How to get Scala List from Java List?

java

conversion-strategies

scala-collections

functional-programming

byAnton Shumikhin·Feb 18, 2025

Potently convert a Java List to a Scala List by deploying .asScala and sealing it with .toList from the JavaConverters machinery:

import scala.collection.JavaConverters._

// Place your Java faith into the Scala altar of conversions
val scalaList: scala.collection.immutable.List[Int] = javaList.asScala.toList

Post importing the Scala Conversion tools, awaken .asScala.toList directly on your Java List to transpose it to the Scala List enlightenment. Remember, for Scala versions posterior and anterior, consult the respective documentations, wink wink.

Mechanics of Conversions

Transmuting Java collections to Scala collections imbibes different conversion strategies across Scala versions. For instance, JavaConverters is the blessed instrument in Scala 2.12 and earlier, while scala.jdk.CollectionConverters._ succeeds it starting Scala 2.13.

The master method .asScala transfigures Java into a mutable Scala Buffer like a homunculus, which can wax and wane at will. Add .toList to your commandment and, voilà, an Immutable Scala List rises like a phoenix.

Using these conversion tools is like putting on Scala-tinted glasses, allowing you to see the Java collection in a new light. You're looking at the same object but now you can utilize Scala methods like foreach, map, and so forth - stylish and functional!

Beware of the Ides of JavaConversions

The ancient way, the JavaConversions._, walks the path of implicit conversions, a slippery slope riddled with ambiguities and unexpected outcomes when converging with Java collections. It's akin to a mysterious Pandora's box - can be useful, but handle with care!

Also, if your forces involve an armada of complex types or case classes, make sure to align the apply() / unapply() methods and Java's getters/setters on the same battle line, else face the calamities of instantiation and data manipulation.

Profiting from Scala Collections

One of the crowning glories of embracing Scala collections is the treasure trove of operations now at your disposal - .map, .filter, .flatMap and more! Think of them as your personal army of minions doing your collection's bidding.

Command these minions with Scala's pattern matching can provide the in-depth control of logic, further enhancing the expressiveness of your code. And let's not forget the sacred functional programming practices that Scala collections zealously abide by.

Gathering Experience from the Field

Complex Types and Custom Objects

When dealing with complex types or custom objects, ensure you construct and employ implicit conversions or custom converter methods to navigate the labyrinth of such conundrums.

Optimizing Performance

Converting collections has a price: performance overhead. It's like paying for a ticket to a Scala opera, but if you're on a budget (or dealing with large scales or highly iterative scenarios), alternative strategies might be music to your ears.

Thread Safety

Java collections are a wild west when it comes to concurrency, unlike Scala's immutable collections, which operate under the meticulous rule of law, offering indispensable safety guarantees. A town worth a visit during your Scala journey.