The easiest way to transform collection to array

Question

Suppose we have a Collection lt Foo gt   What is the best  shortest in LoC in current context  way to transform it to Foo    Any well-known libraries are allowed   UPD   one more case in this section  leave comments if you think it s worth to create another thread for it   What about transforming Collection lt Foo gt  to Bar   where Bar has constructor with 1 parameter of type Foo i e  public Bar Foo foo

User · Accepted Answer

Where x is the collection   Foo   foos   x toArray new Foo x size

User · Answer

For example  you have collection ArrayList with elements Student class   List stuList   new ArrayList    Student s1   new Student  Raju    Student s2   new Student  Harish    stuList add s1   stuList add s2     now you can convert this collection stuList to Array like this Object   stuArr   stuList toArray                  lt ----- toArray   function will convert collection to array

User · Answer

If you use it more than once or in a loop  you could define a constant  public static final Foo   FOO   new Foo        and do the conversion it like  Foo   foos   fooCollection toArray FOO     The toArray method will take the empty array to determine the correct type of the target array and create a new array for you     Here s my proposal for the update   Collection lt Foo gt  foos   new ArrayList lt Foo gt     Collection lt Bar gt  temp   new ArrayList lt Bar gt     for  Foo foo foos       temp add new Bar foo    Bar   bars   temp toArray new Bar

User · Answer

For the original see doublep answer   Foo   a   x toArray new Foo x size        As for the update    int i   0  Bar   bars   new Bar fooCollection size     for  Foo foo   fooCollection        where fooCollection is Collection lt Foo gt      bars i      new Bar foo

User · Answer

Alternative solution to the updated question using Java 8   Bar   result   foos stream        map x - gt  new Bar x        toArray size - gt  new Bar size

User · Answer

If you use Guava in your project you can use Iterables  toArray    Foo   foos   Iterables toArray x  Foo class

User · Answer

Here s the final solution for the case in update section  with the help of Google Collections    Collections2 transform  fooCollection  new Function lt Foo  Bar gt          public Bar apply  Foo foo            return new Bar  foo            toArray  new Bar fooCollection size        But  the key approach here was mentioned in the doublep s answer  I forgot for toArray method

User · Answer

With JDK 11  an alternate way of converting a Collection lt Foo gt  to an Foo   could be to make use of Collection toArray IntFunction lt T   gt  generator  as   Foo   foos   fooCollection toArray new Foo 0       before JDK 11 Foo   updatedFoos   fooCollection toArray Foo    new      after JDK 11     As explained by  Stuart on the mailing list emphasis mine   the performance of this should essentially be the same as that of the existing Collection toArray new T 0   --     The upshot is that implementations that use Arrays copyOf   are the   fastest   probably because it s an intrinsic        It can avoid zero-filling the freshly allocated array because it knows the    entire array contents will be overwritten   This is true regardless of what   the public API looks like     The implementation of the API within the JDK reads   default  lt T gt  T   toArray IntFunction lt T   gt  generator        return toArray generator apply 0           The default  implementation calls generator apply 0  to get a zero-length array   and then  simply calls toArray T     This goes through the Arrays copyOf      fast path  so it s essentially the same speed as toArray new T 0        Note - Just that the API use shall be guided along with a backward incompatibility when used for code with null values e g  toArray null  since these calls would now be ambiguous because of existing toArray T   a  and would fail to compile

[java] The easiest way to transform collection to array?

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