AlarmManager

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.application.alarmmanagerproject.MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@id/button_timepicker"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="30dp"
        android:text="No Alarm set"
        android:textSize="30sp" />

    <Button
        android:id="@+id/button_timepicker"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:text="open time picker" />

    <Button
        android:id="@+id/button_cancel"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/button_timepicker"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="27dp"
        android:text="cancel alarm" />

</RelativeLayout>

package com.example.application.alarmmanagerproject;

import android.app.AlarmManager;
import android.app.PendingIntent;
import android.app.TimePickerDialog;
import android.content.Context;
import android.content.Intent;
import android.support.v4.app.DialogFragment;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.TimePicker;

import java.text.DateFormat;
import java.util.Calendar;

public class MainActivity extends AppCompatActivity implements TimePickerDialog.OnTimeSetListener {
    private TextView mTextView;

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

        mTextView = findViewById(R.id.textView);

        Button buttonTimePicker = findViewById(R.id.button_timepicker);
        buttonTimePicker.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                DialogFragment timePicker = new TimePickerFragment();
                timePicker.show(getSupportFragmentManager(), "time picker");
            }
        });

        Button buttonCancelAlarm = findViewById(R.id.button_cancel);
        buttonCancelAlarm.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                cancelAlarm();
            }
        });
    }

    @Override
    public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
        Calendar c = Calendar.getInstance();
        c.set(Calendar.HOUR_OF_DAY, hourOfDay);
        c.set(Calendar.MINUTE, minute);
        c.set(Calendar.SECOND, 0);

        updateTimeText(c);
        startAlarm(c);
    }

    private void updateTimeText(Calendar c) {
        String timeText = "Alarm set for: ";
        timeText += DateFormat.getTimeInstance(DateFormat.SHORT).format(c.getTime());

        mTextView.setText(timeText);
    }

    private void startAlarm(Calendar c) {
        AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
        Intent intent = new Intent(this, AlertReceiver.class);
        PendingIntent pendingIntent = PendingIntent.getBroadcast(this, 1, intent, 0);

        if (c.before(Calendar.getInstance())) {
            c.add(Calendar.DATE, 1);
        }

        alarmManager.setExact(AlarmManager.RTC_WAKEUP, c.getTimeInMillis(), pendingIntent);
    }

    private void cancelAlarm() {
        AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
        Intent intent = new Intent(this, AlertReceiver.class);
        PendingIntent pendingIntent = PendingIntent.getBroadcast(this, 1, intent, 0);

        alarmManager.cancel(pendingIntent);
        mTextView.setText("Alarm canceled");
    }
}

package com.example.application.alarmmanagerproject;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.support.v4.app.NotificationCompat;


public class AlertReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        NotificationHelper notificationHelper = new NotificationHelper(context);
        NotificationCompat.Builder nb = notificationHelper.getChannelNotification();
        notificationHelper.getManager().notify(1, nb.build());
    }
}

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

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

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

        <receiver android:name=".AlertReceiver" />

    </application>

</manifest>

package com.example.application.alarmmanagerproject;

import android.annotation.TargetApi;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.content.Context;
import android.content.ContextWrapper;
import android.os.Build;
import android.support.v4.app.NotificationCompat;


public class NotificationHelper extends ContextWrapper {
    public static final String channelID = "channelID";
    public static final String channelName = "Channel Name";

    private NotificationManager mManager;

    public NotificationHelper(Context base) {
        super(base);
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            createChannel();
        }
    }

    @TargetApi(Build.VERSION_CODES.O)
    private void createChannel() {
        NotificationChannel channel = new NotificationChannel(channelID, channelName, NotificationManager.IMPORTANCE_HIGH);

        getManager().createNotificationChannel(channel);
    }

    public NotificationManager getManager() {
        if (mManager == null) {
            mManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        }

        return mManager;
    }

    public NotificationCompat.Builder getChannelNotification() {
        return new NotificationCompat.Builder(getApplicationContext(), channelID)
                .setContentTitle("Alarm!")
                .setContentText("Your AlarmManager is working.")
                .setSmallIcon(R.drawable.ic_android);
    }
}

package com.example.application.alarmmanagerproject;

import android.app.Dialog;
import android.app.TimePickerDialog;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v4.app.DialogFragment;
import android.text.format.DateFormat;

import java.util.Calendar;


public class TimePickerFragment extends DialogFragment {

    @NonNull
    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        Calendar c = Calendar.getInstance();
        int hour = c.get(Calendar.HOUR_OF_DAY);
        int minute = c.get(Calendar.MINUTE);

        return new TimePickerDialog(getActivity(), (TimePickerDialog.OnTimeSetListener) getActivity(), hour, minute, DateFormat.is24HourFormat(getActivity()));
    }
}