What is activity in a mobile application?

Activity in Android is a separate screen of an application that interacts with the user. Each activity represents a distinct UI component and has its own lifecycle. An application can contain multiple activities, which may be linked to each other.

Each activity has a lifecycle managed by the Android system:

State	Description
`onCreate()`	The activity is created. Initialization is performed.
`onStart()`	The activity becomes visible to the user.
`onResume()`	The activity gains focus and is ready for interaction.
`onPause()`	The activity loses focus but is still visible. It may be partially covered.
`onStop()`	The activity is no longer visible to the user.
`onDestroy()`	The activity is destroyed.
`onRestart()`	An activity in the `onStop()` state becomes visible again.

An example structure of an Android project with activities:

// MainActivity.java
package com.example.myapp;

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.content.Intent;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

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

        Button nextButton = findViewById(R.id.next_button);
        nextButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // Transition to another activity
                Intent intent = new Intent(MainActivity.this, SecondaryActivity.class);
                startActivity(intent);
            }
        });
    }
}

// SecondaryActivity.java
package com.example.myapp;

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

public class SecondaryActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_secondary); // Link to layout file
    }
}

The application manifest (AndroidManifest.xml) defines all activities and entry points:

<!-- AndroidManifest.xml -->
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
          package="com.example.myapp">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.MyApp">

        <!-- Declaration of the main activity -->
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>
                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
        </activity>

        <!-- Declaration of the second activity -->
        <activity android:name=".SecondaryActivity"/>

    </application>
</manifest>

Thus, activities are the building blocks of the user interface in Android applications.