How to convert List Integer to int in Java

Question

This is similar to this question  How to convert int   to Integer   in Java   I m new to Java  How can i convert a List lt Integer gt  to int   in Java  I m confused because List toArray   actually returns an Object    which can be cast to nether Integer   or int     Right now I m using a loop to do so   int   toIntArray List lt Integer gt  list     int   ret   new int list size       for int i   0 i  lt  ret length i        ret i    list get i     return ret      I m sure there s a better way to do this

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This simple loop is always correct  no bugs    int   integers   new int myList size       for  int i   0  i  lt  integers length  i            integers i    myList get i

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With Eclipse Collections  you can do the following if you have a list of type  java util List lt Integer gt    List lt Integer gt  integers   Lists mutable with 1  2  3  4  5   int   ints   LazyIterate adapt integers  collectInt i - gt  i  toArray     Assert assertArrayEquals new int   1  2  3  4  5   ints     If you already have an Eclipse Collections type like MutableList  you can do the following   MutableList lt Integer gt  integers   Lists mutable with 1  2  3  4  5   int   ints   integers asLazy   collectInt i - gt  i  toArray     Assert assertArrayEquals new int   1  2  3  4  5   ints     Note  I am a committer for Eclipse Collections

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Using a lambda you could do this  compiles in jdk lambda    public static void main String ars              TransformService transformService    inputs  - gt                int   ints   new int inputs size                 int i   0              for  Integer element   inputs                    ints  i       element                            return ints                      List lt Integer gt  inputs   new ArrayList lt Integer gt  5     add 10   add 10                int   results   transformService transform inputs              public interface TransformService           int   transform List lt Integer gt  inputs

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Here is Java 8 single line code for this  public int   toIntArray List lt Integer gt  intList          return intList stream   mapToInt Integer  intValue  toArray

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Java 8 has given us an easy way to do this via streams     Using the collections stream   function and then mapping to ints  you ll get an IntStream  With the IntStream we can call toArray   which gives us int     int    ints   list stream   mapToInt Integer  intValue  toArray      to int     to IntStream

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try also Dollar  check this revision    import static com humaorie dollar Dollar        List lt Integer gt  source        int   ints     source  convert   toIntArray

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There is really no way of  one-lining  what you are trying to do because toArray returns an Object   and you cannot cast from Object   to int   or Integer   to int

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Unfortunately  I don t believe there really is a better way of doing this due to the nature of Java s handling of primitive types  boxing  arrays and generics  In particular    List lt T gt  toArray won t work because there s no conversion from Integer to int You can t use int as a type argument for generics  so it would have to be an int-specific method  or one which used reflection to do nasty trickery     I believe there are libraries which have autogenerated versions of this kind of method for all the primitive types  i e  there s a template which is copied for each type   It s ugly  but that s the way it is I m afraid     Even though the Arrays class came out before generics arrived in Java  it would still have to include all the horrible overloads if it were introduced today  assuming you want to use primitive arrays

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In addition to Commons Lang  you can do this with Guava s method Ints toArray Collection lt Integer gt  collection    List lt Integer gt  list       int   ints   Ints toArray list     This saves you having to do the intermediate array conversion that the Commons Lang equivalent requires yourself

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int   ret   new int list size            Iterator lt Integer gt  iter   list iterator    for  int i 0  iter hasNext    i                 ret i    iter next                                                              return ret

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No one mentioned yet streams added in Java 8 so here it goes  int   array   list stream   mapToInt i- gt i  toArray      OR   int   array   list stream   mapToInt Integer  intValue  toArray      Thought process   simple Stream toArray returns Object    so it is not what we want  Also Stream toArray IntFunction lt A   gt  generator  doesn t do what we want because generic type A can t represent primitive int  so it would be nice to have some stream which could handle primitive type int instead of wrapper Integer  because its toArray method will most likely also return int   array  returning something else like Object   or even boxed Integer   would be unnatural here   And fortunately Java 8 has such stream which is IntStream  so now only thing we need to figure out is how to convert our Stream lt Integer gt   which will be returned from list stream    to that shiny IntStream  Here Stream mapToInt ToIntFunction lt   super T gt  mapper  method comes to a rescue  All we need to do is pass to it mapping from Integer to int  We could use something like Integer intValue which returns int like     mapToInt   Integer i  - gt  i intValue           or if someone prefers mapToInt Integer  intValue    but similar code can be generated using unboxing  since compiler knows that result of this lambda must be int  lambda in mapToInt is implementation of ToIntFunction interface which expects body for int applyAsInt T value  method which is expected to return int   So we can simply write     mapToInt  Integer i - gt i   Also since Integer type in  Integer i  can be inferred by compiler because List lt Integer gt  stream   returns Stream lt Integer gt  we can also skip it which leaves us with     mapToInt i - gt  i

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int   toIntArray List lt Integer gt  list         int   ret   new int list size         int i   0      for  Integer e   list            ret i      e      return ret      Slight change to your code to avoid expensive list indexing  since a List is not necessarily an ArrayList  but could be a linked list  for which random access is expensive

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I ll throw one more in here  I ve noticed several uses of for loops  but you don t even need anything inside the loop  I mention this only because the original question was trying to find less verbose code   int   toArray List lt Integer gt  list        int   ret   new int  list size          int i   0      for  Iterator lt Integer gt  it   list iterator              it hasNext              ret i      it next          return ret      If Java allowed multiple declarations in a for loop the way C   does  we could go a step further and do for int i   0  Iterator it     In the end though  this part is just my opinion   if you are going to have a helping function or method to do something for you  just set it up and forget about it  It can be a one-liner or ten  if you ll never look at it again you won t know the difference

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If you are simply mapping an Integer to an int then you should consider using parallelism  since your mapping logic does not rely on any variables outside its scope   int   arr   list parallelStream   mapToInt Integer  intValue  toArray      Just be aware of this      Note that parallelism is not automatically faster than performing operations serially  although it can be if you have enough data and processor cores  While aggregate operations enable you to more easily implement parallelism  it is still your responsibility to determine if your application is suitable for parallelism      There are two ways to map Integers to their primitive form    Via a ToIntFunction   mapToInt Integer  intValue   Via explicit unboxing with lambda expression   mapToInt i - gt  i intValue     Via implicit  auto-  unboxing with lambda expression   mapToInt i - gt  i       Given a list with a null value  List lt Integer gt  list   Arrays asList 1  2  null  4  5     Here are three options to handle null    Filter out the null values before mapping   int   arr   list parallelStream   filter Objects  nonNull  mapToInt Integer  intValue  toArray     Map the null values to a default value   int   arr   list parallelStream   map i - gt  i    null   -1   i  mapToInt Integer  intValue  toArray     Handle null inside the lambda expression   int   arr   list parallelStream   mapToInt i - gt  i    null   -1   i intValue    toArray

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The easiest way to do this is to make use of Apache Commons Lang   It has a handy ArrayUtils class that can do what you want   Use the toPrimitive method with the overload for an array of Integers   List lt Integer gt  myList       assign and fill the list int   intArray   ArrayUtils toPrimitive myList toArray new Integer myList size         This way you don t reinvent the wheel   Commons Lang has a great many useful things that Java left out   Above  I chose to create an Integer list of the right size   You can also use a 0-length static Integer array and let Java allocate an array of the right size   static final Integer   NO INTS   new Integer 0           int   intArray2   ArrayUtils toPrimitive myList toArray NO INTS

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I would recommend you to use List lt   gt  skeletal implementation from the java collections API  it appears to be quite helpful in this particular case   package mypackage   import java util AbstractList  import java util Arrays  import java util Collections  import java util List   public class Test      Helper method to convert int arrays into Lists static List lt Integer gt  intArrayAsList final int   a        if a    null          throw new NullPointerException        return new AbstractList lt Integer gt                Override         public Integer get int i                return a i    autoboxing                    Override         public Integer set int i  Integer val                final int old   a i               a i    val   auto-unboxing             return old   autoboxing                    Override         public int size                 return a length                      public static void main final String   args        int   a    1  2  3  4  5       Collections reverse intArrayAsList a        System out println Arrays toString a          Beware of boxing unboxing drawbacks

[java] How to convert List<Integer> to int[] in Java?

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