Question

我正在开发以下小实用程序类：

public class Collections2 {
    public static <T, K extends Comparable<? super K>> Comparator<T>
            comparatorBy(Function<T, K> selector) {
        return (a, b) -> selector.apply(a).compareTo(selector.apply(b));
    }

    public static <T, K extends Comparable<? super K>> void
                sortBy(List<T> list, Function<? super T, K> selector) {
        Collections.sort(list, comparatorBy(selector));
    }
}

这允许按python style key functions排序，所以我不必为我想要排序的每个字段编写完整的比较器，而是我可以写：

List<Person> persons = ...;
Collections2.sortBy(persons, Person::getName);

这很好，我想用这种风格编写了很多实用方法，如minBy，maxBy，groupBy等等。

但是如果有人想要使用这些新的实用功能并进行比较呢？我想到了以下几点：

public static class KeyByComparator<T> implements Comparable<KeyByComparator<T>> {
    private T target;
    private Comparator<? super T> comparator;
    public KeyByComparator(T target, Comparator<? super T> comparator) {
        this.target = target;
        this.comparator = comparator;
    }
    @Override
    public int compareTo(KeyByComparator<T> other) {
        return this.comparator.compare(this.target, other.target);
    }
}

public static <T> Function<T, KeyByComparator<T>> keysByComparator(Comparator<? super T> comparator) {
    return t -> new KeyByComparator<T>(t, comparator);
}

所以他们可以这样使用它：

public static final Comparator<Person> LEGACY_COMPARATOR = ...;
// ...

Collections2.sortBy(persons, Collections2.keysByComparator(LEGACY_COMPARATOR));
// Ignore the fact that regular sort would be shorter
// Collections.sort(persons, LEGACY_COMPARATOR)

但要实现这一点，我必须公开KeyByComparator<T>。

如果我不想公开KeyByComparator<T>课怎么办？这是我试过的：

public static <T, K extends Comparable<? super K>> Function<T, K>
        keysByComparator2(Comparator<? super T> comparator) {
    return t -> new KeyByComparator<T>(t, comparator);
}

但是它给出了这个编译错误：

Type mismatch: cannot convert from Collections2.KeyByComparator<T> to K

返回值是否有一些奇特的泛型类型约束？

返回与自身相当的值而不暴露类

0 个答案: