Collections emptyList vs new instance

Question

In practice  is it better to return an empty list like this   return Collections emptyList      Or like this   return new ArrayList lt Foo gt       Or is this completely dependent upon what you re going to do with the returned list

User · Answer

Use Collections emptyList   if you want to make sure that the returned list is never modified  This is what is returned on calling emptyList           The empty list  immutable        public static final List EMPTY LIST   new EmptyList

User · Answer

Starting with Java 5 0 you can specify the type of element in the container   Collections  lt Foo gt emptyList     I concur with the other responses that for cases where you want to return an empty list that stays empty  you should use this approach

User · Answer

I would go with Collections emptyList   if the returned list is not being modified in any way  as the list is immutable   otherwise I would go with option 2   The benefit of Collections emptyList   is that the same static instance is returned each time and so there is not instance creation occurring for each call

User · Answer

The main difference is that Collections emptyList   returns an immutable list  i e   a list to which you cannot add elements   Same applies to the List of   introduced in Java 9    In the rare cases where you do want to modify the returned list  Collections emptyList   and List of   are thus not a good choices   I d say that returning an immutable list is perfectly fine  and even the preferred way  as long as the contract  documentation  does not explicitly state differently     In addition  emptyList   might not create a new object with each call      Implementations of this method need not create a separate List object for each call  Using this method is likely to have comparable cost to using the like-named field   Unlike this method  the field does not provide type safety     The implementation of emptyList looks as follows   public static final  lt T gt  List lt T gt  emptyList         return  List lt T gt   EMPTY LIST      So if your method  which returns an empty list  is called very often  this approach may even give you slightly better performance both CPU and memory wise

User · Answer

The given answers stress the fact that emptyList   returns an immutable List but do not give alternatives  The Constructor ArrayList int initialCapacity  special cases 0 so returning new ArrayList lt  gt  0  instead of new ArrayList lt  gt    might also be a viable solution          Shared empty array instance used for empty instances      private static final Object   EMPTY ELEMENTDATA                       Constructs an empty list with the specified initial capacity         param  initialCapacity  the initial capacity of the list     throws IllegalArgumentException if the specified initial capacity            is negative     public ArrayList int initialCapacity        if  initialCapacity  gt  0            this elementData   new Object initialCapacity         else if  initialCapacity    0            this elementData   EMPTY ELEMENTDATA        else           throw new IllegalArgumentException  Illegal Capacity                                                initialCapacity              sources from Java 1 8 0 72

User · Answer

Be carefully though  If you return Collections emptyList   and then try to do some changes with it like add   or smth like that  u will have an UnsupportedOperationException   because Collections emptyList   returns an immutable object

User · Answer

Collections emptyList is immutable so there is a difference between the two versions so you have to consider users of the returned value   Returning new ArrayList lt Foo gt  always creates a new instance of the object so it has a very slight extra cost associated with it which may give you a reason to use Collections emptyList   I like using emptyList just because it s more readable

[java] Collections.emptyList() vs. new instance

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