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What are the potential risks of creating a shared array with varargs in Java?

I found a way to create a shared inline array with varargs and an anonymous inner class:

import java.util.Arrays; import java.util.Objects; public class GenericArrayCreate<C> { public void method() { C[] ans = new Object() { C[] c(int length, C ... cs) { return Arrays.copyOf(cs, length); }}.c(10); System.out.println(ans.length); System.out.println(Objects.toString(ans)); } public static void main(String[] args) { new GenericArrayCreate<Class<? extends Integer>>().method(); } }

The Java compiler does not raise any warnings in this code, and the code check (in IntelliJ) says:

 Unchecked generics array creation for varargs parameter

What is going on here and why does it swear?

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2 answers

Inside, you create an array of "Object" instead of an array of "C"

Try this code:

  public static <C> C[] method(int number) { return new Object() { C[] create(int length, C ... cs) { return Arrays.copyOf(cs, length); } }.create(number); } public static void main(String[] args) { System.out.println(Main.<Integer>method(10)); System.out.println(new Integer[10]); }

[Ljava.lang.Object; @ 6bc7c054

[Ljava.lang.Integer; @ 232204a1

As you can see, they do not match.

This is dangerous because if you run something like this:

  public static void main(String[] args) { Integer[] integerArray1 = Main.<Integer>method(10); Integer[] integerArray2 = new Integer[10]; }

And you will have a ClassCastException

[Ljava.lang.Object; cannot be applied to [Ljava.lang.Integer

If you want to create any array, you need to send a class to build the array, you can do it as follows:

 public static <C> C[] method(Class<C> clazz, int number) { return (C[]) Array.newInstance(clazz, number); } public static void main(String[] args) { Integer[] integerArray1 = Main.<Integer>method(Integer.class, 10); Integer[] integerArray2 = new Integer[10]; }
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Checking the code will notice that since varargs are implemented using an array, and the array does not support generics. But you can suppress this warning by adding the following:

 @SuppressWarnings("unchecked")

The reason why this happens can lead to loss of information. You can learn more about this here .

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