Android 7（API 25）及以前的版本

public void notify(View view) {
        //点击通知时，所使用的Intent
        Intent intent=new Intent(this,MainActivity.class);
        PendingIntent pi=PendingIntent.getActivity(this,0,intent,0);
        //获取通知管理器对象
        NotificationManager manager=(NotificationManager)getSystemService(NOTIFICATION_SERVICE);

        Notification notification=new Notification.Builder(this)
                .setContentTitle("这是一条通知")
                .setContentText("这是通知的正文")
                .setSmallIcon(android.R.drawable.ic_dialog_alert)
                .setContentIntent(pi)
                .build();
        //发送通知
        manager.notify(1,notification);
    }

Android 8+的版本

/**
     * 
      * 创建两个频道
     */    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_notification);
        
        
        //当版本大于Oreo(Android 8.0)
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            //创建频道
            String channelId = "chat";
            String channelName = "聊天消息";
            int importance = NotificationManager.IMPORTANCE_HIGH;
            createNotificationChannel(channelId, channelName, importance);
            //创建频道
            channelId = "subscribe";
            channelName = "订阅消息";
            importance = NotificationManager.IMPORTANCE_DEFAULT;
            createNotificationChannel(channelId, channelName, importance);
        }
    }

    @TargetApi(Build.VERSION_CODES.O)
    private void createNotificationChannel(String channelId, String channelName, int importance) {
        NotificationChannel channel = new NotificationChannel(channelId, channelName, importance);
        NotificationManager notificationManager = (NotificationManager) getSystemService(
                NOTIFICATION_SERVICE);
        notificationManager.createNotificationChannel(channel);
    }

    /**
     * 
     * 这是一条高等级频道上的通知
     */
    public void sendChatMsg(View view) {
        NotificationManager manager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        Notification notification = new NotificationCompat.Builder(this, "chat")
                .setContentTitle("收到一条聊天消息")
                .setContentText("今天中午吃什么？")
                .setWhen(System.currentTimeMillis())
                .setSmallIcon(android.R.drawable.ic_input_add)
                .setAutoCancel(true)
                .build();
        manager.notify(1, notification);
    }

    /**
     *
     * 这是一条普通级频道上的通知
     */

    public void sendSubscribeMsg(View view) {
        NotificationManager manager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        Notification notification = new NotificationCompat.Builder(this, "subscribe")
                .setContentTitle("收到一条订阅消息")
                .setContentText("地铁沿线30万商铺抢购中！")
                .setWhen(System.currentTimeMillis())
                .setSmallIcon(android.R.drawable.ic_input_add)
                .setAutoCancel(true)
                .build();
        manager.notify(2, notification);
    }