Android Components

Main Components of Android Activities -> It provides a GUI(graphical user interface) for android application Activities are used by users to interact with the smartPhones. Services -> It is used to handle the background work with in an android application. Ex: WhatsApp New Message Arrive Notification Broadcast Receivers -> It helps you to communicate from Android Application to the Android hardware via Android OS Ex: Battery Low Warning Content Providers -> Its basically provides you the data from the other application Ex Contacts in WhatsApp

Activities

public class MainActivity extends Activity { 
}

or

public class MainActivity extends AppCompatActivity { 
}

Services

public class MyService extends Service {
}

Broadcast Receivers

A broadcast receiver is implemented as a subclass of BroadcastReceiverclass and each message is broadcaster as an Intent object.

public class MyReceiver  extends  BroadcastReceiver {
   public void onReceive(context,intent){}
}

Content Providers

A content provider is implemented as a subclass of ContentProvider class and must implement a standard set of APIs that enable other applications to perform transactions.

public class MyContentProvider extends  ContentProvider {
   public void onCreate(){}
}

Additional Components in Android

Fragments -> Part of an Activity like an Iframe in HTML Views -> UI elements that are drawn on-screen example text,button,image,etc. Layouts -> View hierarchies that control screen format and appearance of the views. Intents -> An Intent is a messaging object you can use to request an action from another app component.

Activity-lifecycle concepts

The Activity class defines the following call backs i.e. events. You don't need to implement all the callbacks methods. However, it's important that you understand each one and implement those that ensure your app behaves the way users expect. onCreate() -> This is the first method called when the activity is first created. onStart() -> It is method called when the activity becomes visible to the user. onResume() -> This method is called when the user starts interacting with the application. onPause() -> In this state activity does not receive user input and cannot execute any code.  onStop() -> This method is called when the activity is no longer visible. onDestroy() -> This method is called before the activity is destroyed by the system. onRestart() -> This method is called when the activity restarts after stopping it.  Example of Activity Lifecycle

package com.example.helloworld;

import android.os.Bundle;
import android.app.Activity;
import android.util.Log;

public class MainActivity extends Activity {
   String msg = "Android : ";
   
   @Override
   public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      Log.d(msg, "The onCreate() event");
   }

   @Override
   protected void onStart() {
      super.onStart();
 Log.d(msg, "The onStart() event"); 
   }

   @Override
   protected void onResume() {
      super.onResume();
 Log.d(msg, "The onResume() event"); 
//the activity has become visible
   }

   @Override
   protected void onPause() {
      super.onPause();
 Log.d(msg, "The onPause() event"); 
//activity is taking focus.
   }

   @Override
   protected void onStop() {
      super.onStop();
      Log.d(msg, "The onStop() event");
//the activity is no longer visible.
   }

   @Override
   public void onDestroy() {
      super.onDestroy();
       // the activity is destroyed.
      Log.d(msg, "The onDestroy() event");

   }
}

setContentView() Method is responsible for displaying the XML Layout in your android app, so always use it in onCreate() Method

setContentView(R.layout.activity_main); it will display Emulator window and you should see following log messages in LogCat window in Android studio −

The output somewhat will look like this in the text because here we are not dealing with the XML  Layout file

com.example.helloworld D/Android :: The onCreate() event

com.example.helloworld D/Android :: The onStart() event

com.example.helloworld D/Android :: The onResume() event