Learn about Activity lifecycle. Play with the app watching logcat to see the logs. Comments from offcicial documentation (https://developer.android.com/reference/android/app/Activity).

MyLifecycleActivity.java

package com.udacity.myapplicationtest;

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

/**
 *  Official documentation:
 *  https://developer.android.com/reference/android/app/Activity
 */
public class MyLifecycleActivity extends AppCompatActivity {

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

        /* TODO: See Logcat output to see when onCreate() is called.
         * Called when the activity is first created. This is where you should do all of your normal static set up: create views, bind data to lists, etc.
         * This method also provides you with a Bundle containing the activity's previously frozen state, if there was one. Always followed by onStart().
         *
         * NEXT: onStart()
         *
         */
        Log.d("lifecycle",">\n>>>>>>>>>>>>>>>>>>>>>\n>>>>>>>>>>>>>>>>>>>>> onCreate invoked\n>>>>>>>>>>>>>>>>>>>>>");

    }

    /**
     * Called when the activity is becoming visible to the user.
     * Followed by onResume() if the activity comes to the foreground, or onStop() if it becomes hidden.
     *
     * NEXT: onResume() or onStop()
     *
     */
    @Override
    protected void onStart() {
        super.onStart();
        Log.d("lifecycle",">\n>>>>>>>>>>>>>>>>>>>>>\n>>>>>>>>>>>>>>>>>>>>> onStart invoked\n>>>>>>>>>>>>>>>>>>>>>");
    }


    /** TODO: See Logcat output to see when onResume() is called.
     * Called when the activity will start interacting with the user. At this point your activity is at the top of the activity stack,
     * with user input going to it. Always followed by onPause().
     *
     * NEXT: onPause()
     *
     */
    @Override
    protected void onResume() {
        super.onResume();
        Log.d("lifecycle",">\n>>>>>>>>>>>>>>>>>>>>>\n>>>>>>>>>>>>>>>>>>>>> onResume invoked\n>>>>>>>>>>>>>>>>>>>>>");
    }


    /** TODO: See Logcat output to see when onStop() is called.
     * Called when the activity is no longer visible to the user, because another activity has been resumed and is covering this one.
     * This may happen either because a new activity is being started, an existing one is being brought in front of this one, or this one is being destroyed.
     * Followed by either onRestart() if this activity is coming back to interact with the user, or onDestroy() if this activity is going away.
     *
     * NEXT: onRestart() or onDestroy()
     *
     */
    @Override
    protected void onStop() {
        super.onStop();
        Log.d("lifecycle",">\n>>>>>>>>>>>>>>>>>>>>>\n>>>>>>>>>>>>>>>>>>>>> onStop invoked\n>>>>>>>>>>>>>>>>>>>>>");
    }



    /** TODO: See Logcat output to see when onRestart() is called.
     * Called after your activity has been stopped, prior to it being started again. Always followed by onStart()
     *
     * NEXT: onStart()
     *
     */
    @Override
    protected void onRestart() {
        super.onRestart();
        Log.d("lifecycle",">\n>>>>>>>>>>>>>>>>>>>>>\n>>>>>>>>>>>>>>>>>>>>> onRestart invoked\n>>>>>>>>>>>>>>>>>>>>>");
    }



    /** TODO: See Logcat output to see when onPause() is called.
     * Called when the system is about to start resuming a previous activity.
     * This is typically used to commit unsaved changes to persistent data, stop animations and other things that may be consuming CPU, etc.
     * Implementations of this method must be very quick because the next activity will not be resumed until this method returns.
     * Followed by either onResume() if the activity returns back to the front, or onStop() if it becomes invisible to the user.
     *
     * NEXT: onResume() or onStop()
     *
     */
    @Override
    protected void onPause() {
        super.onPause();
        Log.d("lifecycle",">\n>>>>>>>>>>>>>>>>>>>>>\n>>>>>>>>>>>>>>>>>>>>> onPause invoked\n>>>>>>>>>>>>>>>>>>>>>");
    }



    /** TODO: See Logcat output to see when onDestroy() is called.
     * The final call you receive before your activity is destroyed. This can happen either because the activity is finishing (someone called finish() on it,
     * or because the system is temporarily destroying this instance of the activity to save space. You can distinguish between these two scenarios with the isFinishing() method.
     *
     * NEXT: NOTHING
     *
     */
    @Override
    protected void onDestroy() {
        super.onDestroy();
        Log.d("lifecycle",">\n>>>>>>>>>>>>>>>>>>>>>\n>>>>>>>>>>>>>>>>>>>>> onDestroy invoked\n>>>>>>>>>>>>>>>>>>>>>");
    }


}