Difference between a Seq and a List in Scala

Question

I ve seen in many examples that sometimes a Seq is being used  while other times is the List     Is there any difference  other than the former one being a Scala type and the List coming from Java

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In Scala  a List inherits from Seq  but implements Product  here is the proper definition of List     sealed abstract class List  A  extends AbstractSeq A  with Product with        Note  the actual definition is a tad bit more complex  in order to fit in with and make use of Scala s very powerful collection framework

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A Seq is an Iterable that has a defined order of elements  Sequences provide a method apply   for indexing  ranging from 0 up to the length of the sequence  Seq has many subclasses including Queue  Range  List  Stack  and LinkedList   A List is a Seq that is implemented as an immutable linked list  It s best used in cases with last-in first-out  LIFO  access patterns   Here is the complete collection class hierarchy from the Scala FAQ

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Seq is a trait that List implements   If you define your container as Seq  you can use any container that implements Seq trait    scala gt  def sumUp s  Seq Int    Int     s sum   sumUp   s  Seq Int  Int  scala gt  sumUp List 1 2 3   res41  Int   6  scala gt  sumUp Vector 1 2 3   res42  Int   6  scala gt  sumUp Seq 1 2 3   res44  Int   6   Note that   scala gt  val a   Seq 1 2 3  a  Seq Int    List 1  2  3    Is just a short hand for   scala gt  val a  Seq Int    List 1 2 3  a  Seq Int    List 1  2  3    if the container type is not specified  the underlying data structure defaults to List

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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 equivalent 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 parallelism of a code doesn t matter  They are both relatively newly introduced  so people doesn t use them much yet

User · Answer

As  daniel-c-sobral said  List extends the trait Seq and is an abstract class implemented by scala collection immutable  colon colon  or    for short   but technicalities aside  mind that most of lists and seqs we use are initialized in the form of Seq 1  2  3  or List 1  2  3  which both return scala collection immutable  colon colon  hence one can write   var x  scala collection immutable  colon colon Int    null x   Seq 1  2  3  asInstanceOf scala collection immutable  colon colon Int   x   List 1  2  3  asInstanceOf scala collection immutable  colon colon Int     As a result  I d argue than the only thing that matters are the methods you want to expose  for instance to prepend you can use    from List that I find redundant with    from Seq and I personally stick to Seq by default

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