Message

public class MainActivity extends AppCompatActivity {

    TextView textView;

    //UI thread(MainActivity)Create a Handler instance with
    //Now both Message Queue and Looper are UI threads.
    //Since the instance handler can be referenced from another thread
    //If you refer to the handler in another thread and post a message
    //UI thread (MainActivity)And Looper will handle it for you!
    //It's cool! !!
    //Then, please come to handler!
    Handler handler = new MyHandler();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView = findViewById(R.id.textView1);

        MyLovelyMessage();
    }

    private void MyLovelyMessage() {
        //UI thread (MainActivity)Access the handler generated in
        //Put Message in handler and sendMessage!
        //The UI thread Looper should now do just fine!
        Message message = handler.obtainMessage(1);
        handler.sendMessage(message);
    }

    class MyHandler extends Handler{
        @Override
        public void handleMessage(Message message){
            //Defines what handle does in the UI thread
            //It will be executed automatically by Looper!
            String msg = String.valueOf(message.what);
            textView.setText(msg);
        }
    }
}

Simplify the above code as an internal function

Message

public class MainActivity extends AppCompatActivity {

    TextView textView;

    Handler handler = new Handler(){
      @Override
      public void handleMessage(Message message){
          String msg = String.valueOf(message.what);
          textView.setText(msg);
      }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView = findViewById(R.id.textView1);

        MyLovelyRunnable();
    }

    private void MyLovelyRunnable() {
        Message message = handler.obtainMessage(1);
        handler.sendMessage(message);
    }
}

Example using Handler and Runnable

public class MainActivity extends AppCompatActivity {

    TextView textView;

    //UI thread Handler(MainAvtivity.class)Generate with
    //Now both MessageQueue and Looper are in this thread(UI thread)Associated with
    //
    //Since the Handler instance can be referenced from other threads as well
    //In another thread, using the instance handler
    //Runnable in this thread(UI thread)If you give it to
    //This thread(UI thread side)Will be able to run Runnable
    //It's nice!
    //Now let's create an instance of Handler! !!
    Handler handler = new Handler();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView = findViewById(R.id.textView1);

        MyLovelyHandler();

    }

    private void MyLovelyHandler(){
        new Thread(new Runnable() {
            @Override
            public void run() {
                //UI thread(MainActivity)The instance of Handler created in
                //It can also be referenced from other threads
                //Put Runnable on the instance handler
                //UI thread (MainActivity)Let's hand it over to and have it processed!
                handler.post(new Runnable() {
                    @Override
                    public void run() {
                        textView.setText("I Love Handler");
                    }
                });
            }
        }).start();
    }
}

Handler and Service: Example of dividing a file into multiple files

Min target: api28 or above.

Created file

• MainActivity.java
  Create Handler instance
  Run Foreground Service
• MyService.java
  Create a Message and sendMessage
• MyHandler sendMessage Process the sent message

public class MainActivity extends AppCompatActivity {
    public static TextView textView;

    //Create a Handler object and make MessageQueue and Looper a UI thread!
    //This will store the message sent to the Handle object in the Message Queue.
    //Looper will take it out and process it! (The content of the process is MyHandler.Define it in class! )
    static MyHandler myHandler = new MyHandler();

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

        textView = findViewById(R.id.textView);

        //Create an Intent object and call Service!
        Intent intent = new Intent(this,MyService.class);
        startForegroundService(intent);

    }
}

public class MyService extends Service {
    public MyService() {
    }

    @Override
    public int onStartCommand(Intent intent,int flags,int startid){
        //SendMessage by putting a message on the Handle object of MainActivity!!
        //The destination is the UI thread Message Queue!
        //After that, Looper is MyHandler.Execute class and process it!!
        Message message = MainActivity.myHandler.obtainMessage(1);
        MainActivity.myHandler.sendMessage(message);
        return START_STICKY;
    }

    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }
}

//Message stored in Message Queue of UI thread
//The UI thread Looper has the following handleMessage()Will be executed!

public class MyHandler extends Handler {
    @Override
    public void handleMessage(Message message){
        String msg = String.valueOf(message.what);
        MainActivity.textView.setText(msg);
    }
}

public class MainActivity extends AppCompatActivity {
    public static TextView textView;
    static Handler myHandler = new Handler();
    boolean signal =false;

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

        Button btnStart= findViewById(R.id.btnStart);
        Button btnStop = findViewById(R.id.btnStop);
        textView = findViewById(R.id.textView);

        final Intent intent = new Intent(this,MyService.class);
        startForegroundService(intent);
        
        btnStart.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(signal == false){
                    MyService.runnable.run();
                    signal = true;
                }
            }
        });

        btnStop.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                myHandler.removeCallbacks(MyService.runnable);
                signal = false;
            }
        });
    }
}

public class MyService extends Service {
    static Runnable runnable;
    public MyService() {
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startid) {

        runnable = new Runnable() {
            int i=0;
            @Override
            public void run() {
                i++;
                String st = String.valueOf(i);
                MainActivity.textView.setText(st);
                MainActivity.myHandler.postDelayed(this,1000);
            }
        };
        
//        MainActivity.myHandler.postDelayed(runnable,1000);
        return START_STICKY;
    }

    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }
}