Java6,番石榴,泛型,类型推断

Question

我在Java中编写了一个实用工具方法:

public static final ImmutableSortedSet<TimeUnit> REVERSED_TIMEUNITS = ImmutableSortedSet.copyOf(
        Collections.<TimeUnit>reverseOrder(),
        EnumSet.allOf(TimeUnit.class)
);


/**
 * Get the number of ..., minutes, seconds and milliseconds
 *
 * You can specify a max unit so that you don't get days for exemple
 * and can get more than 24 hours if you want to display the result in hours
 *
 * The lowest unit is milliseconds
 * @param millies
 * @param maxTimeUnit
 * @return the result map with the higher unit first
 */
public static Map<TimeUnit,Long> getCascadingDateDiff(long millies,TimeUnit maxTimeUnit) {
    if ( maxTimeUnit == null ) {
        maxTimeUnit = TimeUnit.DAYS;
    }
    Map<TimeUnit,Long> map = new TreeMap<TimeUnit,Long>(Collections.<TimeUnit>reverseOrder());
    long restInMillies = millies;
    Iterable<TimeUnit> forUnits = REVERSED_TIMEUNITS.subSet(maxTimeUnit,TimeUnit.MICROSECONDS); // micros not included
    // compute the number of days, then number of hours, then minutes...
    for ( TimeUnit timeUnit : forUnits ) {
        long numberForUnit = timeUnit.convert(restInMillies,TimeUnit.MILLISECONDS);
        map.put(timeUnit,numberForUnit);
        restInMillies = restInMillies - timeUnit.toMillis(numberForUnit);
    }
    return map;
}

它适用于:

    Map<TimeUnit,Long> map = new TreeMap<TimeUnit,Long>(Collections.reverseOrder());

但我第一次尝试

    Map<TimeUnit,Long> map = Maps.newTreeMap(Collections.reverseOrder());

我的IntelliJ没有说什么,而我的编译器说:

DateUtils.java:[302,48]不兼容的类型; 没有类型变量的实例K,V存在,以便java.util.TreeMap符合java.util.Map [ERROR] found:java.util.TreeMap [ERROR] required:java.util.Map

没有比较器,它工作正常:

   Map<TimeUnit,Long> map = Maps.newTreeMap();

但我尝试过:

Map<TimeUnit,Long> map = Maps.newTreeMap(Collections.<TimeUnit>reverseOrder());

与:

Map<TimeUnit,Long> map = Maps.newTreeMap(new Comparator<TimeUnit>() {
    @Override
    public int compare(TimeUnit timeUnit, TimeUnit timeUnit1) {
        return 0; 
    }
});

我得到了同样的错误.所以每当我在TreeMap中使用比较器时,类型推断似乎不再起作用.为什么？

---

番石榴方法的签名是:

  public static <C, K extends C, V> TreeMap<K, V> newTreeMap(Comparator<C> comparator)

预期的返回类型是类型,因此没有比较器,Java能够推断出K = TimeUnit和V = Long.

使用TimeUnit类型的比较器,Java知道C是TimeUnit.它还知道预期的返回类型是类型,因此K = TimeUnit,V = Long.K扩展C受到尊重,因为TimeUnit扩展了TimeUnit(无论如何,如果你认为它是错误的话,我还尝试使用对象比较器......)

所以我只是想知道为什么类型推断在这种情况下不起作用？

Answer 1

像Michael Laffargue建议的那样,它是一个OpenJDK6类型的推理错误:

https://bugs.openjdk.java.net/show_bug.cgi?id=100167

http://code.google.com/p/guava-libraries/issues/detail?id=635

它在我的IntelliJ中运行良好,在版本7中使用OpenJDK,在版本6中使用其他JDK.

以下关于kutschkem的建议:

    Map<TimeUnit,Long> map = Maps.<TimeUnit,TimeUnit,Long>newTreeMap(Collections.<TimeUnit>reverseOrder());

注意<TimeUnit,TimeUnit,Long>哪个允许显式强制输入类型的参数.查看相关主题:Java中的这种泛型用法是什么？X. <Y>()的方法

谢谢大家