Casting to a Class which is determined at run-time

问题

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.

---

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.

回答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.

回答2:

So why not use

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

Note. Modified the code from above.

来源：https://stackoverflow.com/questions/380952/casting-to-a-class-which-is-determined-at-run-time