope.launch {
        try {
            val posts = withContext(Dispatchers.IO) { repo.getPosts() }
            _state.value = UiState.Success(posts)
        } catch (e: Exception) {
            _state.value = UiState.Error(e.message ?: "Unknown error")
        }
    }
}

Rendering State in the UI

Observe the state and show the right view for each case:

viewModel.state.observe(this) { state ->
    when (state) {
        is UiState.Loading -> {
            binding.progressBar.visibility = View.VISIBLE
            binding.recyclerView.visibility = View.GONE
            binding.tvError.visibility = View.GONE
        }
        is UiState.Success -> {
            binding.progressBar.visibility = View.GONE
            binding.recyclerView.visibility = View.VISIBLE
            adapter.submitList(state.data)
        }
        is UiState.Error -> {
            binding.progressBar.visibility = View.GONE
            binding.tvError.text = state.message
            binding.tvError.visibility = View.VISIBLE
        }
    }
}

ProgressBar for Loading

Show a ProgressBar while waiting for data:

Use android:indeterminate="true" for a spinning loader
Toggle visibility between View.VISIBLE and View.GONE
GONE removes the view from layout (not just invisible — it doesn't take up space)

Snackbar vs Toast

Prefer Snackbar over Toast for error messages:

Snackbar can have an action button (e.g. "Retry")
It's attached to the current screen — doesn't float over other apps
Dismisses automatically or on tap

Snackbar with Retry

Show an error with a Retry action button:

import com.google.android.material.snackbar.Snackbar

fun showError(message: String) {
    Snackbar.make(
        binding.root,  // root view of the screen
        message,
        Snackbar.LENGTH_INDEFINITE  // stays until dismissed
    ).setAction("Retry") {
        viewModel.loadPosts()  // retry the request
    }.show()
}

Input Validation

Validate user input before sending it to the server:

Use TextInputLayout with error property to show inline errors below the field
Check empty fields, email format, password length before calling the API
Clear errors when the user starts typing

TextInputLayout Error

Show inline validation errors on form fields:

fun validateAndSubmit() {
    val email = binding.etEmail.text.toString().trim()
    val password = binding.etPassword.text.toString()

    var hasError = false

    if (email.isBlank() || !email.contains("@")) {
        binding.tilEmail.error = "Enter a valid email"
        hasError = true
    } else {
        binding.tilEmail.error = null  // clear error
    }

    if (password.length < 6) {
        binding.tilPassword.error = "Minimum 6 characters"
        hasError = true
    } else {
        binding.tilPassword.error = null
    }

    if (!hasError) viewModel.login(email, password)
}

Quick Check

Which view visibility state removes a view from the layout completely (takes no space)?

Answer it in the interactive lesson

Recap: Error Handling & UX

Your apps can now handle errors gracefully:

Model state as Loading / Success / Error with a sealed class
Show a ProgressBar during loading
Snackbar with Retry action for network errors
TextInputLayout inline errors for form validation
View.GONE vs View.INVISIBLE — remove vs hide

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Frequently Asked Questions

Is the “Error Handling & UX” lesson free?

Yes — any 3 lessons of the Android Academy learning path, including “Error Handling & UX”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Android Academy course includes 6 lessons in total.

What will I learn in “Error Handling & UX”?

Model UI states with sealed classes (Loading/Success/Error). Show ProgressBar, Snackbar with Retry actions, and inline TextInputLayout validation errors. You practise Android Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Android Academy?

No prior experience is required. Android Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 6.

How long does the “Error Handling & UX” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Android Academy lesson?

Yes. Every Android Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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