Java Generics Cast Error

Question

corresponding to the title, my problem is to get the array-length of an generic array, which is inside an instance of an class, which has been initialized with this generic type. When i want to get the length of my array (ak.array.length), java throws this error:

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String; at Testclass.main(Testclass.java:15)

So where is my fault in this code? Thanks in advance

public class Testclass {

    static class ArrayKeeper<T> {
        protected T[] array;

        public ArrayKeeper() {
            array = (T[])new Object[5];
        }
    }

    public static void main(String[] args) {
        //create a new instance of inner class ArrayKepper (type String)
        Testclass.ArrayKeeper<String> ak = new Testclass.ArrayKeeper<String>();
        System.out.println(ak.array.length); //cast error here.
    }
}

Óscar López · Accepted Answer · 2012-05-22 17:49:37Z

4

The problem is in this line:

array = (T[]) new Object[5];

That's not the way to create a generic array, you're creating an Object[] and not a String[] as you intended. You can not create an Object[] and then cast it to any type you want, you have to create an array of the correct class. Try this instead:

public ArrayKeeper(Class<T> clazz, int size) {
    array = (T[]) Array.newInstance(clazz, size);
}

And then, when you need to instantiate an ArrayKeeper do this:

Testclass.ArrayKeeper<String> ak =
    new Testclass.ArrayKeeper<String>(String.class, 5);

edited May 22, 2012 at 17:49

answered May 22, 2012 at 16:37

Óscar López

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8 Comments

Louis Wasserman Over a year ago

Meh. Better than the reflection-based approach is just to hide the casts with an API. Look at e.g. the implementation of ArrayList in Java: it's backed by a plain old Object[] and objects are cast on calls to get(int) and the like. Just avoid exposing the casts, and you'll be set.

Louis Wasserman Over a year ago

Reflection is slow and requires you to pass around and track lots of perfectly redundant Class objects. The best option of all is probably to use a List instead of an array, but failing that, cluttering your API with passing around Class objects is just awkward.

Óscar López Over a year ago

@LouisWasserman Reflection is not as slow as it used to be (that's a common misconception), and besides, how do you know if this is going to be slow if you don't know the intended usage, and have not profiled it yet? W.r.t. passing around Class objects, that's a very common idiom, in use all over the place in Java's API itself.

Louis Wasserman Over a year ago

In what parts of the Java APIs? Nowhere in the collections framework, albeit with the notable (and deserving) exception of EnumMap and EnumSet. I'll...grant that reflection is much faster than it used to be, and I believe the Array.newInstance idiom actually ends up getting used in AbstractCollection.toArray(T[]). But...if you insist, here's another argument: what if T itself is a generic type? The approach you describe in this answer stops you from using a generic T.

Óscar López Over a year ago

@LouisWasserman read the FAQ first, please.

|

Louis Wasserman · Accepted Answer · 2012-05-22 16:35:04Z

2

I think you need to implement the length getter in ArrayKeeper, just because you can't cast an Object[] to a String[], and calling ak.array does try to do that cast.

answered May 22, 2012 at 16:35

Louis Wasserman

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2 Comments

Maximilian Hils Over a year ago

For my understanding: Doesn't array = (T[])new Object[5]; perform a cast already?

Louis Wasserman Over a year ago

Nope. Due to type erasure at runtime, no actual casting occurs at that point. (After type erasure, T becomes Object.)

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