public class AnonymousAssign {

    public static void main(String[] args) {
        new AnonymousAssign().testAnonymousAssign();
    }

    public void testAnonymousAssign() {
        var list1 = new ArrayList<String>() {{
            add("");
        }};
        list1 = new ArrayList<String>(); // this line produces an error
        System.out.println(list1.getClass()); // print `class AnonymousAssign$1`

    }
}

 error: incompatible types: ArrayList<String> cannot be converted to <anonymous ArrayList<String>>
        list1 = new ArrayList<String>();

Why is var assign make such type inference and not allow a seemingly straightforward and reasonable value reassignment ?

Some more infos

System.out.println(list1.getClass()); would print class AnonymousAssign$1

but not a class java.util.ArrayList

Below is a tip from IntelliJ IDEA's 2022.3, it isn't meaningful.

CodePudding user response：

The expression

new ArrayList<String>() {{
        add("");
    }};

creates an instance of an anonymous sub-class of ArrayList<String>. Therefore the inferred type of it is a sub-class of ArrayList, so you can't assign an instance of ArrayList to it later (unless that instance was also of the same sub-class of ArrayList, which is not possible, since it's an anonymous sub-class).

If you want the type of the variable to be ArrayList, you'll have to explicitly declare it so:

ArrayList<String> list1 = new ArrayList<String>() {{
        add("");
    }};

or even better:

List<String> list1 = new ArrayList<String>() {{
        add("");
    }};

Now the second assignment will work.

CodePudding user response：

This error message is indicating that the variable "list1" is being assigned a new instance of an ArrayList of Strings, but it has been declared as a more specific type of ArrayList (an anonymous one) which is not compatible with the type of ArrayList being created. To fix this error, you can either change the declaration of "list1" to match the type of ArrayList being created, or you can change the type of ArrayList being created to match the declared type of "list1".