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Thread: Do Class Casts Work With Arrays?

  1. #1
    Member snowguy13's Avatar
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    Default Do Class Casts Work With Arrays?

    Hello, everyone! I have recently run into a problem with a soccer team manager program I'm writing.

    Here's the code:

       public Player[] getPlayersThatMeetQuery(Player[] playersToTest)
       {
     
          ArrayList<Player> validPlayers = new ArrayList<Player>();
     
          for(Player p : playersToTest)
          {
     
             if(this.isMetBy(p)) //Ignore this method; something I've written; not the problem
             {
     
                validPlayers.add(p);
     
             }
          }
     
          return (Player[])validPlayers.toArray(); //Here's the error. Class casting with arrays?
     
       }

    And here's the error:

    Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [LObjects.Player;
    	at Objects.PlayerQuery.getPlayersThatMeetQuery(PlayerQuery.java:193)
    	at Objects.PlayerQueryTest.main(PlayerQueryTest.java:36)
    Java Result: 1

    Don't worry about the Player class that I have created; the problem lies not in that class. Think of it as any other object.

    validPlayers is an ArrayList of players, yet when I try to class cast its toArray() method to a Player[], I get the above error. Does anyone see my error? Or is it illegal in Java to class cast with arrays?
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  2. #2
    Super Moderator pbrockway2's Avatar
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    Default Re: Do Class Casts Work With Arrays?

    You'll get a runtime exception if you try and cast an array of Object to an array of Player. That's why Collection provides the toArray(T[]) method as well. Try

    return validPlayers.toArray(new Player[0]);

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    snowguy13 (February 19th, 2012)

  4. #3
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    Default Re: Do Class Casts Work With Arrays?

    Okay. Hm... That organization strikes me as odd...

    But, anyway, thank you!
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    Super Moderator Sean4u's Avatar
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    Default Re: Do Class Casts Work With Arrays?

    Quote Originally Posted by snowguy13 View Post
    Okay. Hm... That organization strikes me as odd...
    It is odd, it creates an object which is never used to prompt the method call to instantiate one just like it, only the right size. You can also write:
    return validPlayers.toArray(new Player[validPlayers.size()]);
    ... although if you're writing a lot of methods that take one collection of objects and returns a smaller collection of objects filtered from it, you could consider using the collections (Collection, Set, List etc) interface instead.

    Another alternative might be to return an Iterator which *doesn't* create a new collection (or array), but returns objects from the original collection skipping those that fail the filter. Here's an example, just for the sake of typing some Java:

    package com.javaprogrammingforums.domyhomework;
     
    import java.util.*;
     
    public class IteratorFilter
    {
      public static void main(String args[])
      {
        List<Integer> ints = new ArrayList<Integer>();
        for (int i = 0; i < 10; i++)
          ints.add(new Integer(i));
        OddIterator odds = new OddIterator(ints);
        while (odds.hasNext())
          System.out.println(odds.next());
      }
     
      static class OddIterator extends ProxyIterator<Integer>
      {
        public OddIterator(Collection<Integer> theIntegers)
        { super(theIntegers); }
        protected boolean isValid(Integer value)
        { return value % 2 == 1; } // odd?
      }
     
      static abstract class ProxyIterator<V> implements Iterator<V>
      {
        private final Iterator<V> backingIterator;
        private V next = null;
        protected ProxyIterator(Collection<V> collection)
        { backingIterator = collection.iterator(); }
        protected abstract boolean isValid(V value);
        public void remove()
        { throw new UnsupportedOperationException("No ProxyIterator.remove()"); }
        public boolean hasNext()
        {
          if (next != null)
            return true;
          while (backingIterator.hasNext())
          {
            next = backingIterator.next();
            if (isValid(next))
              return true;
          }
          next = null;
          return false;
        }
        public V next()
        {
          if (hasNext())
          {
            V wasNext = next;
            next = null;
            return wasNext;
          }
          // do whatever backing iterator does when finished
          // (throw a NoSuchElementException)
          return backingIterator.next();
        }
      }
    }

    ProxyIterator is a bit ugly / kludge-y, but it works as a filter on a collection of data without creating a new collection.

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    snowguy13 (February 19th, 2012)

  7. #5
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    Default Re: Do Class Casts Work With Arrays?

    That's a very interesting idea. I'd have to read more into Iterators (though I think I get the gist of them). Thank you!
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    Default Re: Do Class Casts Work With Arrays?

    Quote Originally Posted by Sean4u View Post
    It is odd
    I agree.

    You'd think that an ArrayList<Foo> would have commonsense enough to return a Foo[] when told to convert itself toArray(). But all knowledge of its Foo-ness is erased away at runtime. That's just how generics is (are?) in Java.

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    Default Re: Do Class Casts Work With Arrays?

    You'd think that an ArrayList<Foo> would have commonsense enough to return a Foo[] when told to convert itself toArray().
    That's exactly how I feel! At least the get() method returns the right object type.
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    Default Re: Do Class Casts Work With Arrays?

    It's instructive to (try and) create such a method yourself in order to see what happens. Something along the lines of:

    public class Foo<E> {
        private E data;
     
            // get() is fine...
        public E get() {return data;}
        public void set(E e) {data = e;}
     
            // ... but this one is not: "Cannot create a generic array of E"
        public E[] toArray() {return new E[]{data};}
    }

    Angelika Langer's excellent web site on generics has a discussion of what would go wrong.

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    snowguy13 (February 23rd, 2012)

  12. #9
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    Default Re: Do Class Casts Work With Arrays?

    Angelika Langer's excellent web site on generics has a discussion of what would go wrong.
    Ah, so it basically says that the traces of parameterized classes are lost declaring an instance of the class; if I declared an array of Foo<Integer>, the program still wouldn't be able to pick up an error if I tried to enter a Foo<String> in the array. In other words, the array only notices the Foo and doesn't pay enough attention to catch the differences in Foo's class parameter.

    Very interesting article, thanks pbrockway2!
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