使用泛型返回接口时应使用哪种数据类型？

Question

With this interface:

interface SomeInterface<E> {
  List<E> getList();
}

And this implementation:

private void someMethod(SomeInterface someImplementation) {
  Object item = someImplementation.getList().get(0);  // here
}

What would be the appropriate datatype to use? Just Object , as shown?

Answer 1

private <T> void someMethod(SomeInterface<T> someImplementation) {
  T item = someImplementation.getList().get(0);  // here
}

Object is fine too, as long as you don't need to do something like someImplementation.getList().add(item) later. The key to this example is that it avoids the use of a raw type, so that the compiler can ensure that you've written type-safe code.

Answer 2

An alternative way to avoid the use of raw types is to use a wildcard ? in the signature. If you did it that way the appropriate type for item would be Object :

private void someMethod(SomeInterface<?> someImplementation) {
    Object item = someImplementation.getList().get(0);  
}

The downside of doing it this way is that you cannot use the generic type E in the body of the method.