Why would I use java.lang.Class.cast [duplicate]

Answer 1

class.cast is designed for generics type.

When you construct a class with generic parameter T, you can pass in a Class. You can then do the cast with both static and dynamic checking, which (T) does not give you. It also doesn't produce unchecked warnings, because it is checked (at that point).

Answer 2

A "cast" in Java, e.g. (Number)var, where the thing inside the parentheses is a reference type, really consists of two parts:

• Compile time: the result of the cast expression has the type of the type you cast to
• Run time: it inserts a check operation, which basically says, if the object is not an instance of that class, then throw a ClassCast Exception (if the thing you're casting to is a type variable, then the class it checks would be the lower bound of the type variable)

To use the syntax, you need to know the class at the time you write the code. Suppose you don't know at compile-time what class you want to cast to; you only know it at runtime.

Now you would ask, then what is the point of casting? Isn't the point of casting to turn the expression into the desired type at compile time? So if you don't know the type at compile time, then there is no benefit at compile-time, right? True, but that is just the first item above. You're forgetting the runtime component of a cast (second item above): it checks the object against the class.

Therefore, the purpose of a runtime cast (i.e. Class.cast()) is to check that the object is an instance of the class, and if not, throw an exception. It is roughly equivalent to this but shorter:

if (!clazz.isInstance(var))
    throw new ClassCastException();

Some people have mentioned that Class.cast() also has a nice return type that is based on the type parameter of the class passed in, but that is just a compile-time feature that is provided by a compile-time cast anyway. So for that purpose there is no point in using Class.cast().

Answer 3

In some cases, you only know the type to cast an object to during runtime, and that's when you have to use the cast method.

Answer 4

The common sample for that is when you retrieve from persistent layer a collection of entity referenced with a Class Object and some conditions. The returned collection could contain unchecked objects, so if you just cast it as pointed G_H, you will throw the Cast Exception at this point, and not when the values are accessed.

One example for this is when you retrieve a collection from a DAO that returns an unchecked collection and on your service you iterate over it, this situation can lead to a ClassCastException.

One way to solve it, as you have the wanted class and the unchecked collection is iterate over it and cast it inside the DAO transforming the collection in a checked collection and afterwards return it.

Answer 5

I don't think it's often used exactly as you have shown. Most common use I have seen is where folks using generics are trying to do the equivalent of this:

public static <T extends Number> T castToNumber(Object o) {
    return (T)o;
}

Which doesn't really do anything useful because of type erasure.

Whereas this works, and is type safe (modulo ClassCastExceptions):

public static <T extends Number> T castToNumber(Object o, Class<T> clazz) {
    return clazz.cast(o);
}

---

EDIT: Couple of examples of use from google guava:

• MutableClassToInstanceMap
• Cute use in Throwables#propagateIfInstanceOf, for type safe generic throw spec

Answer 6

Because you might have something this:

Number fooMethod(Class<? extends Number> clazz) {
    return clazz.cast(var);
}