Java泛型：创建扩展Throwable的类对象集合

Question

Why does the first line work, but the second one doesn't?

Collection<Class<? extends Throwable>> exs =
    new ArrayList<Class<? extends Throwable>>() {{ add(MyOwnException.class); }};
Collection<Class<? extends Throwable>> exs = Arrays.asList(MyOwnException.class);

Answer 1

The reason it's an error is that java is inferring the wrong type, but you can make it compile, without casting , by specifying the type in the call to the typed method Arrays.asList() :

Collection<Class<? extends Throwable>> exs 
    = Arrays.<Class<? extends Throwable>>asList(Exception.class); // compiles

Without specifying the type, java infers the element type of the collection to be Class<Exception> , which is not assignable to Collection<Class<? extends Throwable>> Collection<Class<? extends Throwable>> .

Remember with generics that if B extends A , List<B> does not extend List<A> .

Answer 2

Generics are very tricky.

In this case Arrays.asList will return a List<Class<MyOwnException>> , which is not the same than List<Class<? extends Throwable>> List<Class<? extends Throwable>> .

Their behavior will differ since the type List<Class<? extends Throwable>> List<Class<? extends Throwable>> would allow you to add objects that extend Throwable but List<Class<MyOwnException>> only accept objects of type MyOwnException.

Answer 3

The first line works because the type of the element of the right side of the assignement (of the ArrayList ) is Class<? extends Throwable> Class<? extends Throwable> that allows to add any kind of Class<? extends Throwable> Class<? extends Throwable> object, including your MyOwnException . You could also add to the first line Collection an Exception.class or a NullPointerException.class , etc.

But the second line is much stricter: you have a Collection<Class<? extends Throwable>> Collection<Class<? extends Throwable>> that might contain a Class<Exception> , a Class<NullPointerException> , etc. and you want to put in place a Collection<Class<MyOwnException>> that don't allows for Class<Exception> , etc.

Answer 4

Arrays.asList(MyOwnException.class) is inferred to have type List<Class<MyOwnException>> , which is not compatible with List<Class<? extends Throwable>> List<Class<? extends Throwable>> since the type parameters are different.

If you put a wildcard at the first-level, it should work:

Collection<? extends Class<? extends Throwable>> exs = Arrays.asList(MyOwnException.class);

Answer 5

If the second example were to work, it would allow for code like this:

Collection<Class<? extends Throwable>> exs = Arrays.asList(MyOwnException.class);
exs.add(IOException.class);

See the problem? Arrays.asList() would return a List<Class<MyOwnException>> , and then you'd be trying to insert a Class<IOException> into that. That's obviously a type mismatch.