[2019 oracle code one] Lambda, Streams and Collectors Lab

Lambda, Streams and Collectors Lab

Speakers: Stuart Marks, Maurice Naftalin, Jose Paumard & Gustavo Durand

For more blog posts, see The Oracle Code One table of contents

The lab is self paced


I like that it is organized by topic so you can pick what you want to learn. Since “someone” believed I didn’t need to be here, I decided to blog about what I did and learned.

  • O_SimpleCollectors
    • I forgot Comparator.naturalOrder() exists because I hardly use it.
    • I forgot you can’t use Function.identity() with primitives and instead have to write a lambda: ex: x -> x.
  • P_HarderCollectors
    • I almost never use flatMap. I didn’t think to use it combined with splitAsStream to read words from a file. I do a lot of file processing though so this is definitely an idiom I need to remember! I shall type it in for each exercise in this lab that uses it (vs copy/paste) in order to ingrain it in my fingers! reader.lines().flatMap(SPLIT_PATTERN::splitAsStream) [edit: I think I’ve typed this enough times to remember it forever!]
    • The extra challenge to write a groupingBy using toMap. I knew I needed to use a merge function, but the types didn’t match my expectations. I learned that:
      • if you write a value instead of a lambda for the value function, you get a compiler error on the merge function (about the + operator being invalid ) and not the value function for not being a lambda
      • if you write a value function that returns an int instead of a long, you get a compiler error on the merge function and not the value function (because the merge function result is what gets set to the return value)
    • If you try to read from a reader that has already called reader.lines(), you don’t get any lines. Doh!
    • flatMap(String::chars) doesn’t work because chars() returns an IntStream. flatMapToInt(String::chars) does work
    • Entry has comparingByKey() and comparingByValue() methods
    • Remember to use groupingBy when aggregating and toMap when one to one
    • Need to call boxed() on an IntStream to be able to use partitioning by. An IntStream doesn’t have a collect method that takes a Collector as a parameter
    • Partitioning by can take a nested collector (ex: summing int)
  • L_HarderStreams
    • Convert type using mapToInt() before calling max
    • Forgot about Comparator.comparing() – I knew about this one earlier today! I think I’m getting tired :). I’m a morning person. Coding at 6:30pm is less than ideal for me.
    • Character.toString(Int) exists in Java 11. This means you can call mapToInt(String::chars).mapToObj(Character::toString)
    • The concept of using IntStream for an index and referring to a separate list (I would use a for loop for this since I need the index, but it’s a good tool to have in the toolbox)

My take

The lab is great. I like that it can be as easy or as hard as you want. I like they support multiple versions of Java and multiple IDEs. I completed the hard stream and collectors hard exercises

The room is terrible. It’s not really a room. It’s a grid of pipe/drape separated areas. I can hear every word in the room next to us. It got better. Once I got into the lab I was able to tune out the surroundings.

Also, my back hurts. Live blogging was fine. Different angle and I hardly look at the laptop. Coding for two hours (and I did take a break and stretch) was an awkward neck/back angle. How do people code on a laptop full time?

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