16 September 2021

Nifty Bits of Java

Mostly discovered via Intellij's very helpful suggestions.

I'm building me first real(-ish) java project. I did some C# development about a decade ago. Since then I've mostly used (and enjoyed) Go. Now that I'm diving into Clojure I think having some direct experience with Java and the JVM is going to help out. For the most part, it feels a lot like C#, but there are some noticeably nice, fancy features. Here's one I quite like:


Given, say, a HashMap<String, ArrayList<String>>, and you want to add a value to the ArrayList that corresponds to a key that already exists, or maybe the value in question is the first for that key, so we have to be careful. My first draft came out looking like this:

var m = new HashMap<String, ArrayList<String>>();
var listing = m.get("key");
if (listing == null) {
	listing = new ArrayList<String>();
	m.put("key", listing);

(Don't ask me how many times I've written that kind of code in other languages like C# and Go...)

Well, my nifty editor, Intellij, made the following nifty suggestion:

var m = new HashMap<String, ArrayList<String>>();
m.computeIfAbsent("key", k -> new ArrayList<>());

(Hopefully now that generics are [soon?] coming to Go, similar behavior will appear for that community...)

UPDATE: 2022-10-24

As alluded to above, here's a little something I whipped up the other day in Go (now that generics are a thing):

func GetOrSetDefault[K comparable, V any](m map[K]V, key K, new_ func() V) V {
	value, found := m[key]
	if !found {
		value = new_()
		m[key] = value
	return value


a := make(map[int]string)
GetOrSetDefault(a, 42, func() string { return "hi" })
fmt.Println(a[42]) // hi