Руководство Android для начинающих - Hello Android

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="org.o7planning.helloandroid" >

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

package org.o7planning.helloandroid;

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

public class MainActivity extends AppCompatActivity {

    public static final String TAG ="MyMessage";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // Print Log
        Log.i(TAG,"onCreate");
    }


    @Override
    protected void onStop() {
        super.onStop();
        // Print Log
        Log.i(TAG,"onStop");
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // Print Log
        Log.i(TAG,"onDestroy");
    }

    @Override
    protected void onPause() {
        super.onPause();
        // Print Log
        Log.i(TAG,"onPause");
    }

    @Override
    protected void onResume() {
        super.onResume();
        // Print Log
        Log.i(TAG,"onResume");      }

    @Override
    protected void onStart() {
        super.onStart();
        // Print Log
        Log.i(TAG,"onStart");
    }

    @Override
    protected void onRestart() {
        super.onRestart();
        // Print Log
        Log.i(TAG,"onRestart");
    }

}

package org.o7planning.helloandroid;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends AppCompatActivity {

   public static final String TAG ="MyMessage";

   @Override
   protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);

       // Read activity_main.xml to draw user interface.
       setContentView(R.layout.activity_main);
   }
   
   // .....
}  

package org.o7planning.helloandroid;

import androidx.appcompat.app.AppCompatActivity;

import android.graphics.Color;
import android.os.Bundle;
import android.widget.Button;
import android.widget.RelativeLayout;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Layout
        RelativeLayout layout = new RelativeLayout(this);
        layout.setBackgroundColor(Color.GREEN);

        // Tạo Button
        Button button = new Button(this);
        button.setText("My Button");

        // Add button to Layout
        layout.addView(button);

        // Content View
        setContentView(layout);
    }


}