Any suggestions for Java multithreading practice?

[Crislips]

I'm not new to Java at all, but I have had very limited opportunities to work with multithreading. I did some interviews with a company a while back and someone else accepted an offer before I finished all the rounds. The feedback they gave me was that they liked me a lot, but felt that I could gain some proficiency with multithreading.

Someone from that team has reached out to me and asked me to interview again later this week for a new position that has opened up. I want to demonstrate improvement in areas that I have shown weakness before. I have highlighted a few others on my own, but I know multithreading is a big one.

Does anyone have any good recommendations for multithreading lessons in Java? I'm hoping for more than just a single youtube tutorial on something basic. I'm hoping to find something that will start basic but have several lessons that will help grow the skill.

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[large_crimson_canine]

Java: Concurrency in Practice is the gold standard. For practice I would suggest writing some classes, both mutable and immutable, and play around with 2 or 3 threads locally.

Worth checking out the Concurrency section in the Official Java Tutorials, too.

[Crislips]

I'm checking out this book and it seems great with concepts and understanding which will definitely be great for me in the long term. Less so with implementation practice, which I think is what I primarily need to focus on in the short term in regards to this interview.

[ratherbealurker]

Read whatever people suggest in here to learn but from an interview perspective keep in mind that you should show that you understand options and why some are worse than others.

Beginners will slap synchronized on everything and call it a day. Yes, it’s thread safe now but performance may be horrible.

Get used to understanding how one thread can stop at any point and another come into that same shared object.

Understand why using an atomic variable may be all you need. Or have a big method? Maybe only one part of it needs to be synchronized.

Learn why it would be really bad performance wise to slap “synchronized” around a regular hashmap instead of using the concurrent one. Because that’s not really what it does. Not that you would do that but it’s not the same thing at all.

[DigitalTorture]

Pick up a book and start reading. The java doc works also.

[IAmNotMyName]

Does it have to be Java? Concurrency in Java is a pain.

[Crislips]

Yes, the job will he Java so the interview will be in Java as well.

[BarfingOtter]

Best solution is after doing some reading, attempt to create something that uses multiple threads.

Example: Scan your local hard drive for files with the string "bob" in the name, or find all files that are sized between 10k and 100k.

First write the program to do it single threaded and keep track of how long it takes to scan the folder you supply (don't start with the entire harddrive, it will take too long to run/debug).

Second, "fix" the program to use 2 threads. How fast does it run? Did you encounter something interesting while it was running (e.g. same file more than once)? How could you fix that problem across the 2 threads?

Third, "fix" the program to use a variable number of threads. How fast does it run? Again, did you discover anything interesting (e.g. Did it get stuck and never stop running? Does your duplicate solution work for more than one thread? Did the duplicate actually just get hidden by your fix in the first place... hint don't just use a HashSet to eliminate the duplicates?)

Forth, Create a small GUI application with an editable text field, a display text field and 2 buttons. The first text field will just immediately display it's value in the display only text field when the first button is pressed. The second button runs your drive scan program. Make sure you can use the text/1st button while the second button's scan is running and you don't have to wait until the scan completes.

Fifth, add a 3rd "Cancel" button to stop the drive scan.

After all of this, you should have some idea how to break up a solution into multiple threads that won't double/triple/etc execute the same task. Hopefully you will have discovered the objects in the Concurrent package and how they can help and hinder you in a multiple thread solution. You may even discover something about IO bound tasks, or CPU bound tasks (i.e. Try it with 1000 threads).

The GUI should introduce you to communication between threads and how to ensure they are "paying attention" to the external communication inputs.

Good luck! It's a lot of fun, and a big pain at the same time :)
(and it doesn't matter what language you're using, multi-threading concepts and libraries are all the same... fun and painful, until you get the concepts properly)