App.javaAndroidManifest.xmlactivity_main.xmlMainActivity.java
package com.codinginflow.notificationsexample;

import android.app.Application;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.os.Build;


public class App extends Application {
    public static final String CHANNEL_1_ID = "channel1";
    public static final String CHANNEL_2_ID = "channel2";

    @Override
    public void onCreate() {
        super.onCreate();

        createNotificationChannels();
    }

    private void createNotificationChannels() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            NotificationChannel channel1 = new NotificationChannel(
                    CHANNEL_1_ID,
                    "Channel 1",
                    NotificationManager.IMPORTANCE_HIGH
            );
            channel1.setDescription("This is Channel 1");

            NotificationChannel channel2 = new NotificationChannel(
                    CHANNEL_2_ID,
                    "Channel 2",
                    NotificationManager.IMPORTANCE_LOW
            );
            channel2.setDescription("This is Channel 2");

            NotificationManager manager = getSystemService(NotificationManager.class);
            manager.createNotificationChannel(channel1);
            manager.createNotificationChannel(channel2);
        }
    }
}

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

    <application
        android:name=".App"
        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>
    </application>

</manifest>

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:orientation="vertical"
    android:padding="16dp"
    tools:context="com.codinginflow.notificationsexample.MainActivity">

    <EditText
        android:id="@+id/edit_text_title"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Title" />

    <EditText
        android:id="@+id/edit_text_message"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Message" />

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:onClick="sendOnChannel1"
        android:text="Send on Channel 1" />

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:onClick="sendOnChannel2"
        android:text="Send on Channel 2" />

</LinearLayout>

package com.codinginflow.notificationsexample;

import android.app.Notification;
import android.support.v4.app.NotificationCompat;
import android.support.v4.app.NotificationManagerCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

import static com.codinginflow.notificationsexample.App.CHANNEL_1_ID;
import static com.codinginflow.notificationsexample.App.CHANNEL_2_ID;

public class MainActivity extends AppCompatActivity {
    private NotificationManagerCompat notificationManager;
    private EditText editTextTitle;
    private EditText editTextMessage;

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

        notificationManager = NotificationManagerCompat.from(this);

        editTextTitle = findViewById(R.id.edit_text_title);
        editTextMessage = findViewById(R.id.edit_text_message);
    }

    public void sendOnChannel1(View v) {
        String title = editTextTitle.getText().toString();
        String message = editTextMessage.getText().toString();

        Notification notification = new NotificationCompat.Builder(this, CHANNEL_1_ID)
                .setSmallIcon(R.drawable.ic_one)
                .setContentTitle(title)
                .setContentText(message)
                .setPriority(NotificationCompat.PRIORITY_HIGH)
                .setCategory(NotificationCompat.CATEGORY_MESSAGE)
                .build();

        notificationManager.notify(1, notification);
    }

    public void sendOnChannel2(View v) {
        String title = editTextTitle.getText().toString();
        String message = editTextMessage.getText().toString();

        Notification notification = new NotificationCompat.Builder(this, CHANNEL_2_ID)
                .setSmallIcon(R.drawable.ic_two)
                .setContentTitle(title)
                .setContentText(message)
                .setPriority(NotificationCompat.PRIORITY_LOW)
                .build();

        notificationManager.notify(2, notification);
    }
}