Initialization of an ArrayList in one line

Question

I wanted to create a list of options for testing purposes  At first  I did this   ArrayList lt String gt  places   new ArrayList lt String gt     places add  Buenos Aires    places add  C  rdoba    places add  La Plata      Then  I refactored the code as follows   ArrayList lt String gt  places   new ArrayList lt String gt       Arrays asList  Buenos Aires    C  rdoba    La Plata       Is there a better way to do this

User · Answer

With Eclipse Collections you can write the following:

List<String> list = Lists.mutable.with("Buenos Aires", "Córdoba", "La Plata");

You can also be more specific about the types and whether they are Mutable or Immutable.

MutableList<String> mList = Lists.mutable.with("Buenos Aires", "Córdoba", "La Plata");
ImmutableList<String> iList = Lists.immutable.with("Buenos Aires", "Córdoba", "La Plata");

You can also do the same with Sets and Bags:

Set<String> set = Sets.mutable.with("Buenos Aires", "Córdoba", "La Plata");
MutableSet<String> mSet = Sets.mutable.with("Buenos Aires", "Córdoba", "La Plata");
ImmutableSet<String> iSet = Sets.immutable.with("Buenos Aires", "Córdoba", "La Plata");

Bag<String> bag = Bags.mutable.with("Buenos Aires", "Córdoba", "La Plata");
MutableBag<String> mBag = Bags.mutable.with("Buenos Aires", "Córdoba", "La Plata");
ImmutableBag<String> iBag = Bags.immutable.with("Buenos Aires", "Córdoba", "La Plata");

Note: I am a committer for Eclipse Collections.

User · Answer

In Java 9 we can easily initialize an ArrayList in a single line   List lt String gt  places   List of  Buenos Aires    C  rdoba    La Plata      or  List lt String gt  places   new ArrayList lt  gt  List of  Buenos Aires    C  rdoba    La Plata       This new approach of Java 9 has many advantages over the previous ones    Space Efficiency Immutability Thread Safe   See this post for more details -  What is the difference between List of and Arrays asList

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public static  lt T gt  List lt T gt  asList T    a        return new ArrayList lt T gt  a       This is the implementation of Arrays asList  so you could go with  ArrayList lt String gt  arr    ArrayList lt String gt   Arrays asList  1    2

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List lt String gt  names   Arrays asList  2    2234   21   11

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Yes with the help of Arrays you can initialize array list in one line   List lt String gt  strlist  Arrays asList  aaa    bbb    ccc

User · Answer

Here is another way   List lt String gt  values   Stream of  One    Two   collect Collectors toList

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The simple answer In Java 9 or later  after List of   was added  List lt String gt  strings   List of  quot foo quot    quot bar quot    quot baz quot     With Java 10 or later  this can be shortened with the var keyword  var strings   List of  quot foo quot    quot bar quot    quot baz quot     This will give you an immutable List  so it cannot be changed  Which is what you want in most cases where you re prepopulating it   Java 8 or earlier  List lt String gt  strings   Arrays asList  quot foo quot    quot bar quot    quot baz quot     This will give you a List backed by an array  so it cannot change length  But you can call List set  so it s still mutable   You can make Arrays asList even shorter with a static import  List lt String gt  strings   asList  quot foo quot    quot bar quot    quot baz quot     The static import  import static java util Arrays asList     Which any modern IDE will suggest and automatically do for you  For example in IntelliJ IDEA you press Alt Enter and select Static import method      However  i don t recommend shortening the List of method to of  because that becomes confusing  List of is already short enough and reads well   Using Streams Why does it have to be a List  With Java 8 or later you can use a Stream which is more flexible  Stream lt String gt  strings   Stream of  quot foo quot    quot bar quot    quot baz quot     You can concatenate Streams  Stream lt String gt  strings   Stream concat Stream of  quot foo quot    quot bar quot                                           Stream of  quot baz quot    quot qux quot      Or you can go from a Stream to a List  import static java util stream Collectors toList   List lt String gt  strings   Stream of  quot foo quot    quot bar quot    quot baz quot   collect toList      But preferably  just use the Stream without collecting it to a List   If you really specifically need a java util ArrayList  You probably don t   To quote JEP 269  emphasis mine    There is a small set of use cases for initializing a mutable collection instance with a predefined set of values  It s usually preferable to have those predefined values be in an immutable collection  and then to initialize the mutable collection via a copy constructor    If you want to both prepopulate an ArrayList and add to it afterwards  why    use ArrayList lt String gt  strings   new ArrayList lt  gt  List of  quot foo quot    quot bar quot     strings add  quot baz quot     or in Java 8 or earlier  ArrayList lt String gt  strings   new ArrayList lt  gt  asList  quot foo quot    quot bar quot     strings add  quot baz quot     or using Stream  import static java util stream Collectors toCollection   ArrayList lt String gt  strings   Stream of  quot foo quot    quot bar quot                                 collect toCollection ArrayList  new    strings add  quot baz quot     But again  it s better to just use the Stream directly instead of collecting it to a List   Program to interfaces  not to implementations You said you ve declared the list as an ArrayList in your code  but you should only do that if you re using some member of ArrayList that s not in List  Which you are most likely not doing  Usually you should just declare variables by the most general interface that you are going to use  e g  Iterable  Collection  or List   and initialize them with the specific implementation  e g  ArrayList  LinkedList or Arrays asList     Otherwise you re limiting your code to that specific type  and it ll be harder to change when you want to  For example  if you re passing an ArrayList to a void method          Iterable if you just need iteration  for  String s   strings   void method Iterable lt String gt  strings         for  String s   strings                 Collection if you also need  size     isEmpty    or  stream    void method Collection lt String gt  strings        if   strings isEmpty      strings stream              List if you also need  get index   void method List lt String gt  strings        strings get            Don t declare a specific list implementation    unless you re sure you need it  void method ArrayList lt String gt  strings               You don t want to limit yourself to just ArrayList     Another example would be always declaring variable an InputStream even though it is usually a FileInputStream or a BufferedInputStream  because one day soon you or somebody else will want to use some other kind of InputStream

User · Answer

You could create a factory method   public static ArrayList lt String gt  createArrayList String     elements      ArrayList lt String gt  list   new ArrayList lt String gt       for  String element   elements        list add element         return list           ArrayList lt String gt  places   createArrayList     S  o Paulo    Rio de Janeiro    Bras  lia      But it s not much better than your first refactoring   For greater flexibility  it can be generic   public static  lt T gt  ArrayList lt T gt  createArrayList T     elements      ArrayList lt T gt  list   new ArrayList lt T gt       for  T element   elements        list add element         return list

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In Java  you can t do  ArrayList lt String gt  places   new ArrayList lt String gt   Arrays asList  Buenos Aires    C  rdoba    La Plata       As was pointed out  you d need to do a double brace initialization   List lt String gt  places   new ArrayList lt String gt       add  x    add  y          But this may force you into adding an annotation  SuppressWarnings  serial   or generate a serial UUID which is annoying  Also most code formatters will unwrap that into multiple statements lines   Alternatively you can do  List lt String gt  places   Arrays asList new String     x    y        but then you may want to do a  SuppressWarnings  unchecked     Also according to javadoc you should be able to do this   List lt String gt  stooges   Arrays asList  Larry    Moe    Curly      But I m not able to get it to compile with JDK 1 6

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Collection literals didn t make it into Java 8  but it is possible to use the Stream API to initialize a list in one rather long line   List lt String gt  places   Stream of  Buenos Aires    C  rdoba    La Plata   collect Collectors toList       If you need to ensure that your List is an ArrayList   ArrayList lt String gt  places   Stream of  Buenos Aires    C  rdoba    La Plata   collect Collectors toCollection ArrayList  new

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Collections singletonList messageBody    If you d need to have a list of one item   Collections is from java util package

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Like Tom said   List lt String gt  places   Arrays asList  Buenos Aires    C  rdoba    La Plata      But since you complained of wanting an ArrayList  you should firstly know that ArrayList is a subclass of List and you could simply add this line   ArrayList lt String gt  myPlaces   new ArrayList places     Although  that might make you complain of  performance    In that case it doesn t make sense to me  why  since your list is predefined it wasn t defined as an array  since the size is known at time of initialisation   And if that s an option for you   String   places     Buenos Aires    C  rdoba    La Plata      In case you don t care of the minor performance differences then you can also copy an array to an ArrayList very simply   ArrayList lt String gt  myPlaces   new ArrayList Arrays asList places      Okay  but in future you need a bit more than just the place name  you need a country code too  Assuming this is still a predefined list which will never change during run-time  then it s fitting to use an enum set  which would require re-compilation if the list needed to be changed in the future   enum Places  BUENOS AIRES  CORDOBA  LA PLATA    would become   enum Places       BUENOS AIRES  Buenos Aires  123       CORDOBA  C  rdoba  456       LA PLATA  La Plata  789        String name      int code      Places String name  int code          this name name        this code code            Enum s have a static values method that returns an array containing all of the values of the enum in the order they are declared  e g    for  Places p Places values          System out printf  The place  s has code  d n                     p name  p code       In that case I guess you wouldn t need your ArrayList   P S  Randyaa demonstrated another nice way using the static utility method Collections addAll

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Here is code by AbacusUtil     ArrayList List lt String gt  list   N asList  Buenos Aires    C  rdoba    La Plata       HashSet Set lt String gt  set   N asSet  Buenos Aires    C  rdoba    La Plata       HashMap Map lt String  Integer gt  map   N asMap  Buenos Aires   1   C  rdoba   2   La Plata   3       Or for Immutable List Set Map ImmutableList of  Buenos Aires    C  rdoba    La Plata    ImmutableSet of  Buenos Aires    C  rdoba    La Plata    ImmutableSet of  Buenos Aires   1   C  rdoba   2   La Plata   3       The most efficient way  which is similar with Arrays asList      in JDK      but returns a flexible-size list backed by the specified array  List lt String gt  set   Array asList  Buenos Aires    C  rdoba    La Plata      Declaration  I m the developer of AbacusUtil

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About the most compact way to do this is   Double array       1 0  2 0  3 0   List lt Double gt  list   Arrays asList array

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You can use the below statements  Code Snippet  String    arr     quot Sharlock quot    quot Homes quot    quot Watson quot     List lt String gt  names   Arrays asList arr

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interestingly no one-liner with the other overloaded Stream  collect method is listed  ArrayList lt String gt  places   Stream of   Buenos Aires    C  rdoba    La Plata    collect  ArrayList  new  ArrayList  add  ArrayList  addAll

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For me Arrays asList   is the best and convenient one  I always like to initialize that way  If you are a beginner into Java Collections then I would like you to refer ArrayList initialization

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If you need a simple list of size 1   List lt String gt  strings   new ArrayList lt String gt  Collections singletonList  A       If you need a list of several objects   List lt String gt  strings   new ArrayList lt String gt     Collections addAll strings  A   B   C   D

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It would be simpler if you were to just declare it as a List - does it have to be an ArrayList   List lt String gt  places   Arrays asList  Buenos Aires    C  rdoba    La Plata      Or if you have only one element   List lt String gt  places   Collections singletonList  Buenos Aires      This would mean that places is immutable  trying to change it will cause an UnsupportedOperationException exception to be thrown    To make a mutable list that is a concrete ArrayList you can create an ArrayList from the immutable list   ArrayList lt String gt  places   new ArrayList lt  gt  Arrays asList  Buenos Aires    C  rdoba    La Plata

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Try with this code line   Collections singletonList provider

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import com google common collect ImmutableList         List lt String gt  places   ImmutableList of  Buenos Aires    C  rdoba    La Plata

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With Guava you can write   ArrayList lt String gt  places   Lists newArrayList  Buenos Aires    C  rdoba    La Plata      In Guava there are also other useful static constructors  You can read about them here

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Why not make a simple utility function that does this   static  lt A gt  ArrayList lt A gt  ll A    a      ArrayList l   new ArrayList a length     for  A x   a  l add x     return l       ll  stands for  literal list    ArrayList lt String gt  places   ll  Buenos Aires    C  rdoba    La Plata

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Java 9 has the following method to create an immutable list   List lt String gt  places   List of  Buenos Aires    C  rdoba    La Plata      which is easily adapted to create a mutable list  if required   List lt String gt  places   new ArrayList lt  gt  List of  Buenos Aires    C  rdoba    La Plata       Similar methods are available for Set and Map

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Simply use below code as follows   List lt String gt  list   new ArrayList lt String gt                   add  A                add  B                add  C

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The best way to do it   package main package   import java util ArrayList    public class Stackkkk       public static void main String   args            ArrayList lt Object gt  list   new ArrayList lt Object gt             add list   1    2    3    4    5    6            System out println  I added     list size       element in one line               public static void add ArrayList lt Object gt  list Object   objects           for Object object objects              list add object             Just create a function that can have as many elements as you want and call it to add them in one line

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Actually  probably the  best  way to initialize the ArrayList is the method you wrote  as it does not need to create a new List in any way   ArrayList lt String gt  list   new ArrayList lt String gt     list add  A    list add  B    list add  C      The catch is that there is quite a bit of typing required to refer to that list instance   There are alternatives  such as making an anonymous inner class with an instance initializer  also known as an  double brace initialization     ArrayList lt String gt  list   new ArrayList lt String gt           add  A        add  B        add  C          However  I m not too fond of that method because what you end up with is a subclass of ArrayList which has an instance initializer  and that class is created just to create one object -- that just seems like a little bit overkill to me   What would have been nice was if the Collection Literals proposal for Project Coin was accepted  it was slated to be introduced in Java 7  but it s not likely to be part of Java 8 either     List lt String gt  list     A    B    C      Unfortunately it won t help you here  as it will initialize an immutable List rather than an ArrayList  and furthermore  it s not available yet  if it ever will be

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With java-9 and above  as suggested in JEP 269  Convenience Factory Methods for Collections  this could be achieved using collection literals now with - List lt String gt  list   List of  quot A quot    quot B quot    quot C quot     Set lt String gt  set   Set of  quot A quot    quot B quot    quot C quot     A similar approach would apply to Map as well - Map lt String  String gt  map   Map of  quot k1 quot    quot v1 quot    quot k2 quot    quot v2 quot    quot k3 quot    quot v3 quot    which is similar to Collection Literals proposal as stated by  coobird  Further clarified in the JEP as well -  Alternatives  Language changes have been considered several times  and rejected  Project Coin Proposal  29 March 2009 Project Coin Proposal  30 March 2009 JEP 186 discussion on lambda-dev  January-March 2014 The language proposals were set aside in preference to a library-based proposal as summarized in this message   Related  What is the point of overloaded Convenience Factory Methods for Collections in Java 9

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You can use StickyList from Cactoos   List lt String gt  names   new StickyList lt  gt      Scott Fitzgerald    Fyodor Dostoyevsky

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Should be a comment  but too long  so new reply    As others have mentioned  the Arrays asList method is fixed size  but that s not the only issue with it   It also doesn t handle inheritance very well   For instance  suppose you have the following   class A   class B extends A    public List lt A gt  getAList        return Arrays asList new B         The above results in a compiler error  because List lt B gt  which is what is returned by Arrays asList  is not a subclass of List lt A gt   even though you can add Objects of type B to a List lt A gt  object   To get around this  you need to do something like   new ArrayList lt A gt  Arrays  lt A gt asList b1  b2  b3     This is probably the best way to go about doing this  esp  if you need an unbounded list or need to use inheritance

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Actually  it s possible to do it in one line   Arrays asList new MyClass    new MyClass  arg1    new MyClass  arg2

[java] Initialization of an ArrayList in one line

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