Type List vs type ArrayList in Java

Question

1  List lt   gt  myList   new ArrayList lt   gt       2  ArrayList lt   gt  myList   new ArrayList lt   gt       I understand that with  1   implementations of the List interface can be swapped   It seems that  1  is typically used in an application regardless of need  myself I always use this      I am wondering if anyone uses  2      Also  how often  and can I please get an example  does the situation actually require using  1  over  2   i e  where  2  wouldn t suffice  aside coding to interfaces and best practices etc

User · Answer

I would say that 1 is preferred  unless    you are depending on the implementation of optional behavior  in ArrayList  in that case explicitly using ArrayList is more clear You will be using the ArrayList in a method call which requires ArrayList  possibly for optional behavior or performance characteristics   My guess is that in 99  of the cases you can get by with List  which is preferred    for instance removeAll  or add null

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It is considered good style to store a reference to a HashSet or TreeSet in a variable of type Set   Set lt String gt  names   new HashSet lt String gt      This way  you have to change only one line if you decide to use a TreeSet instead   Also  methods that operate on sets should specify parameters of type Set   public static void print Set lt String gt  s   Then the method can be used for all set implementations   In theory  we should make the same recommendation for linked lists  namely to save LinkedList references in variables of type List  However  in the Java library  the List interface is common to both the ArrayList and the LinkedList class  In particular  it has get and set methods for random access  even though these methods are very inefficient for linked lists   You can   t write efficient code if you don   t know whether random access is efficient or not   This is plainly a serious design error in the standard library  and I cannot recommend using the List interface for that reason    To see just how embarrassing that error is  have a look at the source code for the binarySearch method of the Collections class  That method takes a List parameter  but binary search makes no sense for a linked list  The code then clumsily tries to discover whether the list is a linked list  and then switches to a linear search   The Set interface and the Map interface  are well designed  and you should use them

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When you write List  you actually tell  that your object implements List interface only  but you don t specify what class your object belongs to    When you write ArrayList  you specify that your object class is a resizable-array   So  the first version makes your code more flexible in future   Look at Java docs    Class ArrayList - Resizable-array implementation of the List interface   Interface List - An ordered collection  also known as a sequence   The user of this interface has precise control over where in the list each element is inserted   Array - container object that holds a fixed number of values of a single type

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Actually there are occasions where  2  is not only preferred but mandatory and I am very surprised  that nobody mentions this here   Serialization   If you have a serializable class and you want it to contain a list  then you must declare the field to be of a concrete and serializable type like ArrayList because the List interface does not extend java io Serializable  Obviously most people do not need serialization and forget about this   An example   public class ExampleData implements java io Serializable       The following also guarantees that strings is always an ArrayList  private final ArrayList lt String gt  strings   new ArrayList lt  gt

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Somebody asked this again  duplicate  which made me go a little deeper on this issue    public static void main String   args        List lt String gt  list   new ArrayList lt String gt         list add  a        list add  b         ArrayList lt String gt  aList   new ArrayList lt String gt         aList add  a        aList add  b         If we use a bytecode viewer  I used http   asm ow2 org eclipse index html  well see the following  only list initialization and assignment  for our list snippet      L0     LINENUMBER 9 L0     NEW ArrayList     DUP     INVOKESPECIAL ArrayList  lt init gt       void     ASTORE 1    L1     LINENUMBER 10 L1     ALOAD 1  list     LDC  a      INVOKEINTERFACE List add  Object    boolean     POP    L2     LINENUMBER 11 L2     ALOAD 1  list     LDC  b      INVOKEINTERFACE List add  Object    boolean     POP   and for alist      L3     LINENUMBER 13 L3     NEW java util ArrayList     DUP     INVOKESPECIAL java util ArrayList  lt init gt    V     ASTORE 2    L4     LINENUMBER 14 L4     ALOAD 2     LDC  a      INVOKEVIRTUAL java util ArrayList add  Ljava lang Object  Z     POP    L5     LINENUMBER 15 L5     ALOAD 2     LDC  b      INVOKEVIRTUAL java util ArrayList add  Ljava lang Object  Z     POP   The difference is list ends up calling INVOKEINTERFACE whereas aList calls INVOKEVIRTUAL  Accoding to the Bycode Outline Plugin reference       invokeinterface is used to invoke a method declared within a Java   interface   while invokevirtual      invokes all methods except interface methods  which use   invokeinterface   static methods  which use invokestatic   and the few   special cases handled by invokespecial    In summary  invokevirtual pops objectref off the stack while for invokeinterface      the interpreter pops  n  items off the operand stack  where  n  is an 8-bit unsigned   integer parameter taken from the bytecode  The first of these items is   objectref  a reference to the object whose method is being called    If I understand this correctly  the difference is basically how each way retrieves objectref

User · Answer

Almost always List is preferred over ArrayList because  for instance  List can be translated into a LinkedList without affecting the rest of the codebase   If one used ArrayList instead of List  it s hard to change the ArrayList implementation into a LinkedList one because ArrayList specific methods have been used in the codebase that would also require restructuring   You can read about the List implementations here   You may start with an ArrayList  but soon after discover that another implementation is the more appropriate choice

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I am wondering if anyone uses  2      Yes   But rarely for a sound reason  IMO    And people get burned because they used ArrayList when they should have used List    Utility methods like Collections singletonList      or Arrays asList      don t return an ArrayList  Methods in the List API don t guarantee to return a list of the same type      For example of someone getting burned  in https   stackoverflow com a 1481123 139985 the poster had problems with  slicing  because ArrayList sublist      doesn t return an ArrayList     and he had designed his code to use ArrayList as the type of all of his list variables   He ended up  solving  the problem by copying the sublist into a new ArrayList   The argument that you need to know how the List behaves is largely addressed by using the RandomAccess marker interface   Yes  it is a bit clunky  but the alternative is worse      Also  how often does the situation actually require using  1  over  2   i e  where  2  wouldn t suffice  aside  coding to interfaces  and best practices etc     The  how often  part of the question is objectively unanswerable         and can I please get an example     Occasionally  the application may require that you use methods in the ArrayList API that are not in the List API   For example  ensureCapacity int   trimToSize   or removeRange int  int     And the last one will only arise if you have created a subtype of ArrayList that declares the method to be public    That is the only sound reason for coding to the class rather than the interface  IMO      It is theoretically possible that you will get a slight improvement in performance     under certain circumstances     on some platforms     but unless you really need that last 0 05   it is not worth doing this   This is not a sound reason  IMO         You can   t write efficient code if you don   t know whether random access is efficient or not    That is a valid point   However  Java provides better ways to deal with that  e g   public  lt T extends List  amp  RandomAccess gt  void test T list           do stuff     If you call that with a list that does not implement RandomAccess you will get a compilation error   You could also test dynamically     using instanceof     if static typing is too awkward   And you could even write your code to use different algorithms  dynamically  depending on whether or not a list supported random access   Note that ArrayList is not the only list class that implements RandomAccess   Others include CopyOnWriteList  Stack and Vector   I ve seen people make the same argument about Serializable  because List doesn t implement it      but the approach above solves this problem too    To the extent that it is solvable at all using runtime types   An ArrayList will fail serialization if any element is not serializable

User · Answer

List interface have several different classes - ArrayList and LinkedList  LinkedList is used to create an indexed collections and ArrayList - to create sorted lists  So  you can use any of it in your arguments  but you can allow others developers who use your code  library  etc  to use different types of lists  not only which you use  so  in this method  ArrayList lt Object gt  myMethod  ArrayList lt Object gt  input          body     you can use it only with ArrayList  not LinkedList  but you can allow to use any of List classes on other places where it method is using  it s just your choise  so using an interface can allow it   List lt Object gt  myMethod  List lt Object gt  input          body     In this method arguments you can use any of List classes which you want to use   List lt Object gt  list   new ArrayList lt Object gt       list add   string     myMethod  list     CONCLUSION   Use the interfaces everywhere when it possible  don t restrict you or others to use different methods which they want to use

User · Answer

Out of the following two    1  List lt   gt  myList   new ArrayList lt   gt      2  ArrayList lt   gt  myList   new ArrayList lt   gt       First is generally preferred  As you will be using methods from List interface only  it provides you the freedom to use some other implementation of List e g  LinkedList in future  So it decouples you from specific implementation  Now there are two points worth mentioning    We should always program to interface  More here  You will almost always end up using ArrayList over LinkedList  More here       I am wondering if anyone uses  2    Yes sometimes  read rarely   When we need methods that are part of implementation of ArrayList but not part of the interface List  For example ensureCapacity      Also  how often  and can I please get an example  does the situation   actually require using  1  over  2    Almost always you prefer option  1   This is a classical design pattern in OOP where you always try to decouple your code from specific implementation and program to the interface

User · Answer

I think the people who use  2  don t know the Liskov substitution principle or the Dependency inversion principle  Or they really have to use ArrayList

User · Answer

The only case that I am aware of where  2  can be better is when using GWT  because it reduces application footprint  not my idea  but the google web toolkit team says so   But for regular java running inside the JVM  1  is probably always better

User · Answer

I use  2  if code is the  owner  of the list  This is for example true for local-only variables  There is no reason to use the abstract type List instead of ArrayList  Another example to demonstrate ownership   public class Test           This object is the owner of strings  so use the concrete type      private final ArrayList lt String gt  strings   new ArrayList lt  gt             This object uses the argument but doesn t own it  so use abstract type      public void addStrings List lt String gt  add            strings addAll add                 Here we return the list but we do not give ownership away  so use abstract type  This also allows to create optionally an unmodifiable list      public List lt String gt  getStrings             return Collections unmodifiableList strings                 Here we create a new list and give ownership to the caller  Use concrete type      public ArrayList lt String gt  getStringsCopy             return new ArrayList lt  gt  strings

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For example you might decide a LinkedList is the best choice for your application  but then later decide ArrayList might be a better choice for performance reason   Use   List list   new ArrayList 100      will be better also to set the initial capacity of a collection    Instead of   ArrayList list   new ArrayList        For reference      posted mostly for Collection diagram

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3  Collection myCollection   new ArrayList lt   gt      I am using this typically  And only if I need List methods  I will use List  Same with ArrayList  You always can switch to more  quot narrow quot  interface  but you can t switch to more  quot wide quot

User · Answer

List is an interface  It doesn t have methods  When you call a method on a List reference  it in fact calls the method of ArrayList in both cases  And for the future you can change List obj   new ArrayList lt  gt  to  List obj   new LinkList lt  gt  or other types which implement List interface

[java] Type List vs type ArrayList in Java

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