Is this valid Java code: List nums = new ArrayList();

Question

Is this valid Java code: List nums = new ArrayList<>();

BlackTom AI · Accepted Answer

Let’s break down the question by examining what the code is attempting to do and what Java generics require. Option 1: Yes. This is incorrect because List uses a primitive type (int) as the generic type, which Java does not permit. Generics in Java require reference types, so you would use List instead. Therefore, claiming it’s valid is wrong. Option 2: No, since List and ArrayList types don't match up. While it’s true that List is an interface and ArrayList is a concrete implementation, the code would not compile for a different reason (the primitive type issue). The mismatch of interface to implementation isn’t the actual blocker here; even if you declared List nums = new ArrayList<>(); it would be fine. So this justification is partially misguided and not the primary defect. Option 3: No, since 'new ArrayList<>();' should instead be 'new ArrayList();'. This is incorrect because Java uses type inference with generics and cannot take a primitive type parameter like int; you should not specify at all. The proper use would be new ArrayList(), or with diamond operator, new ArrayList<>(), but the real prohibition is the primitive type in the generic parameter, not the constructor syntax per se. Option 4: No, because list elements cannot be of primitive type. This is the accurate explanation: Java generics require reference types, so you cannot parameterize a List with int. You must use Integer (the wrapper type) if you want to store integers as a generic type. In summary, the code fails due to the primitive type in the generic parameter, not due to a direct mismatch between List and ArrayList or the specific constructor form. The canonical correction would be: List nums = new ArrayList<>();

Is this valid Java code: List<int> nums = new ArrayList<>();单项选择题

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