Navigation Between Activities and Fragments

# CHAPTER 14

Navigation Between Activities and Fragments #

1. Introduction #

2. Learning Objectives #

• Differentiate between Explicit Intents and Implicit Intents.

• Launch a new Activity using startActivity().

• Transmit string and integer data between screens using putExtra().

• Understand how Android manages screen history via the "Back Stack".

• Conceptualize the visual workflow of the Jetpack Navigation Component.

3. The Concept of "Intents" #

There are two types of Intents:

1. 1. Explicit Intent: You tell the OS exactly which screen inside your app to open (e.g., "Open SettingsActivity").

1. 2. Implicit Intent: You ask the OS to perform a general action (e.g., "Open a web browser", "Take a photo"), and the OS decides which external app should handle it.

4. Basic Navigation (Explicit Intents) #

Here is how we launch the DetailsActivity when a button is clicked:

// Inside MainActivity.kt
val navigateButton = findViewById<Button>(R.id.navBtn)

navigateButton.setOnClickListener {
    // 1. Create the Intent. 
    // Context = 'this' (Where we are), Target = 'DetailsActivity::class.java' (Where we are going)
    val intent = Intent(this, DetailsActivity::class.java)
    
    // 2. Tell the OS to execute the transition!
    startActivity(intent)
}

5. Passing Data Between Screens (Intent Extras) #

Sending the Data (Screen A):

val intent = Intent(this, DetailsActivity::class.java)
// Stuffing data into the envelope using Key-Value pairs!
intent.putExtra("USERNAME", "Alex")
intent.putExtra("USER_AGE", 25)

startActivity(intent)

Receiving the Data (Screen B / DetailsActivity):

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

    // Opening the envelope!
    // We look for the Key "USERNAME". If we can't find it, we return a default value.
    val incomingName = intent.getStringExtra("USERNAME") ?: "Unknown User"
    
    // For Ints, the fallback default value is mandatory!
    val incomingAge = intent.getIntExtra("USER_AGE", 0)

    println("Received: $incomingName, Age: $incomingAge")
}

6. The Back Stack #

val closeButton = findViewById<Button>(R.id.closeBtn)
closeButton.setOnClickListener {
    finish() // Destroys this Activity and returns to the previous screen!
}

7. The Modern Approach: Jetpack Navigation Component #

8. Mini Project: The Login Handshake #

MainActivity.kt (Login):

loginButton.setOnClickListener {
    val userEmail = emailEditText.text.toString()
    
    if(userEmail.isNotEmpty()) {
        val intent = Intent(this, DashboardActivity::class.java)
        intent.putExtra("EMAIL_KEY", userEmail)
        startActivity(intent)
        // We call finish() here so the user cannot hit the 'Back' button to return to the Login screen!
        finish() 
    }
}

DashboardActivity.kt:

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

    val welcomeText = findViewById<TextView>(R.id.welcomeText)
    
    // Extract the email
    val email = intent.getStringExtra("EMAIL_KEY") ?: "Guest"
    welcomeText.text = "Welcome to the Dashboard, $email!"
}

9. Common Mistakes #

• Typos in Intent Keys: If you send data using the key intent.putExtra("USERID", 5) and try to receive it using intent.getIntExtra("userid", 0), it will fail silently and return 0. Keys are strictly case-sensitive. Always define keys as global constant variables (e.g., const val EXTRAUSERID = "userid") to prevent typos.

• Passing Massive Objects: You can pass Strings and Ints via Intents easily. Beginners often try to pass massive Lists of downloaded images or entire Custom Objects through an Intent. The app will crash with a TransactionTooLargeException. The Intent payload must remain under 1 Megabyte. Pass the *ID* of the data, and let the next screen fetch the data itself.

10. Best Practices #

• Implicit Intents for System Actions: Do not build your own Web Browser if the user just wants to view a link. Use an Implicit Intent!

val webIntent = Intent(Intent.ACTION_VIEW, Uri.parse("https://google.com"))
startActivity(webIntent) // The OS will automatically open Chrome!

11. Exercises #

1. 1. Write the Kotlin code required to launch an Activity named SettingsActivity via an Explicit Intent.

1. 2. Add an Intent Extra to the above code containing a String key THEME with a value of "DARK".

12. Coding Challenges #

13. MCQ Quiz with Answers #

14. Interview Questions #

• Q: Explain the fundamental difference between an Explicit Intent and an Implicit Intent. In what scenario would an Implicit Intent be securely rejected by the Android OS?

• Q: Describe the architectural mechanics of the Android "Back Stack" (Task Stack). How does the OS utilize LIFO (Last-In-First-Out) architecture to manage screen history?

• Q: Detail the limitation of Intent Extras regarding payload size. If a developer needs to pass a massive JSON object representing 500 database entries to a new screen, what is the architecturally correct approach instead of using Intent Extras?

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #