转换ArrayList <Comparable> 变成另一个ArrayList类型

Question

I have a sorting method that takes an ArrayList<Comparable>, sorts it using the compateTo() method, then returns a sorted ArrayList<Comparable>. Here it is:

public static ArrayList<Comparable> insertionSort(ArrayList<Comparable>
                                                          input) {
    Comparable temp;
    ArrayList<Comparable> result;

    result = (ArrayList<Comparable>) input.clone();

    if (result.size() > 1) {
        for (int k = 1; k < result.size(); k++) {
            for (int j = 1; j <= k; j++) {
                if (result.get(k - j).compareTo(result.get(k - j + 1)) >0){
                    temp = result.get(k - j + 1);
                    result.set(k - j + 1, result.get(k - j));
                    result.set(k - j, temp);
                }
            }
        }
    }

    return result;
}

Somewhere else in my program, I define DVD objects that implement the Comparable interface, create a bunch of them, and store them in an ArrayList<DVD> called members. Now, when I try to sort members like this:

members = (ArrayList<DVD>) YaSort.insertionSort(members);

I get the following error: Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Comparable; cannot be cast to [LDVD;

How do I solve this? Thanks for your time.

Answer 1

The point of generics is that you shouldn't need to cast anything reference types. Also, rare types, mixing generic and raw types, is bad. And generally List is preferred to ArrayList , by convention more than anything else.

The start of your method should look something like:

public static <T extends Comparable<? super T>> List<T> insertionSort(
    List<T> input
) {
    List<T> result  = new ArrayList<T>(input);

(Actually for maximum performance, probably not from an insertion sort over much data, doing something hacky with arrays is better. BTW: The error you are quoting appears to be from the use of arrays rather the collections.)

Answer 2

It seems that you want to sort a list of objects that implement Comparable and return a list of the same objects, not just Comparable . In this case you can use method parameter that can be used by a compiler to determine the type of return result:

public static <T extends Comparable<? super T>> ArrayList<T>
    insertionSort(ArrayList<T> input) {
    ....
}

Then you can assign to members without cast:

members = YaSort.insertionSort(members);

Answer 3

You need to define members as ArrayList<Comparable> , because even thought DVD implements/extends Comparable ArrayList<DVD> does not implement/extend ArrayList<Comparable> - this is called covariance/contravariance and is not supported in Java. (It is actually supported in C#)