Question

Why does the following code throw ArrayStoreException?

double[] a = {2.0,3.4,3.6,2.7,5.6};
int[] b = {2,3,4,5};
System.arraycopy(b,0,a,1,4);

Answer 1

From the docs for System.arraycopy:

Otherwise, if any of the following is
true, an ArrayStoreException is thrown
and the destination is not modified:

[…]

The src argument and dest argument
refer to arrays whose component types
are different primitive types.

That’s exactly the case here – int and double are different primitive types, so the exception is thrown as documented.

The point of arraycopy is that it can work blindingly fast by copying the raw data blindly, without having to apply any conversions. In your case it would have to apply conversions, so it fails.