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subList for arrays - ClassCastException when casting to Generic Type

Answer 1

+4 A:

T[] isn't Object[] - try String[] ar = new Object[1];

You'd better use System.arraycopy(..). And the utility method Arrays.copyOf uses Array.newInstance(newType.getComponentType(), newLength); to create a new array, so combine these two methods to achieve your goal

Update: As noted in the comments, and in BalusC's answer, there is already Arrays.copyOfRange(..)

Bozho 2010-06-21 16:01:45

Another option is using `Arrays.copyOfRange(...)`, possibly performing slightly worse than `System.arraycopy(...)`

MarcoS 2010-06-21 16:11:33

@MarcoS: `Arrays.copyOfRange(...)` is only available in Java 1.6+

newacct 2010-06-22 02:27:41

@newacct: you're right, but the question did not state any restriction on Java version

MarcoS 2010-06-22 06:20:35

Answer 2

+5 A:

I suspect the problem is you are returning an Object[]. Instead of using new Object[size], you need to create a new array of the correct type. One way to do that is to use Array.newInstance(Class<?> componentType, int length).

Something like:

<T> T[] subArray(int begin, int end, T[] array) {
    int size = end - begin;
    T[] newArray = (T[])Array.newInstance(array.getClass().getComponentType(), size);
    for (int i = 0; i < size; i++) {
        newArray[i] = array[begin + i];
    }
    return  newArray;
}

ILMTitan 2010-06-21 16:02:46

How come the array does know what is its component type?

OscarRyz 2010-06-21 16:19:48

@Sup: the exact `Class` is always known during runtime.

BalusC 2010-06-21 17:36:24

Answer 3

+1 A:

Yes, it's quite straightforward. You're creating an Object[] array, and then casting it to T[]. If T is not Object, this cast will fail.

Generics and arrays don't really mix very well. As ILMTitan says, you can use Array.newInstance, though then you'll have to have the caller pass an instance of Class<T> into your method. Although a better approach would be to use System.arrayCopy as this seems to be exactly what you're doing here.

Andrzej Doyle 2010-06-21 16:05:28

Answer 4

+5 A:

Use Arrays#copyOfRange()

<T> T[] subArray(int begin, int end, T[] array) {
    return Arrays.copyOfRange(array, begin, end);
}

Heck, it even makes the whole subArray() method superfluous :) It only requires JDK 1.6. If you're not on it yet for some reason, here's an extract of relevance:

public static <T> T[] copyOfRange(T[] original, int from, int to) {
    int newLength = to - from;
    if (newLength < 0) throw new IllegalArgumentException(from + " > " + to);
    T[] copy = (T[]) Array.newInstance(original.getClass().getComponentType(), newLength);
    System.arraycopy(original, from, copy, 0, Math.min(original.length - from, newLength));
    return copy;
}

BalusC 2010-06-21 16:09:46

how I tend to forget these useful methods :) (+1)

Bozho 2010-06-21 16:11:39

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