Why does Java ArrayList use per-element casting instead of per-array casting?

Question

What happens inside Java\'s ArrayList (and probably many other classes) is that there is an internal Object[] array = new Object[n];, to w

Answer 1

Whenever an element is read from it, a cast return (T) array[i]; is done. So, a cast on every single read.

Generic is a compile time check. At runtime the type T extends is used instead. In this case T implicitly extends Object so what you have at runtime is effectively.

return (Object) array[i];

or

return array[i];

Wouldn't it be more logical and also slightly faster to just create a

T[] array = (T[]) new Object[n]

Not really. Again at runtime this becomes

Object[] array = (Object[]) new Object[n];

or

Object[] array = new Object[n];

What you are really fishing for is

T[] array = new T[n];

except this doesn't compile, mostly because T isn't known at runtime.

What you can do is

private final Class tClass; // must be passed in the constructor

T[] array = (T[]) Array.newInstance(tClass, n);

only then will the array actually be the type expected. This could make reads faster, but at the cost of writes. The main benefit would be a fail fast check, i.e. you would stop a collection being corrupted rather than wait until you detect it was corrupted to throw an Exception.