java - 将泛型 Collection<Object> 转换为 Collection<Integer>

Question

欢迎! 所以，我目前正在研究集合到数组转换器，但我遇到了问题。

    private static Collection<Object> resolveDesiredCollection(String desiredCollection){
        if (desiredCollection == null || desiredCollection.equals("")){
            desiredCollection = "al";
       }
       Collection<Object> collection;
       switch (desiredCollection) {
        case "al":
            collection = new ArrayList<>();
            break;
        case "all":
            collection = new LinkedList<>();
            break;
        case "hs":
            collection = new HashSet<>();
            break;
        case "ts":
            collection = new TreeSet<>();
            break;
        default:
            collection = new ArrayList<>();
        }
        return collection;
    }

    public static Collection<Integer> intArrayToCollection(int [] array, String 
            desiredCollection){

        Collection<Integer> collection = (Collection<Integer>) 
            (resolveDesiredCollection(desiredCollection));

        Arrays.stream(array).forEach(i -> collection.add(new Integer(i)));

        return collection;
    }

我怎样才能转换它？显示的方式不起作用，因为编译器尖叫着“不可转换类型”，这甚至可能吗？我想避免代码重复，所以我不想为 Integer、Double、String 等创建resolveDesiredCollection 方法。

Answer 1

如果您创建 resolveDesiredCollection方法通用:

private static <T> Collection<T> resolveDesiredCollection(String desiredCollection)
{
    if (desiredCollection == null || desiredCollection.equals("")) {
        desiredCollection = "al";
    }
    Collection<T> collection;
    switch (desiredCollection) {
    case "al":
        collection = new ArrayList<>();
        break;
    case "all":
        collection = new LinkedList<>();
        break;
    case "hs":
        collection = new HashSet<>();
        break;
    case "ts":
        collection = new TreeSet<>();
        break;
    default:
        collection = new ArrayList<>();
    }
    return collection;
}

您可以简单地调用它:

Collection<Integer> collection = resolveDesiredCollection(desiredCollection);

你的第二种方法也可以变得通用:

public static <T> Collection<T> ArrayToCollection(T[] array, String desiredCollection)
{
    Collection<T> collection = resolveDesiredCollection(desiredCollection);

    Arrays.stream(array).forEach(i -> collection.add(i));

    return collection;
}

当然，您可以消除这些方法并使用现有的JDK方法来实现相同的功能。

例如，要转换 Integer[]到ArrayList<Integer> :

ArrayList<Integer> list =
    Arrays.stream(array).collect(Collectors.toCollection(ArrayList::new));