How to create a generic array

Question

I don t understand the connection between generics and arrays   I can create array reference with generic type   private E   elements    GOOD   But can t create array object with generic type   elements   new E 10     ERROR   But it works   elements    E    new Object 10     GOOD

User · Answer

Here is the implementation of LinkedList lt T gt  toArray T      public  lt T gt  T   toArray T   a        if  a length  lt  size          a    T   java lang reflect Array newInstance                              a getClass   getComponentType    size       int i   0      Object   result   a      for  Node lt E gt  x   first  x    null  x   x next          result i      x item       if  a length  gt  size          a size    null       return a      In short  you could only create generic arrays through Array newInstance Class  int  where int is the size of the array

User · Answer

checked    public Constructor Class lt E gt  c  int length         elements    E    Array newInstance c  length       or unchecked    public Constructor int s        elements   new Object s

User · Answer

You should not mix-up arrays and generics  They don t go well together  There are differences in how arrays and generic types enforce the type check  We say that arrays are reified  but generics are not  As a result of this  you see these differences working with arrays and generics   Arrays are covariant  Generics are not   What that means  You must be knowing by now that the following assignment is valid   Object   arr   new String 10     Basically  an Object   is a super type of String    because Object is a super type of String  This is not true with generics  So  the following declaration is not valid  and won t compile   List lt Object gt  list   new ArrayList lt String gt        Will not compile    Reason being  generics are invariant   Enforcing Type Check   Generics were introduced in Java to enforce stronger type check at compile time  As such  generic types don t have any type information at runtime due to type erasure  So  a List lt String gt  has a static type of List lt String gt  but a dynamic type of List   However  arrays carry with them the runtime type information of the component type  At runtime  arrays use Array Store check to check whether you are inserting elements compatible with actual array type  So  the following code   Object   arr   new String 10   arr 0    new Integer 10     will compile fine  but will fail at runtime  as a result of ArrayStoreCheck  With generics  this is not possible  as the compiler will try to prevent the runtime exception by providing compile time check  by avoiding creation of reference like this  as shown above   So  what s the issue with Generic Array Creation   Creation of array whose component type is either a type parameter  a concrete parameterized type or a bounded wildcard parameterized type  is type-unsafe   Consider the code as below   public  lt T gt  T   getArray int size        T   arr   new T size       Suppose this was allowed for the time being      return arr      Since the type of T is not known at runtime  the array created is actually an Object    So the above method at runtime will look like   public Object   getArray int size        Object   arr   new Object size       return arr      Now  suppose you call this method as   Integer   arr   getArray 10     Here s the problem  You have just assigned an Object   to a reference of Integer    The above code will compile fine  but will fail at runtime   That is why generic array creation is forbidden   Why typecasting new Object 10  to E   works   Now your last doubt  why the below code works   E   elements    E    new Object 10     The above code have the same implications as explained above  If you notice  the compiler would be giving you an Unchecked Cast Warning there  as you are typecasting to an array of unknown component type  That means  the cast may fail at runtime  For e g  if you have that code in the above method   public  lt T gt  T   getArray int size        T   arr    T   new Object size               return arr      and you call invoke it like this   String   arr   getArray 10     this will fail at runtime with a ClassCastException  So  no this way will not work always   What about creating an array of type List lt String gt      The issue is the same  Due to type erasure  a List lt String gt    is nothing but a List    So  had the creation of such arrays allowed  let s see what could happen   List lt String gt    strlistarr   new List lt String gt  10       Won t compile  but just consider it Object   objarr   strlistarr        this will be fine objarr 0    new ArrayList lt Integer gt        This should fail but succeeds    Now the ArrayStoreCheck in the above case will succeed at runtime although that should have thrown an ArrayStoreException  That s because both List lt String gt    and List lt Integer gt    are compiled to List   at runtime    So can we create array of unbounded wildcard parameterized types   Yes  The reason being  a List lt   gt  is a reifiable type  And that makes sense  as there is no type associated at all  So there is nothing to loose as a result of type erasure  So  it is perfectly type-safe to create an array of such type   List lt   gt    listArr   new List lt   gt  10   listArr 0    new ArrayList lt String gt         Fine  listArr 1    new ArrayList lt Integer gt        Fine   Both the above case is fine  because List lt   gt  is super type of all the instantiation of the generic type List lt E gt   So  it won t issue an ArrayStoreException at runtime  The case is same with raw types array  As raw types are also reifiable types  you can create an array List     So  it goes like  you can only create an array of reifiable types  but not non-reifiable types  Note that  in all the above cases  declaration of array is fine  it s the creation of array with new operator  which gives issues  But  there is no point in declaring an array of those reference types  as they can t point to anything but null  Ignoring the unbounded types    Is there any workaround for E     Yes  you can create the array using Array newInstance   method   public  lt E gt  E   getArray Class lt E gt  clazz  int size         SuppressWarnings  unchecked       E   arr    E    Array newInstance clazz  size        return arr      Typecast is needed because that method returns an Object  But you can be sure that it s a safe cast  So  you can even use  SuppressWarnings on that variable

User · Answer

Problem is that while runtime generic type is erased so new E 10  would be equivalent to new Object 10     This would be dangerous because it would be possible to put in array other data than of E type  That is why you need to explicitly say that type you want by either   creating Object array and cast it to E   array  or  useing Array newInstance Class componentType  int length  to create real instance of array of type passed in componentType argiment

[java] How to create a generic array?

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