How it is possible to add element of List in List of Integer in Java

Question

I am confused with this behavior of Array List. Can someone please explain this

List list = new ArrayList();
list.add(1);
list.add("test");

List<Integer> integerList = new ArrayList<>();
integerList.add(123);
integerList.add(456);
integerList.addAll(list);

System.out.println(integerList);

How can I add String in Integer arrayList Can someone please share some resource to understand these things?

It works because Java doesn't check the list at runtime, and list was declared with a raw type, which basically lets you bypass compile time checks for generics-related stuff. You should have gotten a warning, though. — user
– user, Commented Sep 30, 2020 at 17:42
Thanks for the comment @user. But I didn't see any warning there. I am very confused with this. Can you please explain this is more details. — Saurav Dudeja
– Saurav Dudeja, Commented Sep 30, 2020 at 17:46
I get a warning on this line of your code: integerList.addAll(list); Are you saying that you don't get a warning for that line? — Abra
– Abra, Commented Sep 30, 2020 at 17:57
@SauravDudeja Here, the details: What is a raw type and why shouldn't we use it? — akuzminykh
– akuzminykh, Commented Sep 30, 2020 at 18:04

martinspielmann · Accepted Answer · 2020-09-30 17:47:43Z

1

The reason is discussed here: Are generics removed by the compiler at compile time

Generics are checked by the compiler, but not afterwards during runtime.

As @user mentioned, your compiler/IDE will most likely show a warning e.g.

List is a raw type. References to generic type List should be parameterized

answered Sep 30, 2020 at 17:47

martinspielmann

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2 Comments

user4910279 Over a year ago

Why does the compiler warn integerList.addAll(list);, not an error?

martinspielmann Over a year ago

Because the first ArrayList is declared with a raw type. See stackoverflow.com/a/2770692/12323248. If the first line would be typed instead like List<Integer> list = new ArrayList<>(); or List<String> list = new ArrayList<>(); then the compiler would safely indicate an error as soon you try to add another type.

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