Converting array to list in Java

Question

How do I convert an array to a list in Java   I used the Arrays asList   but the behavior  and signature  somehow changed from Java SE 1 4 2  docs now in archive  to 8 and most snippets I found on the web use the 1 4 2 behaviour   For example   int   spam   new int     1  2  3    Arrays asList spam     on 1 4 2 returns a list containing the elements 1  2  3 on 1 5 0  returns a list containing the array spam   In many cases it should be easy to detect  but sometimes it can slip unnoticed   Assert assertTrue Arrays asList spam  indexOf 4     -1

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In Java 8  you can use streams   int   spam   new int     1  2  3    Arrays stream spam         boxed          collect Collectors toList

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you have to cast in to array  Arrays asList  Object    array

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Using Arrays  This is the simplest way to convert an array to List  However  if you try to add a new element or remove an existing element from the list  an UnsupportedOperationException will be thrown   Integer   existingArray    1  2  3   List lt Integer gt  list1   Arrays asList existingArray   List lt Integer gt  list2   Arrays asList 1  2  3       WARNING  list2 add 1          Unsupported operation  list2 remove 1       Unsupported operation    Using ArrayList or Other List Implementations  You can use a for loop to add all the elements of the array into a List implementation  e g  ArrayList   List lt Integer gt  list   new ArrayList lt  gt     for  int i   new int   1  2  3       list add i       Using Stream API in Java 8  You can turn the array into a stream  then collect the stream using different collectors  The default collector in Java 8 use ArrayList behind the screen  but you can also impose your preferred implementation   List lt Integer gt  list1  list2  list3  list1   Stream of 1  2  3  collect Collectors toList     list2   Stream of 1  2  3  collect Collectors toCollection ArrayList  new    list3   Stream of 1  2  3  collect Collectors toCollection LinkedList  new      See also    Why do we use autoboxing and unboxing in Java  When to use LinkedList over ArrayList

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Speaking about conversion way  it depends on why do you need your List  If you need it just to read data  OK  here you go   Integer   values     1  3  7    List lt Integer gt  list   Arrays asList values     But then if you do something like this   list add 1     you get java lang UnsupportedOperationException  So for some cases you even need this   Integer   values     1  3  7    List lt Integer gt  list   new ArrayList lt Integer gt  Arrays asList values      First approach actually does not convert array but  represents  it like a List  But array is under the hood with all its properties like fixed number of elements  Please note you need to specify type when constructing ArrayList

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If you are targeting Java 8  or later   you can try this   int   numbers   new int    1  2  3  4   List lt Integer gt  integers   Arrays stream numbers                           boxed   collect Collectors  lt Integer gt toList       NOTE   Pay attention to the Collectors  lt Integer gt toList    this generic method helps you to avoid the error  Type mismatch  cannot convert from List lt Object gt  to List lt Integer gt

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The problem is that varargs got introduced in Java5 and unfortunately  Arrays asList   got overloaded with a vararg version too  So Arrays asList spam  is understood by the Java5 compiler as a vararg parameter of int arrays   This problem is explained in more details in Effective Java 2nd Ed   Chapter 7  Item 42

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So it depends on which Java version you are trying-  Java 7   Arrays asList 1  2  3     OR         final String arr     new String      G    E    E    K            final List lt String gt  initialList   new ArrayList lt String gt                  add  C               add  O               add  D               add  I               add  N                          Elements of the array are appended at the end        Collections addAll initialList  arr     OR   Integer   arr   new Integer     1  2  3    Arrays asList arr     In Java 8  int   num   new int    1  2  3   List lt Integer gt  list   Arrays stream num                           boxed   collect Collectors  lt Integer gt toList      Reference - http   www codingeek com java how-to-convert-array-to-list-in-java

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Given Array       int   givenArray    2 2 3 3 4 5     Converting integer array to Integer List  One way    boxed   -  returns the IntStream      List lt Integer gt  givenIntArray1   Arrays stream givenArray                                     boxed                                      collect Collectors toList       Second Way   map each element of the stream to Integer and then collect  NOTE   Using mapToObj you can covert each int element into string stream  char stream etc by casing i to  char i      List lt Integer gt  givenIntArray2   Arrays stream givenArray                                            mapToObj i- gt i                                            collect Collectors toList       Converting One array Type to Another Type Example   List lt Character gt  givenIntArray2   Arrays stream givenArray                                                mapToObj i- gt  char i                                                collect Collectors toList

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One-liner   List lt Integer gt  list   Arrays asList new Integer    1  2  3  4

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use two line of code to convert array to list if you use it in integer value you must use autoboxing type for primitive data type    Integer    arr  1 2     Arrays asList arr

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Another workaround if you use apache commons-lang   int   spam   new int     1  2  3    Arrays asList ArrayUtils toObject spam      Where ArrayUtils toObject converts int   to Integer

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It seems little late but here are my two cents  We cannot have List lt int gt  as int is a primitive type so we can only have List lt Integer gt   Java 8  int array  int   ints   new int    1 2 3 4 5   List lt Integer gt  list11  Arrays stream ints  boxed   collect Collectors toList       Java 8 and below  Integer array  Integer   integers   new Integer    1 2 3 4 5   List lt Integer gt  list21    Arrays asList integers      returns a fixed-size list backed by the specified array  List lt Integer gt  list22   new ArrayList lt  gt  Arrays asList integers       good List lt Integer gt  list23   Arrays stream integers  collect Collectors toList       Java 8 only  Need ArrayList and not List  In case we want a specific implementation of List e g  ArrayList then we can use toCollection as  ArrayList lt Integer gt  list24   Arrays stream integers                             collect Collectors toCollection ArrayList  new      Why list21 cannot be structurally modified  When we use Arrays asList the size of the returned list is fixed because the list returned is not java util ArrayList  but a private static class defined inside java util Arrays  So if we add or remove elements from the returned list  an UnsupportedOperationException will be thrown  So we should go with list22 when we want to modify the list  If we have Java8 then we can also go with list23  To be clear list21 can be modified in sense that we can call list21 set index element  but this list may not be structurally modified i e  cannot add or remove elements from the list  You can also check this answer of mine for more explanation   If we want an immutable list then we can wrap it as  List lt Integer gt  list22   Collections unmodifiableList Arrays asList integers     Another point to note is that the method Collections unmodifiableList returns an unmodifiable view of the specified list  An unmodifiable view collection is a collection that is unmodifiable and is also a view onto a backing collection  Note that changes to the backing collection might still be possible  and if they occur  they are visible through the unmodifiable view  We can have a truly immutable list in Java 9 and 10  Truly Immutable list Java 9  String   objects     quot Apple quot    quot Ball quot    quot Cat quot    List lt String gt  objectList   List of objects    Java 10  Truly Immutable list   We can use List of introduced in Java 9  Also other ways   List copyOf Arrays asList integers   Arrays stream integers  collect Collectors toUnmodifiableList

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Even shorter    List lt Integer gt  list   Arrays asList 1  2  3  4

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I recently had to convert an array to a List   Later on the program filtered the list attempting to remove the data   When you use the Arrays asList array  function  you create a fixed size collection   you can neither add nor delete   This entry explains the problem better than I can  Why do I get an UnsupportedOperationException when trying to remove an element from a List    In the end  I had to do a  manual  conversion       List lt ListItem gt  items   new ArrayList lt ListItem gt         for  ListItem item  itemsArray            items add item           I suppose I could have added conversion from an array to a list using an List addAll items  operation

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Can you improve this answer please as this is what I use but im not 100  clear  It works fine but intelliJ added new WeatherStation 0   Why the 0     x000D   x000D      public WeatherStation   removeElementAtIndex WeatherStation   array  int index  x000D        x000D          List lt WeatherStation gt  list   new ArrayList lt WeatherStation gt  Arrays asList array    x000D          list remove index   x000D          return list toArray new WeatherStation 0    x000D        x000D   x000D   x000D

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Using Guava   Integer   array     1  2  3   List lt Integer gt  list   Lists newArrayList sourceArray    Using Apache Commons Collections   Integer   array     1  2  3   List lt Integer gt  list   new ArrayList lt  gt  6   CollectionUtils addAll list  array

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In your example  it is because you can t have a List of a primitive type  In other words  List lt int gt  is not possible    You can  however  have a List lt Integer gt  using the Integer class that wraps the int primitive  Convert your array to a List with the Arrays asList utility method   Integer   spam   new Integer     1  2  3    List lt Integer gt  list   Arrays asList spam     See this code run live at IdeOne com

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If this helps  I ve had the same problem and simply wrote a generic function that takes an array and returns an ArrayList of the same type with the same contents   public static  lt T gt  ArrayList lt T gt  ArrayToArrayList T   array        ArrayList lt T gt  list   new ArrayList lt T gt         for T elmt   array  list add elmt       return list

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In Java 9 you have the even more elegant solution of using immutable lists via the new convenience factory method List of   List lt String gt  immutableList   List of  one   two   three       shamelessly copied from here

[java] Converting array to list in Java

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