Writing Android code involves using Java or Kotlin programming languages, along with the Android SDK (Software Development Kit) and Android Studio, which is the official integrated development environment (IDE) for Android app development、Below is a simple example of an Android app that displays a Hello, World! message on the screen、This example assumes you are familiar with basic programming concepts and have Android Studio installed on your computer.

Step 1: Setting Up Your Project

1、Open Android Studio and start a new project.
2、Choose Empty Activity and click Next.
3、For Name, enter `HelloWorldApp`, and for Package name, enter `com.example.helloworldapp`、Choose a location for your project and click Finish.

Step 2: Understanding the MainActivity.java or MainActivity.kt File

Your main activity file (either `MainActivity.java` or `MainActivity.kt`, depending on your choice of language) will look something like this:

Java Version

java
package com.example.helloworldapp;

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

TextView textView = findViewById(R.id.textView);
textView.setText(Hello, World!);
}
}

Kotlin Version

kotlin
package com.example.helloworldapp

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

class MainActivity : AppCompatActivity() {

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

val textView: TextView = findViewById(R.id.textView)
textView.text = Hello, World!
}
}

Step 3: Understanding the Layout File

Your `activity_main.xml` file should look something like this:

xml
<?xml version=1.0 encoding=utf8?
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android=http://schemas.android.com/apk/res/android
xmlns:app=http://schemas.android.com/apk/resauto
xmlns:tools=http://schemas.android.com/tools
android:layout_width=match_parent
android:layout_height=match_parent
tools:context=.MainActivity

<TextView
android:id=@+id/textView
android:layout_width=wrap_content
android:layout_height=wrap_content
android:textSize=24sp
app:layout_constraintBottom_toBottomOf=parent
app:layout_constraintLeft_toLeftOf=parent
app:layout_constraintRight_toRightOf=parent
app:layout_constraintTop_toTopOf=parent /

</androidx.constraintlayout.widget.ConstraintLayout

Explanation:

MainActivity (Java or Kotlin) is the main entry point of your application、The `onCreate` method is called when the activity is started、You set the content view here, which refers to the UI layout file (`activity_main.xml`).
In `activity_main.xml`, a `TextView` is defined、This is a simple UI component that displays text.
The `TextView` in both Java and Kotlin code examples is being referenced and updated to display Hello, World!.

Running Your App:

1、Connect an Android device to your computer or use an emulator.
2、Make sure your device/emulator is recognized by Android Studio (you might need to enable USB debugging on your device).
3、Run the application by clicking on the green play button in Android Studio or pressing Shift+F10.

Creating a Simple Hello, World! App Programmatically:

If you want to create a simple app programmatically without using a layout file, you can modify your `MainActivity` like this:

Java:

java
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
TextView textView = new TextView(this);
textView.setText(Hello, World!);
setContentView(textView);
}
}

Kotlin:

kotlin
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.TextView

class MainActivity : AppCompatActivity() {

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val textView = TextView(this)
textView.text = Hello, World!
setContentView(textView)
}
}

This will create a simple app that displays Hello, World! on the screen without needing a layout file defined in XML.