Difference between a Seq and a List in Scala

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In Java terms, Scala’s Seq would be Java’s List, and Scala’s List would be Java’s LinkedList.

Note that Seq is a trait, which is similar to Java’s interface, but with the equivalent of up-and-coming defender methods. Scala’s List is an abstract class that is extended by Nil and ::, which are the concrete implementations of List.

So, where Java’s List is an interface, Scala’s List is an implementation.

Beyond that, Scala’s List is immutable, which is not the case of LinkedList. In fact, Java has no equivalent to immutable collections (the read only thing only guarantees the new object cannot be changed, but you still can change the old one, and, therefore, the “read only” one).

Scala’s List is highly optimized by compiler and libraries, and it’s a fundamental data type in functional programming. However, it has limitations and it’s inadequate for parallel programming. These days, Vector is a better choice than List, but habit is hard to break.

Seq is a good generalization for sequences, so if you program to interfaces, you should use that. Note that there are actually three of them: collection.Seq, collection.mutable.Seq and collection.immutable.Seq, and it is the latter one that is the “default” imported into scope.

There’s also GenSeq and ParSeq. The latter methods run in parallel where possible, while the former is parent to both Seq and ParSeq, being a suitable generalization for when there is no concern for code parallelism. They are both relatively new, so people don’t use them as much.

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