java - 将 Comparable 从 Comparator 转换为 Comparator？

Question

我经常有一个 Comparator 类型，而我需要一个 Comparable 类型，反之亦然。是否有可重用的 JDK API 可以相互转换？类似的东西:

    public static <C> Comparable<C> toComparable(final Comparator<C> comparator) {
        // does not compile because Hidden can not extend C, 
        // but just to illustrate the idea
        final class Hidden extends C implements Comparable<C> {
            @Override
            public int compareTo(C another) {
                return comparator.compare((C) this, another);
            }
        };
        return new Hidden();
    }

    public static <C extends Comparable<C>> Comparator<C> toComparator(final Class<C> comparableClass) {
        return new Comparator<C>() {
            @Override
            public int compare(C first, C second) {
                assert comparableClass.equals(first.getClass());
                assert comparableClass.equals(second.getClass());
                return first.compareTo(second);
            }
        };
    }

Answer 1

ComparableComparator 来自 Apache Commons Collections 的地址似乎是 Comparable<T>至 Comparator问题(不幸的是它不是通用类型友好的)。

反向操作不太可能，因为 Comparator<T>代表算法，而 Comparable<T>代表实际数据。您将需要某种组合。快速而肮脏的解决方案:

class ComparableFromComparator<T> implements Comparable<T> {

    private final Comparator<T> comparator;
    private final T instance;

    public ComparableFromComparator(Comparator<T> comparator, T instance) {
        this.comparator = comparator;
        this.instance = instance;
    }

    @Override
    public int compareTo(T o) {
        return comparator.compare(instance, o);
    }

    public T getInstance() {
        return instance;
    }
}

假设你有一个类 Foo那不是Comparable<Foo>但是你有 Comparator<Foo> .你可以这样使用它:

Comparable<Foo> comparable = new ComparableFromComparator<Foo>(foo, comparator);

如您所见(尤其是没有 mixins)，它非常丑陋(我什至不确定它是否会工作...)还要注意 comparable不扩展 Foo , 你必须打电话 .getInstance()相反。