Question

I have a method fetchObjects(String) that is expected to return an array of Contract business objects. The className parameter tells me what kind of business objects I should return (of course this doesn’t make sense in this construed case because I already said I will return Contracts, but it’s basically the situation I have in my real scenario). So I get the set of entries from somewhere and load the class of the collection’s entries (the type of which is specified by className).

Now I need to construct the array to return, so I use Set‘s toArray(T[]) method. Using reflection, I build myself an empty Contracts array. But, this gives me a value of static type Object! So next I need to cast it to the appropriate type, which in this case is Contract[] (see ‘asterisk-underlined’ part in the listing below).

My question is: Is there a way, and how, to cast to Contract[] as I do in the listing, but determining the type of the array elements (Contract) only through className (or entriesType)? In other words, what I’d like to do is basically casting like this: (entriesType[]) valueWithStaticTypeObject, where entriesType be replaced by the class specified through the classname parameter, i.e. Contract.

Is this somehow inherently impossible, or can it be done somehow? Maybe using generics?

package xx.testcode;  import java.util.HashSet; import java.util.Set;  class TypedArrayReflection {      public static void main(String[] args) {         try {             Contract[] contracts = fetchObjects('Contract');             System.out.println(contracts.length);         } catch (ClassNotFoundException e) {}     }      static Contract[] fetchObjects(String className) throws ClassNotFoundException {         Class<?> entriesType = Class.forName('xx.testcode.'+className);         Set<?> entries = ObjectManager.getEntrySet(className);          return entries.toArray(                  (Contract[]) java.lang.reflect.Array.newInstance(                 /********/          entriesType, entries.size()) );     } }  class Contract { } // business object  class ObjectManager {     static Set<?> getEntrySet(String className) {         if (className.equals('Contract'))             return new HashSet<Contract>();         return null; // Error     } } 

Thanks.

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Update: Using the type-safe method toArray, taken from CodeIdol, I updated my fetchObjects method thus:

static Contract[] fetchObjects(String className) throws ClassNotFoundException {     Class<?> entriesType = Class.forName('xx.testcode.'+className);     Set<?> entries = ObjectManager.getEntrySet(className);     return toArray(entries, entriesType); // compile error     // -> 'method not applicable for (Set<capture#3-of ?>, Class<capture#4-of ?>)' }  public static <T> T[] toArray(Collection<T> c, Class<T> k) {     T[] a = (T[]) java.lang.reflect.Array.newInstance(k, c.size());     int i = 0;     for (T x : c)         a[i++] = x;     return a; } 

What do I need to do to get rid of the compiler error quoted in the comment? Do I absolutely have to specify Set<Contract> in the return type of my getEntrySet method so that this can work? Thanks for any pointers.

Answer 1

You may use the class as the parameter rather then the class name.

   static <T extends Contract> T[] buildArray(Class<T> clazz){      ArrayList<T> l=new ArrayList<T>();      return l.toArray((T[]) java.lang.reflect.Array.newInstance(clazz, l.size()));    } 

EDIT: (after read Yang comment)

No, You cannot use generic type with the value of a variable.