import androidx.compose.material3.Button
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Modifier
import androidx.compose.ui.semantics.contentDescription
import androidx.compose.ui.semantics.semantics

@Composable
fun PayButton(amount: String, onPay: () -> Unit) {
    Button(
        onClick = onPay,
        modifier = Modifier.semantics { contentDescription = "Pay $amount rupees" }
    ) {
        Text("Pay ₹$amount")
    }
}

fun validateAmount(input: String, balance: Long): Result<Long> {
    val paise = input.toBigDecimalOrNull()
        ?.movePointRight(2)
        ?.toLong()
        ?: return Result.failure(IllegalArgumentException("Enter a valid amount"))
    return when {
        paise <= 0 -> Result.failure(IllegalArgumentException("Amount must be positive"))
        paise > balance -> Result.failure(IllegalArgumentException("Insufficient balance"))
        else -> Result.success(paise)
    }
}

import androidx.biometric.BiometricManager.Authenticators.BIOMETRIC_STRONG
import androidx.biometric.BiometricPrompt
import androidx.core.content.ContextCompat
import androidx.fragment.app.FragmentActivity

fun showBiometricLogin(activity: FragmentActivity, onSuccess: () -> Unit) {
    val executor = ContextCompat.getMainExecutor(activity)
    val prompt = BiometricPrompt(activity, executor,
        object : BiometricPrompt.AuthenticationCallback() {
            override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
                onSuccess()
            }
        })
    val info = BiometricPrompt.PromptInfo.Builder()
        .setTitle("Log in to your account")
        .setAllowedAuthenticators(BIOMETRIC_STRONG)
        .setNegativeButtonText("Use PIN")
        .build()
    prompt.authenticate(info)
}

// In the Activity showing account details
window.setFlags(
    WindowManager.LayoutParams.FLAG_SECURE,
    WindowManager.LayoutParams.FLAG_SECURE
)

<!-- res/xml/network_security_config.xml -->
<network-security-config>
    <base-config cleartextTrafficPermitted="false" />
</network-security-config>
<!-- Reference it in AndroidManifest.xml:
     android:networkSecurityConfig="@xml/network_security_config" -->

import org.junit.Assert.assertEquals
import org.junit.Assert.assertTrue
import org.junit.Test

class AmountValidationTest {
    @Test
    fun rejectsAmountAboveBalance() {
        val result = validateAmount("1500.00", balance = 100_000) // ₹1000.00 in paise
        assertTrue(result.isFailure)
    }

    @Test
    fun acceptsValidAmountInPaise() {
        val result = validateAmount("250.50", balance = 100_000)
        assertEquals(25_050L, result.getOrThrow())
    }
}

Practice banking app (fake data only)
☐ Biometric login with PIN fallback
☐ Accounts screen with FLAG_SECURE
☐ Transactions list with paging and offline cache
☐ Transfer flow with validation and confirmation
☐ Unit tests for validation; UI test for transfer flow
☐ README: security decisions explained

Weeks 1-2: Kotlin depth, coroutines, clean architecture
Weeks 3-4: Security — Keystore, BiometricPrompt, network security config
Weeks 5-6: Practice banking app — login, accounts, transfer
Week 7:    Testing — unit tests, UI tests, edge cases
Week 8:    Accessibility review, performance profiling, README

☐ No secrets or API keys in source code
☐ Tokens stored using Keystore-backed encryption
☐ Cleartext traffic disabled
☐ FLAG_SECURE on sensitive screens
☐ Session timeout and logout clear sensitive data
☐ Server validates everything the app sends

Prepare three stories
1. A security decision I made and why: ____
2. A bug in a payment-like flow I found through testing: ____
3. How I improved accessibility or performance: ____
Each: situation → action → result