First Android app in Android Kotlin

Introduction

Making your first Android app helps you learn how to create simple screens and show messages. It is the first step to building useful mobile apps.

You want to create a simple app that shows a greeting message.

You want to learn how Android apps start and display content.

You want to test if your development setup works correctly.

You want to understand the basic structure of an Android app.

Syntax

Android Kotlin

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

This code defines the main screen of your app.

setContentView tells Android which layout file to show.

Examples

This is the basic main activity that loads a layout called activity_main.

Android Kotlin

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

This XML code shows a text message centered on the screen.

Android Kotlin

<TextView
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:text="Hello, Android!"
  android:textSize="24sp"
  android:layout_gravity="center" />

Sample App

This app shows a white screen with the text "Hello, Android!" centered in the middle.

Android Kotlin

package com.example.firstapp

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity

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

/* activity_main.xml layout file content:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello, Android!"
        android:textSize="24sp"
        android:layout_gravity="center" />

</LinearLayout>
*/

Important Notes

Always call super.onCreate(savedInstanceState) first in onCreate.

Layouts are defined in XML files inside the res/layout folder.

Use TextView to show text on the screen.

Summary

Your first Android app has a main activity that loads a layout.

The layout file defines what the user sees on the screen.

This simple app shows a greeting message centered on a white background.

Practice

1. What is the role of the MainActivity class in a simple Android app?

easy

A. It controls what the user sees by loading the layout and handling interactions.

B. It stores user data permanently on the device.

C. It manages network connections for the app.

D. It compiles the app code into machine language.

Solution

1. Step 1: Understand MainActivity's purpose

   MainActivity is the entry point of the app that sets the screen content.

2. Step 2: Identify its function in UI

   It loads the layout file which defines what the user sees and handles user actions.

3. Final Answer:

   It controls what the user sees by loading the layout and handling interactions. -> Option A

4. Quick Check:

   MainActivity controls UI = A [OK]

Hint: MainActivity sets the screen content and user interaction [OK]

Common Mistakes:

• Confusing MainActivity with data storage classes
• Thinking MainActivity handles network tasks
• Believing MainActivity compiles code

2. Which of the following is the correct way to set the content view in MainActivity using Kotlin?

easy

A. setContentView(R.id.activity_main)

B. setContentView(R.layout.activity_main)

C. setContentView(activity_main.xml)

D. setContentView("activity_main")

Solution

1. Step 1: Recall setContentView syntax

   In Kotlin, setContentView expects a layout resource ID, accessed as R.layout.filename.

2. Step 2: Match correct resource reference

   R.layout.activity_main correctly points to the layout XML file named activity_main.xml.

3. Final Answer:

   setContentView(R.layout.activity_main) -> Option B

4. Quick Check:

   Layout resource ID = R.layout.filename [OK]

Hint: Use R.layout.filename to reference layout files [OK]

Common Mistakes:

• Using string names instead of resource IDs
• Confusing layout and id resources
• Including file extensions in code

3. Given this Kotlin code in MainActivity:

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

What will the user see when the app runs?

medium

A. A blank white screen with no text

B. An error message because onCreate is missing a return type

C. Nothing, the app will crash immediately

D. The layout defined in activity_main.xml displayed on screen

Solution

1. Step 1: Understand onCreate behavior

   onCreate is called when the app starts and sets the UI with setContentView.

2. Step 2: Effect of setContentView

   It loads the layout file activity_main.xml, so the user sees that layout on screen.

3. Final Answer:

   The layout defined in activity_main.xml displayed on screen -> Option D

4. Quick Check:

   setContentView loads layout = C [OK]

Hint: onCreate sets UI with setContentView(layout) [OK]

Common Mistakes:

• Thinking onCreate needs a return type
• Assuming no UI appears without extra code
• Confusing layout with id resources

4. What is wrong with this MainActivity code snippet?

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

medium

A. setContentView should be called outside onCreate

B. savedInstanceState should be nullable (Bundle?)

C. super.onCreate(savedInstanceState) must be called before setContentView

D. onCreate must return a value

Solution

1. Step 1: Check onCreate call order

   super.onCreate must be called first to properly initialize the activity.

2. Step 2: Identify correct order

   Calling setContentView before super.onCreate causes errors or unexpected behavior.

3. Final Answer:

   super.onCreate(savedInstanceState) must be called before setContentView -> Option C

4. Quick Check:

   super.onCreate first = B [OK]

Hint: Always call super.onCreate first inside onCreate [OK]

Common Mistakes:

• Calling setContentView before super.onCreate
• Using non-nullable Bundle parameter
• Expecting onCreate to return a value

5. You want your first Android app to show a greeting message centered on a white background. Which layout XML snippet achieves this?

hard

A.

B.

C.

D.

Solution

1. Step 1: Check layout size and background

   uses match_parent for width and height and sets background to white (#FFFFFF).

2. Step 2: Verify text centering

   android:gravity=\"center\" centers the TextView inside the LinearLayout.

3. Final Answer:

   -> Option A

4. Quick Check:

   Center gravity + white background = A [OK]

Hint: Use LinearLayout with gravity=center and background=#FFFFFF [OK]

Common Mistakes:

• Using wrap_content for parent layout size
• Missing gravity=center for centering
• Not setting background color