Android中实现推送通知需要使用第三方库或者服务，比如Firebase Cloud Messaging（FCM），JPush等。以下是使用FCM实现推送通知的代码示例：

1. 在项目根目录下的build.gradle文件中添加以下依赖：

dependencies {
    // ...
    implementation 'com.google.firebase:firebase-messaging:22.0.0'
}

2. 在AndroidManifest.xml文件中添加以下代码：

3. 在MyFirebaseMessagingService类中添加以下代码：

public class MyFirebaseMessagingService extends FirebaseMessagingService {

    private static final String TAG = "MyFirebaseMessagingService";

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        super.onMessageReceived(remoteMessage);
        
        // 获取通知数据
        String title = remoteMessage.getNotification().getTitle();
        String body = remoteMessage.getNotification().getBody();
        Map data = remoteMessage.getData();
        
        // 处理通知
        sendNotification(title, body, data);
    }

    private void sendNotification(String title, String body, Map data) {
        // 构建通知
        NotificationCompat.Builder builder = new NotificationCompat.Builder(this, "default");
        builder.setContentTitle(title)
                .setContentText(body)
                .setSmallIcon(R.mipmap.ic_launcher)
                .setDefaults(Notification.DEFAULT_ALL)
                .setPriority(NotificationCompat.PRIORITY_HIGH)
                .setVisibility(NotificationCompat.VISIBILITY_PUBLIC)
                .setContentIntent(getPendingIntent(data));

        // 发送通知
        NotificationManagerCompat notificationManager =
                NotificationManagerCompat.from(this);
        notificationManager.notify(0, builder.build());
    }

    private PendingIntent getPendingIntent(Map data) {
        Intent intent = new Intent(this, MainActivity.class);
        intent.setAction("OPEN_NOTIFICATION_ACTION");
        for (String key : data.keySet()) {
            intent.putExtra(key, data.get(key));
        }
        return PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_UPDATE_CURRENT);
    }
}

上述代码中，使用MyFirebaseMessagingService类继