Why do we have to cast the List returned by Collectors.toList() to List<Integer> even though the elements of the Stream are already mapped to Integer? [duplicate]

问题

This question already has answers here:

What is a raw type and why shouldn't we use it? (15 answers)

Why does the Streams API need a hint for generic type in this case? (1 answer)

Why does javac complain about generics unrelated to the class' type arguments? [duplicate] (1 answer)

Closed 2 years ago.

I'm mapping a raw Stream to a Stream<Integer> and then collect the elements to a List<Integer>.

Why do I have to cast the result of collect(Collectors.toList()) to List<Integer> if my mapper - .map(str -> ((String)str).length()) - already maps to Integer?

My code:

List list = Arrays.asList("test", "test2");
List<Integer> lengths = (List<Integer>) list.stream()
                                            .map(str -> ((String)str).length())
                                            .collect(Collectors.toList());

If I don't use a raw List, there's no need to cast:

List<String> list = Arrays.asList("test", "test2");
List<Integer> lengths = list.stream()
                            .map(str -> str.length())
                            .collect(Collectors.toList());

回答1:

Calling list.stream() on a raw List produces a raw Stream. calling map on that Stream doesn't change that Stream to a generic Stream<Integer>. It changes it to another raw Stream. Therefore when you call collect(Collectors.toList()), you get a raw List and have to cast it to (List<Integer>).

Conclusion : don't use raw types.

来源：https://stackoverflow.com/questions/45054145/why-do-we-have-to-cast-the-list-returned-by-collectors-tolist-to-listinteger