Using class object as generic type

问题

I'm not entirely sure of how to ask this question, which is also why I'm not sure about the title and so on. Here goes.

Say you have a object Foo foo = new Foo(). Is it possible to write code like new ArrayList<foo.getClass()>(), which would on runtime be equivalent to new ArrayList<Foo>()?

Another, but related question is: Suppose that the class Foo extends Exception. Is it then possible to write something like

try{
    // ...
} catch(foo.getClass() e) {
    //
}

which would translate into

try{
    // ...
} catch(Foo e) {
    //
}

?

Whether this would be horrible to do, is not the important part. However, I would like to hear qualified opinions anyway.

回答1:

No, this is not possible in the Java language specification. Generic parameters are purely a compile-time type-safety mechanism, so defining them at runtime is non-sensical.

Class literals in the code are not interchangeable with Class<T> objects, due to their inherently different roles. The latter only has meaning at runtime, and can vary dynamically.

回答2:

There is a subtle, but existent difference between a type and its Class literal.

The class literal is an object that models a type. Among other things, it provides details about the type's implementation.

When you reference an object you use a type. The same holds for generic parameters. They are represented, on the source code, with a type.

For example, while writing

String foo = "foo";

is legal

"".getClass() foo = "foo"

is not.

I hope this makes sense.

Edit: A usual pattern, when working with Generics, is to pass the Class as an argument.

<T> doSomething(Class<T> clazz);

Edit2: edited according to @millimoose comments

回答3:

Java doesn't support that syntax, but if your goal is to create a Collection whose type is determined by the type of an instance, create a typed method, eg:

public static <T> List<T> createList(T object) {
    List<T> list = new ArrayList<T>();
    list.add(object);
    return list;
}

Whatever type you pass in will determine the type of the List returned. It's not a runtime determination - it's a compile time check, but it's the closest thing to how I interpret your intention.

来源：https://stackoverflow.com/questions/19420617/using-class-object-as-generic-type