Kotlin Intent and Activity

An Intent is a message that tells Android to do something — open a new screen, launch a camera, or share content. Intents are the glue between Activities. Every time a user taps a button to move from one screen to another, an Intent makes it happen.

Types of Intents

Explicit Intent:
  "Open this specific Activity in my app"
  Intent(context, TargetActivity::class.java)

Implicit Intent:
  "Open something that can handle this action"
  Intent(Intent.ACTION_VIEW, Uri.parse("https://google.com"))
  Android finds the right app (browser, email client, etc.)

Navigating to Another Activity

// In MainActivity.kt
class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val openButton: Button = findViewById(R.id.openButton)

        openButton.setOnClickListener {
            val intent = Intent(this, DetailActivity::class.java)
            startActivity(intent)
        }
    }
}

// DetailActivity.kt
class DetailActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_detail)
        println("DetailActivity opened!")
    }
}

Passing Data Between Activities

Sending Activity                     Receiving Activity
─────────────────────────            ─────────────────────────
val intent = Intent(...)             val name = intent.getStringExtra("name")
intent.putExtra("name", "Alice")     val age  = intent.getIntExtra("age", 0)
intent.putExtra("age", 30)
startActivity(intent)

// Sender: MainActivity.kt
val intent = Intent(this, ProfileActivity::class.java).apply {
    putExtra("user_name", "Alice")
    putExtra("user_age", 30)
    putExtra("is_premium", true)
}
startActivity(intent)

// Receiver: ProfileActivity.kt
class ProfileActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_profile)

        val name      = intent.getStringExtra("user_name") ?: "Unknown"
        val age       = intent.getIntExtra("user_age", 0)
        val isPremium = intent.getBooleanExtra("is_premium", false)

        println("Welcome $name, age $age, premium: $isPremium")
    }
}

Implicit Intent Examples

// Open a URL in the browser
val browserIntent = Intent(Intent.ACTION_VIEW, Uri.parse("https://kotlinlang.org"))
startActivity(browserIntent)

// Dial a phone number
val dialIntent = Intent(Intent.ACTION_DIAL, Uri.parse("tel:+1234567890"))
startActivity(dialIntent)

// Share text with another app
val shareIntent = Intent(Intent.ACTION_SEND).apply {
    type = "text/plain"
    putExtra(Intent.EXTRA_TEXT, "Check out Kotlin!")
}
startActivity(Intent.createChooser(shareIntent, "Share via"))

Getting a Result Back (ActivityResultLauncher)

// Modern way to get results back from another Activity
class MainActivity : AppCompatActivity() {

    private val getContent = registerForActivityResult(
        ActivityResultContracts.StartActivityForResult()
    ) { result ->
        if (result.resultCode == RESULT_OK) {
            val message = result.data?.getStringExtra("result_message")
            println("Got back: $message")
        }
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        findViewById<Button>(R.id.launchBtn).setOnClickListener {
            val intent = Intent(this, EditorActivity::class.java)
            getContent.launch(intent)
        }
    }
}

// EditorActivity.kt — sends result back
class EditorActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_editor)

        findViewById<Button>(R.id.doneBtn).setOnClickListener {
            val result = Intent().apply {
                putExtra("result_message", "Editing complete!")
            }
            setResult(RESULT_OK, result)
            finish()   // closes this Activity and returns to caller
        }
    }
}

Back Stack

Activity Back Stack — like a pile of plates:

User navigates:
  MainActivity    ← at bottom
  ListActivity    ← pushed on top
  DetailActivity  ← pushed on top (currently visible)

User presses Back:
  DetailActivity  ← popped (destroyed)
  ListActivity    ← now visible again

  finish() also pops the current Activity off the stack

Practical Example: Login Flow

// LoginActivity.kt
class LoginActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_login)

        val loginBtn: Button   = findViewById(R.id.loginButton)
        val usernameField: EditText = findViewById(R.id.usernameField)

        loginBtn.setOnClickListener {
            val username = usernameField.text.toString().trim()
            if (username.isNotEmpty()) {
                val intent = Intent(this, HomeActivity::class.java).apply {
                    putExtra("logged_in_user", username)
                    // Prevent going back to login screen
                    flags = Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TASK
                }
                startActivity(intent)
            } else {
                Toast.makeText(this, "Enter a username", Toast.LENGTH_SHORT).show()
            }
        }
    }
}

The FLAG_ACTIVITY_CLEAR_TASK flag removes the login screen from the back stack so users cannot press Back to return to the login page after signing in.