Convert Java Array to Iterable

Question

I have an Array of primitives  for example for int  int   foo  It might be a small sized one  or not   int foo      1 2 3 4 5 6 7 8 9 0     What is the best way to create an Iterable lt Integer gt  from it   Iterable lt Integer gt  fooBar   convert foo     Notes   Please do not answer using loops  unless you can give a good explanation on how the compiler do something smart about them    Also note that   int a      1 2 3   List lt Integer gt  l   Arrays asList a     Will not even compile  Type mismatch  cannot convert from List lt int   gt  to List lt Integer gt    Also check Why is an array not assignable to Iterable  before answering   Also  if you use some library  e g   Guava   please explain why this is the Best    Because its from Google is not a complete answer  P    Last  since there seems to be a homework about that  avoid posting homeworkish code

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just my 2 cents   final int a      1 2 3    java lang Iterable lt Integer gt  aIterable new Iterable lt Integer gt           public Iterator lt Integer gt  iterator            return new Iterator lt Integer gt                  private int pos 0               public boolean hasNext                    return a length gt pos                             public Integer next                    return a pos                                public void remove                     throw new UnsupportedOperationException  Cannot remove an element of an array

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In Java 8 or later  Iterable is a functional interface returns Iterator  So you can do this  static Iterable lt Integer gt  convert int   array        return    - gt  Arrays stream array  iterator       and int   array    1  2  3   Iterable lt Integer gt  iterable   convert array   for  int i   iterable      System out println i    output  1 2 3

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Integer foo       1  2  3  4  5  6  7  8  9  0     List lt Integer gt  list   Arrays asList foo      or Iterable lt Integer gt  iterable   Arrays asList foo     Though you need to use an Integer array  not an int array  for this to work   For primitives  you can use guava   Iterable lt Integer gt  fooBar   Ints asList foo      lt dependency gt       lt groupId gt com google guava lt  groupId gt       lt artifactId gt guava lt  artifactId gt       lt version gt 15 0 lt  version gt       lt type gt jar lt  type gt   lt  dependency gt    For Java8   from Jin Kwon s answer   final int   arr    1  2  3   final PrimitiveIterator OfInt i1   Arrays stream arr  iterator    final PrimitiveIterator OfInt i2   IntStream of arr  iterator    final Iterator lt Integer gt  i3   IntStream of arr  boxed   iterator

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You can use IterableOf from Cactoos   Iterable lt String gt  names   new IterableOf lt  gt      Scott Fitzgerald    Fyodor Dostoyevsky       Then  you can turn it into a list using ListOf   List lt String gt  names   new ListOf lt  gt     new IterableOf lt  gt        Scott Fitzgerald    Fyodor Dostoyevsky           Or simply this   List lt String gt  names   new ListOf lt  gt      Scott Fitzgerald    Fyodor Dostoyevsky

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First of all  I can only agree that Arrays asList T     is clearly the best solution for Wrapper types or arrays with non-primtive datatypes  This method calls a constructor of a simple private static AbstractList implementation in the Arrays class which basically saves the given array reference as field and simulates a list by overriding the needed methods  If you can choose between a primtive type or a Wrapper type for your array  I would use the Wrapper type for such situations but of course  it s not always useful or required  There would be only two possibilities you can do  1  You can create a class with a static method for each primitive datatype array  boolean  byte  short  int  long  char  float  double returning an Iterable lt WrapperType   These methods would use anonymous classes of Iterator  besides Iterable  which are allowed to contain the reference of the comprising method s argument  for example an int    as field in order to implement the methods  -  This approach is performant and saves you memory  except for the memory of the newly created methods  even though  using Arrays asList   would take memory in the same way  2  Since arrays don t have methods  as to be read on the side you linked  they can t provide an Iterator instance either  If you really are too lazy to write new classes  you must use an instance of an already existing class that implements Iterable because there is no other way around than instantiating Iterable or a subtype  The ONLY way to create an existing Collection derivative implementing Iterable is to use a loop  except you use anonymous classes as described above  or you instantiate an Iterable implementing class whose constructor allows a primtive type array  because Object   doesn t allow arrays with primitive type elements  but as far as I know  the Java API doesn t feature a class like that The reason for the loop can be explained easily for each Collection you need Objects and primtive datatypes aren t objects  Objects are much bigger than primitive types so that they require additional data which must be generated for each element of the primitive type array  That means if two ways of three  using Arrays asList T     or using an existing Collection  require an aggregate of objects  you need to create for each primitive value of your int   array the wrapper object  The third way would use the array as is and use it in an anonymous class as I think it s preferable due to fast performance  There is also a third strategy using an Object as argument for the method where you want to use the array or Iterable and it would require type checks to figure out which type the argument has  however I wouldn t recommend it at all as you usually need to consider that the Object hasn t always the required type and that you need seperate code for certain cases  In conclusion  it s the fault of Java s problematic Generic Type system which doesn t allow to use primitive types as generic type which would save a lot of code by using simply Arrays asList T      So you need to program for each primitive type array  you need  such a method  which basically makes no difference to the memory used by a C   program which would create for each used type argument a seperate method

User · Answer

While a similar answer has already been sort of posted  I think the reason to use the new PrimitiveIterator OfInt was not clear  A good solution is to use Java 8 PrimitiveIterator since it s specialized for primitive int types  and avoids the extra boxing unboxing penalty        int   arr    1 2 3          If you use Iterator lt Integer gt  here as type then you can t get the actual benefit of being able to use nextInt   later     PrimitiveIterator OfInt iterator   Arrays stream arr  iterator        while  iterator hasNext              System out println iterator nextInt                Use nextInt   instead of next   here to avoid extra boxing penalty         Ref  https   doc bccnsoft com docs jdk8u12-docs api java util PrimitiveIterator OfInt html

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I had the same problem and solved it like this   final YourType   yourArray        return new Iterable lt YourType gt        public Iterator lt YourType gt  iterator          return Iterators forArray yourArray        Iterators is a Google guava utility         The iterator itself is a lazy UnmodifiableIterator but that s exactly what I needed

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Guava provides the adapter you want as Int asList     There is an equivalent for each primitive type in the associated class  e g   Booleans for boolean  etc   int foo      1 2 3 4 5 6 7 8 9 0   Iterable lt Integer gt  fooBar   Ints asList foo   for Integer i   fooBar        System out println i       The suggestions above to use Arrays asList won t work  even if they compile because you get an Iterator lt int   gt  rather than Iterator lt Integer gt    What happens is that rather than creating a list backed by your array  you created a 1-element list of arrays  containing your array

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In java8 IntSteam stream can be boxed to stream of Integers   public static Iterable lt Integer gt  toIterable int   ints        return IntStream of ints  boxed   collect Collectors toList         I think performance matters based on the size of the array

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With Java 8  you can do this   final int   arr    1  2  3   final PrimitiveIterator OfInt i1   Arrays stream arr  iterator    final PrimitiveIterator OfInt i2   IntStream of arr  iterator    final Iterator lt Integer gt  i3   IntStream of arr  boxed   iterator

[java] Convert Java Array to Iterable

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