ArrayStoreException thrown when converting HashSet

Question

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• How to convert int[] into List<Integer> in Java? 19 answers

I have code that looks like this:

    int[] ho = new int[10];
    ho[0]= 1;
    ho[2]= 1;
    ho[4]= 5;

    HashSet<Integer> hs = new HashSet(Arrays.asList(ho));

    Integer[] a = hs.toArray(new Integer[hs.size()]);

This code makes perfect sense to me, but it throws an ArrayStoreException when I run it. Why is that? The HashSet is a set of Integers, and so is the output array.

Answer 1

The problem is that you are skipping generic types so you don't see the error and Java is not able to reject your code even if it's wrong.

The problem is that the signature of Arrays::asList is asList(T... values), but T can't be a primitive so the variadic arguments are collapsed into an int[] (which is now an object) and Arrays.asList returns a List<int[]>.

Then pass it to a generic HashSet constructor which then accepts a Collection<Object> without any problem, and assign it to an HashSet<Integer>, with the compiler warning you about using raw types.

Finally you try to assign the elements in the hash set (which are of type int[]) to elements in an Integer[] array causing the exception, that's like doing

Integer[] data = new Integer[5];
data[0] = new int[] {1, 2, 3};

which is wrong but Java can't realize it at compile time.

If you had constructed the HashSet through new HashSet<> then Java compiler would have risen an error. You can solve the issue by passing an Integer[] to asList method such that it's correctly treated as variadic arguments.

Answer 2

ArrayStoreException: "if the runtime type of the specified array is not a supertype of the runtime type of every element in this set"

ArrayStoreException is thrown to indicate that an attempt has been made to store the wrong type of object into an array of objects.

Try switching int[] to Integer[]