My First Android app activity_main.xml file

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="@dimen/padding"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.aupadhyay.myfirstapp.MainActivity">


    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Enter Details"
        android:layout_marginTop="10dp"
        android:id="@+id/textView"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true" />

    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/editTextName"
        android:hint="Enter Name"
        android:layout_below="@+id/textView"
        android:layout_marginTop="42dp"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true" />

    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/editTextPhone"
        android:hint = "Enter Phone Number"
        android:layout_below="@+id/editTextName"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginTop="46dp" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Submit"
        android:id="@+id/button"
        android:layout_below="@+id/editTextPhone"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="46dp" />
</RelativeLayout>

My First Android app MainActivity.java file

package com.aupadhyay.myfirstapp;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Toast;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity {

    EditText editTextName, editTextPhone;
    Button btnSubmit;

    void initViews()
    {
        editTextName = (EditText)findViewById(R.id.editTextName);
        editTextPhone = (EditText)findViewById(R.id.editTextPhone);

        btnSubmit = (Button)findViewById(R.id.button);

        btnSubmit.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) { // v = btnSubmit;
                String strName = editTextName.getText().toString().trim();
                String strPhone = editTextPhone.getText().toString().trim();

                Toast.makeText(MainActivity.this, "You Entered : "+strName+" : "+strPhone, Toast.LENGTH_LONG).show();

            }
        });
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initViews();
        Toast.makeText(this, "MainActivity - onCreate",Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onStart() {
        super.onStart();
        Toast.makeText(this, "MainActivity - onStart",Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onResume() {
        super.onResume();
        Toast.makeText(this, "MainActivity - onResume",Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onPause() {
        super.onPause();
        Toast.makeText(this, "MainActivity - onPause",Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onStop() {
        super.onStop();
        Toast.makeText(this, "MainActivity - onStop",Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        Toast.makeText(this, "MainActivity - onDestroy",Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onRestart() {
        super.onRestart();
        Toast.makeText(this, "MainActivity - onRestart",Toast.LENGTH_SHORT).show();
    }
}

Example to understand Life Cycle of an Activity in different stages

Below I have written the MainActivity.java file :

package com.aupadhyay.myfirstapp;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toast.makeText(this, "MainActivity - onCreate",Toast.LENGTH_LONG).show();
    }

    @Override
    protected void onStart() {
        super.onStart();
        Toast.makeText(this, "MainActivity - onStart",Toast.LENGTH_LONG).show();
    }

    @Override
    protected void onResume() {
        super.onResume();
        Toast.makeText(this, "MainActivity - onResume",Toast.LENGTH_LONG).show();
    }

    @Override
    protected void onPause() {
        super.onPause();
        Toast.makeText(this, "MainActivity - onPause",Toast.LENGTH_LONG).show();
    }

    @Override
    protected void onStop() {
        super.onStop();
        Toast.makeText(this, "MainActivity - onStop",Toast.LENGTH_LONG).show();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        Toast.makeText(this, "MainActivity - onDestroy",Toast.LENGTH_LONG).show();
    }

    @Override
    protected void onRestart() {
        super.onRestart();
        Toast.makeText(this, "MainActivity - onRestart",Toast.LENGTH_LONG).show();
    }
}

You need to register this MainActivity.java file in your manifest file (AndroidManifest.xml) with the MAIN action and LAUNCHER category.

You need to see the sequence of the output and then you can get to know about the way of order of execution of the different API's on creation and destruction of the activity.