SecondHand C0D1NG

All emulators in AVD failed to run after I accidentally update Android Studio to 4.2.1 on macOS Catalina (10.15.7).

You can find more debug details if you run the emulator in terminal.

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emulator -list-avds
emulator -avd your_avd_name

Solution

Downgrade (sort of) amulator to 30.4.5 (build_id 7140946)
https://dl.google.com/android/repository/emulator-darwin_x64-7140946.zip

First, downlaod the zip file and unzip it.
Then, find the emulator folder under sdk folder. Replace the items inside with what you have downloaded.
Restart Android Studio (for example, go to menu File, select Invalidate Caches / Restart) and try to run the emulators again.

You will see the warnings saying something, like, these files can not be verified due to unknown developer. Remember to go to Mac System Preference, Security & Privacy, under the General tab, allow these apps to run. Eventually, everything will be fine. Good luck.

Ref:
https://issuetracker.google.com/issues/191799887
https://issuetracker.google.com/issues/191805460
https://medium.com/nerd-for-tech/how-to-downgrade-android-emulator-on-macos-6e611d2d2bcb

When this bug happens, it shows that it takes time to open the MySQL Editor, but the fact is that it will never open. So you will wait forever.

I am on MacOS Catalina and the MySQL Workbench version is 8.0.22, just in case you want to know.

Why this error happens (My guess):

You changed the password to the MySQL database user, but it somehow failed to sync the password in the keychain.

The solution:

Go to the /Applicaiton/Utilities/ to find the Keychain Access App and run it. Search and delete the password to the database user. Try search ‘MySQL’. Reopen the MySQL Workbench and you should be fine now.

Update: The typo has been fixed if you are using Dynamic web module version 5.0

When you are using the Servlet Template in Eclipse, you may encounter the error saying “The import javax.servlet cannot be resolved.”

For example, I am using Eclipse for Mac (Version 4.19.0). First, create a Dynamic Web Project, target runtime: Apache Tomcat v10.0, Dynamic web module version 4.0. And inside this project, create a new servlet file using the Create Servlet Wizard. Soon you will see the errors pop up in the servlet file you just created.

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// BEFORE
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

How to fix it? All you have to do is changing the superclass name from javax into jakarta. Why does this happen? I think it is because, as mentioned on Wikipedia page, that from 12 Jun 2020 and on, API moved from package javax.servlet to jakarta.servlet. Let’s hope Eclipse will update the template someday in future. For the exising codes out there, I guess they are doing well.

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// AFTER
import java.io.IOException;
import jakarta.servlet.ServletException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

Of course, you can create an empty class file to build servlet from scratch, to avoid such issues. Just in case you do face the similar errors in Eclipse when coding Java, I hope the simple fix would help you.

Wikipedia page of Jakarta Servlet:
https://en.wikipedia.org/wiki/Jakarta_Servlet

Finally, I got hexo upgraded.

Simple & powerful as it is advertised.

It worked fine, I guess.

I have been thinking about to code myself a blog from scratch because the hexo blog seems out of date. The interactions between Github and Hexo are sometimes failing and I got warnings of it might not be working in some future. Maybe it’s good time to practice some front-end coding with html/css and javascript.

The coding experience on Leetcode is fantastic because you can always get instant feedback that your code is not working. Then you try again and get the feedback again that your codes fail. Back and forth serveral times until you get it.

To be honest, I like Project Euler more, because of its simplicity and the sense of art comes from the simplicity. Still, I think you need the Leetcode to practice to get a job. In this sense, it’s a wonderful website that gives you hope of the employment.

The book of Code Complete has been sitting on my book shelf for a long time. I always feel that it was a big book and the content is too much for my level. But these feelings were purely generated from the imagination because I never ever actually tried to read the book.

Until now. I began the reading. And it turns out the content makes sense to me in a lot of ways. I think it is considered as a classic book for a good reason.

It might take some time before I can finish the book, but I can say it now to anyone hesitate to read it, today is the day, just read it.

I saw something in the shell code as follows.

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cat /dev/random > /dev/null 2>&1

Or something similar.

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export lang="en_us.utf-8" >/dev/null 2>&1

They all have this part.

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>/dev/null 2>&1

I did some research to find out what it means. Basically, the number 1 is standard output, the number 2 is the standard error, the char > is to redirect the flow of information, and 2>&1 means to treat the 2 the same way as the 1, and /dev/null, you know, is in the middle of nowhere.

So, to put them all together, this piece of code means that throw the standard output and standard error away as garbage. You will never see anything stdout or stderr on the screen or anywhere on disk. No, just say goodbye to them. They are gone for good.

CRUD operations in MongoDB, i.e. Create, Read, Update, Delete.

The code is for Pymongo.

Preparation

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import datetime
from pymongo import MongoClient

client = MongoClient()
db = client.test_database
collection = db.test_collection

post = {'author': 'Mike',
        'text': 'My first blog post!',
        'tags': ['mongodb', 'python', 'pymongo'],
        'date': datetime.datetime.utcnow()}

CREATE:

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def create_lines():
    collection.insert_one(post)
    # if we have multiple posts (an array of dictionaries)
    collection.insert_many(posts)

READ:

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def read_lines():
    collection.find_one({'author': 'Mike'})
    # if we want to find multiple records at the same time
    collection.find_many({'author': 'Mike'})

UPDATE:

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def update_lines():
    collection.update_one({'author': 'Mike'},
                          {'$set': {'text': 'My second blog post!'}})
    # if we want to update all the relevant records at the same time
    collection.update_many({'author': 'Mike'},
                          {'$set': {'text': 'My second blog post!'}})

DELETE:

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def delete_lines():
    collection.delete_one({'author': 'Mike'})
    # if we want to delete the matched records all at once
    collection.delete_many({'author': 'Mike'})

I would say MongoDB is designed in a way that is very user-friendly.

Check out the pymongo website for more detailed instructions.
https://pymongo.readthedocs.io/en/stable/tutorial.html

Make the database in a simpler way and make it fast instead of designing complicated architecture of tables. MongoDB can do this with its document model to store things more naturally which can be later fit into your applications. Remember, it’s fast. I think it can make your life easier.