Menu
In Java when casting from an Object to other types, why does the second line produce a warning related to the cast, but the first one doesn't?
void a(Object o) {
Integer i = (Integer) o;
List<Integer> list = (List<Integer>) o;
}
/*Type safety: Unchecked cast from Object to List<Integer>*/
380
The Java compiler won't let you cast a generic type across its type parameters because the target type, in general, is neither a subtype nor a supertype.
The Java Generics programming is introduced in J2SE 5 to deal with type-safe objects. It makes the code stable by detecting the bugs at compile time. Before generics, we can store any type of objects in the collection, i.e., non-generic. Now generics force the java programmer to store a specific type of objects.
Type Casting in Java – An Introduction Here, the concept of Type casting in Java comes into play. Type Casting is a feature in Java using which the form or type of a variable or object is cast into some other kind or Object, and the process of conversion from one type to another is called Type Casting.
Generics Work Only with Reference Types: When we declare an instance of a generic type, the type argument passed to the type parameter must be a reference type. We cannot use primitive data types like int, char. Test<int> obj = new Test<int>(20);
It's because the object won't really be checked for being a List<Integer> at execution time due to type erasure. It'll really just be casting it to List. For example:
List<String> strings = new ArrayList<String>();
strings.add("x");
Object o = strings;
// Warning, but will succeeed at execution time
List<Integer> integers = (List<Integer>) o;
Integer i = integers.get(0); // Bang!
See Angelika Langer's Java Generics FAQ for more info, particularly the type erasure section.
161
Jon's answer is the right one, but occasionally you can't get around that warning (like when you're working with a legacy API). In those cases you can suppress the warning like so:
@SuppressWarnings("unchecked")
List<Integer> list = (List<Integer>) someApiThatReturnsNonGenericList();
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
