How to check if Dart List contains certain object instance? - flutter

Question

I have a Dart List<Book> bookList; Somewhere in the code it has been filled with books.
How can I check if the bookList contains an instance of a Book?
I tried this
if(bookList.contains(Book))
But it didn't work.

Answer 1

You can use is to check the type of the List.
if (bookList is List<Book>) {
print("Yes");
}

Answer 2

You could test the following:
if (bookList.every((item) => item != null && item is Book)) {
...
}
If your bookList is by design a List, testing for nullity is enough:
if (bookList.every((item) => item != null)) {
...
}
If you want to prevent null elements inside the list, you should enforce it also when you add/update element to your list.

Answer 3

First of, you annotated the type of your bookList with List<Book> meaning that any instance should be a Book or null when the list is not empy.
As many others pointed out already, the is is used to test if an object has a specified type. In your case that does not fully solve your problem. If your list contains null, the code
if (bookList is List<Book>) {
print("Yes");
}
will produce Yes. You have to check it like so:
class Book {
String title;
Book(this.title);
}
void main() {
List<Book> bookList = [
Book('foo'),
null,
];
if ((bookList?.length != 0 ?? false) && (!bookList?.contains(null) ?? false) && bookList is List<Book>) {
print("Yes");
} else {
print("No");
}
}
to provide null-safety.
EDIT Updated my answer to be null safe towards bookList being null.
Check the docs:
is test operator: https://dart.dev/guides/language/language-tour#type-test-operators
null-safety: https://dart.dev/null-safety/understanding-null-safety