Initialize ArrayList

Question

Why I can initialize ArrayList, like this:

ArrayList x = new ArrayList(Arrays.asList(1,2));

But got Error whe

Answer 1

That's because 1 and 2 are ints and Arrays.asList(1, 2) creates a List.

And the copy constructor of ArrayList requires the argument to be of the same generic type.

You have several options but the simplest one is to change the ints into longs by adding a L suffix:

List x = new ArrayList(Arrays.asList(1L, 2L));

Note that with Java 9 you can also write:

List x = List.of(1L, 2L);