How to convert List elements to it's super type?

Question

Assume I have the following:

class MyType<T> {
    // ...
}

private void test() {
    Set<MyType<? extends Serializable>> in;
    Set<MyType<Serializable>> out; 
}

then the following assignment is illegal: out = new HashSet<>(in); also out.addAll(in) is illegal, because it cannot be implicitly converted. The following however is completely legal:

out = new HashSet<>();
for (MyType<? extends Serializable> s : in) {
    out.add((MyType<Serializable>) s);
}

Is there a more elegant way to do this in Java 7 without using an add-loop or dirty/uncheckable casts?

Answer 1

Is there a more elegant way to do this in Java 7 without using an add-loop or dirty/uncheckable casts?

No, because you're doing something which isn't type safe.

You're trying to treat a MyType<? extends Serializable> MyType<? extends Serializable> as a MyType<Serializable> - which it's not: a MyType<Serializable> is a MyType<? extends Serializable> MyType<? extends Serializable> , not the other way around.

For example, a MyType<String> is a MyType<? extends Serializable> MyType<? extends Serializable> , but it's not a MyType<Serializable> .

Unless you can provide a method on the MyType to do the cast (and you'll have an unchecked cast there), or otherwise provide a method to give you a MyType<Serializable> from a MyType<? extends Serializable> MyType<? extends Serializable> , you will just have to live with the unchecked cast elsewhere.

---

You haven't defined any methods on MyType , but let's say it's got a getter and a setter:

class MyType<T> {
  T field;
  void set(T field) { this.field = field; }
  T get() { return field; }
}

Now, let's assume that you've got this code:

MyType<String> str = new MyType<>();

If you cast this to a MyType<Serializable> , then you can invoke the setter with anything that implements Serializable :

MyType<Serializable> ser = (MyType<Serializable>) str;
ser.set(Integer.valueOf(0));

You're in trouble now, because:

String got = str.get();

will throw a ClassCastException .

The cast you claim is "absolutely safe" is not. It is only safe if there are no consumer methods on the class which take a parameter T ; and the compiler doesn't consider the existence of any methods to determine if something is type safe.

As such, this cast should generate a warning; if it doesn't, well, that doesn't mean it is actually safe, it just means that either the compiler didn't consider this case, or you are suppressing the warning.