未选中的 Cast 警告 - 显示类型参数但不显示具体类型？

Question

Consider the following snippet:

        Integer a = Integer.valueOf(23);
        Double d = (Double) (Number) a; //not unchecked cast


        List<String> stringList = new ArrayList<>();
        List<Integer> integerList = (List<Integer>)(List<?>)stringList; //unchecked cast

• From what I understand - whenever we convert Supertype -> Subtype Unchecked cast is flagged as the compiler does not know until the runtime if the type represented by the Supertype will ever match the SubType .
• Then over here - why is (Double) (Number)a not being flagged as the Unchecked Cast ?

Answer 1

Unchecked cast is flagged as the compiler does not know until the runtime if the type represented by the Supertype will ever match the SubType .

That is incorrect. Not even the runtime knows whether your list is a ArrayList<String> or a ArrayList<Integer> . As far as the runtime is concerned, your list is an ArrayList (This is because of type erasure ). This is why casting a List<?> to a List<String> cannot be checked by the runtime. The runtime does not know what a List<String> is - it only knows about List . To the runtime, no checks are needed, since you are just casting from List to List , which always succeeds. In fact, nothing is done for this cast at runtime - it is a completely unchecked cast.

As a result, this code runs without throwing an exception:

List<String> stringList = new ArrayList<>();
stringList.add("foo");
List<Integer> integerList = (List<Integer>)(List<?>)stringList;
System.out.println(integerList.size());

It will throw an exception, however, if you did:

List<String> stringList = new ArrayList<>();
stringList.add("foo");
List<Integer> integerList = (List<Integer>)(List<?>)stringList;
System.out.println(integerList.get(0) - 1);

Now, you are getting an integer out of the list, and doing some integer-specific operations to it. But the list contains "foo" , which isn't an integer. The compiler inserts an implicit cast to cast integerList.get(0) to Integer , and that cast failed. Note that this is a checked cast, because the runtime knows about the type Integer .

The cast from Number to Double is also checked, because the runtime knows about Double .

Side note: there are also "partially unchecked casts", which are things like casting from Object to ArrayList<String> . The runtime can check whether the Object is an ArrayList , but it cannot check whether it is an array list of strings .

See all the rules for what is checked and what is unchecked here .