Java中的协方差

Question

Why does the following not work in Java? It would work in C#:

public static final List<String> Split(String str, char delimiter)
    {
        if ((str == null) || "".equals(str))
        {
            return new CopyOnWriteArrayList<String>();
        }
    }

I get an error saying this method has to return List. CopyOnWriteArrayList implements the List interface. Why does covariance not apply on return values in Java?

Answer 1

看来您已经忘记为else分支返回List<String>了。

Answer 2

You either have a else branch explicitly and return whatever you want to return (null maybe ?). Or have a return statement at the end of the method. You program will not compile if you do not have either.

Answer 3

Apart from the correct answers above:

1. use lower case method names in Java (split instead of Split)
2. why return such a heavy object as CopyOnWriteArrayList when your string is empty? That's the kind of situation where you want to return Collections.emptyList(), where no new object is created unnecessarily (assuming you are not going to add values to the list afterwards)

Actually, I'd use apache commons lang and do it like this:

return Arrays.asList(
    StringUtils.stripToEmpty(myString) // converts null into ""
                                 // and strips extra whitespace
        .split(
             Pattern.quote(Character.toString(delimiter))
        )
)

No if / else needed.