How to make a new List in Java

Question

We create a Set as   Set myset   new HashSet     How do we create a List in Java

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Additionally  if you want to create a list that has things in it  though it will be fixed size    List lt String gt  messages   Arrays asList  Hello    World     How    Are    You

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List is just an interface just as Set   Like HashSet is an implementation of a Set which has certain properties in regards to add   lookup   remove performance  ArrayList is the bare implementation of a List   If you have a look at the documentation for the respective interfaces you will find  All Known Implementing Classes  and you can decide which one is more suitable for your needs   Chances are that it s ArrayList

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The following are some ways you can create lists    This will create a list with fixed size  adding removing elements is not possible  it will throw a java lang UnsupportedOperationException if you try to do so   List lt String gt  fixedSizeList   Arrays asList new String     Male    Female           The following version is a simple list where you can add remove any number of elements   List lt String gt  list   new ArrayList lt  gt           This is how to create a LinkedList in java  If you need to do frequent insertion deletion of elements on the list  you should use LinkedList instead of ArrayList  List lt String gt  linkedList   new LinkedList lt  gt

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simple example creating a list form a string array  String   myStrings   new String     Elem1   Elem2   Elem3   Elem4   Elem5     List mylist   Arrays asList myStrings       getting an iterator object to browse list items  Iterator itr  mylist iterator     System out println  Displaying List Elements      while itr hasNext       System out println itr next

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First read this  then read this and this  9 times out of 10 you ll use one of those two implementations   In fact  just read Sun s Guide to the Collections framework

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Sometimes - but only very rarely - instead of a new ArrayList  you may want a new LinkedList   Start out with ArrayList and if you have performance problems and evidence that the list is the problem  and a lot of adding and deleting to that list - then - not before - switch to a LinkedList and see if things improve   But in the main  stick with ArrayList and all will be fine

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List is an interface like Set and has ArrayList and LinkedList as general purpose implementations    We can create List as    List lt String gt  arrayList   new ArrayList lt  gt      List lt String gt  linkedList   new LinkedList lt  gt        We can also create a fixed-size list as   List lt String gt  list   Arrays asList  A    B    C      We would almost always be using ArrayList opposed to LinkedList implementation    LinkedList uses a lot of space for objects and performs badly when we have lots of elements  Any indexed operation in LinkedList requires O n  time compared to O 1  in ArrayList  Check this link for more information    The list created by Arrays asList above can not be modified structurally but its elements can still be modified    Java 8  As per doc  the method Collections unmodifiableList returns an unmodifiable view of the specified list  We can get it like   Collections unmodifiableList Arrays asList  A    B    C       Java 9  In case we are using Java 9 then   List lt String gt  list   List of  A    B      Java 10  In case we are at Java 10 then the method Collectors unmodifiableList will return an instance of truly unmodifiable list introduced in Java 9  Check this answer for more info about the difference in Collections unmodifiableList  vs Collectors unmodifiableList in Java 10

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There are many ways to create a Set and a List  HashSet and ArrayList are just two examples   It is also fairly common to use generics with collections these days  I suggest you have a look at what they are  This is a good introduction for java s builtin collections  http   java sun com javase 6 docs technotes guides collections overview html

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Let me summarize and add something   JDK  1  new ArrayList lt String gt     2  Arrays asList  A    B    C     Guava  1  Lists newArrayList  Mike    John    Lesly    2  Lists asList  A   B   new String      C    D       Immutable List  1  Collections unmodifiableList new ArrayList lt String gt  Arrays asList  A   B      2  ImmutableList builder                                           Guava              add  A                add  B   build    3  ImmutableList of  A    B                                        Guava 4  ImmutableList copyOf Lists newArrayList  A    B    C            Guava   Empty immutable List  1  Collections emptyList    2  Collections EMPTY LIST    List of Characters  1  Lists charactersOf  String                                      Guava 2  Lists newArrayList Splitter fixedLength 1  split  String        Guava   List of Integers  Ints asList 1 2 3                                                  Guava

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More options to do the same thing with Java 8  not better  not worse  just different and if you want to do some extra work with the lists  Streams will provide you more alternatives  filter  map  reduce  etc    List lt String gt  listA   Stream of  a    B    C   collect Collectors toList     List lt Integer gt  listB   IntStream range 10  20  boxed   collect Collectors toList     List lt Double gt  listC   DoubleStream generate    - gt    return new Random   nextDouble       limit 10  boxed   collect Collectors toList     LinkedList lt Integer gt  listD   Stream iterate 0  x - gt  x    limit 10  collect Collectors toCollection LinkedList  new

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Using Eclipse Collections you can create a List like this   List lt String gt  list1   Lists mutable empty    List lt String gt  list2   Lists mutable of  One    Two    Three      If you want an immutable list    ImmutableList lt String gt  list3   Lists immutable empty    ImmutableList lt String gt  list4   Lists immutable of  One    Two    Three      You can avoid auto-boxing by using primitive lists  Here s how you d create int lists   MutableIntList list5   IntLists mutable empty    MutableIntList list6   IntLists mutable of 1  2  3    ImmutableIntList list7   IntLists immutable empty    ImmutableIntList list8   IntLists immutable of 1  2  3     There are variants for all 8 primitives   MutableLongList longList         LongLists mutable of 1L  2L  3L   MutableCharList charList         CharLists mutable of  a    b    c    MutableShortList shortList       ShortLists mutable of  short  1   short  2   short  3   MutableByteList byteList         ByteLists mutable of  byte  1   byte  2   byte  3   MutableBooleanList booleanList   BooleanLists mutable of true  false   MutableFloatList floatList       FloatLists mutable of 1 0f  2 0f  3 0f   MutableDoubleList doubleList     DoubleLists mutable of 1 0  2 0  3 0     Note  I am a committer for Eclipse Collections

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If you need a serializable  immutable list with a single entity you can use   List lt String gt  singList   Collections singletonList  stackoverlow

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Try this    List lt String gt  messages   Arrays asList  bla1    bla2    bla3      Or   List lt String gt  list1   Lists mutable empty       Empty List lt String gt  list2   Lists mutable of  One    Two    Three

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Using Google Collections  you could use the following methods in the Lists class  import com google common collect Lists           List lt String gt  strings   Lists newArrayList     List lt Integer gt  integers   Lists newLinkedList      There are overloads for varargs initialization and initialising from an Iterable lt T gt     The advantage of these methods is that you don t need to specify the generic parameter explicitly as you would with the constructor - the compiler will infer it from the type of the variable

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Since Java 7 you have type inference for generic instance creation  so there is no need to duplicate generic parameters on the right hand side of the assignment   List lt String gt  list   new ArrayList lt  gt       A fixed-size list can be defined as   List lt String gt  list   Arrays asList  foo    bar      For immutable lists you can use the Guava library   List lt String gt  list   ImmutableList of  foo    bar

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List lt Object gt  nameOfList   new ArrayList lt Object gt       You need to import List and ArrayList

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One example   List somelist   new ArrayList      You can look at the javadoc for List and find all known implementing classes of the List interface that are included with the java api

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With Java 9  you are able to do the following to create an immutable List   List lt Integer gt  immutableList   List of 1  2  3  4  5    List lt Integer gt  mutableList   new ArrayList lt  gt  immutableList

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As an option you can use double brace initialization here   List lt String gt  list   new ArrayList lt String gt            add  a       add  b

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As declaration of array list in java is like   public class ArrayList lt E gt  extends AbstractList lt E gt  implements List lt E gt   RandomAccess  Cloneable  Serializable     There is numerous way you can create and initialize array list in java    1  List list   new ArrayList      2  List lt type gt  myList   new ArrayList lt  gt       3  List lt type gt  myList   new ArrayList lt type gt       4  Using Utility class      List lt Integer gt  list   Arrays asList 8  4       Collections unmodifiableList Arrays asList  a    b    c       5  Using static factory method      List lt Integer gt  immutableList   List of 1  2      6  Creation and initializing at a time      List lt String gt  fixedSizeList   Arrays asList new String     Male    Female         Again you can create different types of list  All has their own characteristics   List a   new ArrayList     List b   new LinkedList     List c   new Vector      List d   new Stack      List e   new CopyOnWriteArrayList

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List list   new ArrayList      Or with generics  List lt String gt  list   new ArrayList lt String gt       You can  of course  replace string with any type of variable  such as Integer  also

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List myList   new ArrayList      or with generics  Java 7 or later   List lt MyType gt  myList   new ArrayList lt  gt       or with generics  Old java versions   List lt MyType gt  myList   new ArrayList lt MyType gt

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In Java 8  To create a non-empty list of fixed size  operations like add  remove  etc   are not supported    List lt Integer gt  list   Arrays asList 1  2      but  list set      is supported   To create a non-empty mutable list   List lt Integer gt  list   new ArrayList lt  gt  Arrays asList 3  4      In Java 9  Using a new List of      static factory methods   List lt Integer gt  immutableList   List of 1  2    List lt Integer gt  mutableList   new ArrayList lt  gt  List of 3  4      In Java 10  Using the Local Variable Type Inference   var list1   List of 1  2    var list2   new ArrayList lt  gt  List of 3  4     var list3   new ArrayList lt String gt       And follow best practices     Don t use raw types  Since Java 5  generics have been a part of the language - you should use them   List lt String gt  list   new ArrayList lt  gt        Good  List of String  List list   new ArrayList       Bad  don t do that    Program to interfaces  For example  program to the List interface   List lt Double gt  list   new ArrayList lt  gt       Instead of   ArrayList lt Double gt  list   new ArrayList lt  gt        This is a bad idea

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List arrList   new ArrayList      Its better you use generics as suggested below   List lt String gt  arrList   new ArrayList lt String gt      arrList add  one      Incase you use LinkedList   List lt String gt  lnkList   new LinkedList lt String gt

[java] How to make a new List in Java

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