递归泛型

有没有办法使此方法适当通用并消除警告？

/**
 * <p>Sort a collection by a certain "value" in its entries. This value is retrieved using
 * the given <code>valueFunction</code> which takes an entry as argument and returns
 * its value.</p>
 * 
 * <p>Example:</p>
 * <pre>// sort tiles by number
 *Collects.sortByValue(tileList, true, new Function<Integer,NormalTile>() {
 *  public Integer call(NormalTile t) {
 *      return t.getNumber();
 *  }
 *});</pre>
 *
 * @param list The collection.
 * @param ascending Whether to sort ascending (<code>true</code>) or descending (<code>false</code>).
 * @param valueFunction The function that retrieves the value of an entry.
 */
public static <T> void sortByValue(List<T> list, final boolean ascending, @SuppressWarnings("rawtypes") final Function<? extends Comparable, T> valueFunction) {
    Collections.sort(list, new Comparator<T>() {
        @SuppressWarnings({ "unchecked", "rawtypes" })
        @Override public int compare(T o1, T o2) {
            final Comparable v1 = valueFunction.call(o1);
            final Comparable v2 = valueFunction.call(o2);
            return v1.compareTo(v2) * (ascending ? 1 : -1);
        }
    });
}

I tried Function<? extends Comparable<?>, T> and Function<? extends Comparable<? extends Comparable>, T>但都没有编译，调用时出错compareTo。对于前者来说是：

Comparable 类型中的方法 CompareTo(capture#9-of ?) 不适用于参数 (capture#10-of ? extends Comparable)

尝试这个：

public static <T, C extends Comparable<? super C>> void sortByValue(List<T> list, final boolean ascending, final Function<C, T> valueFunction) {
    Collections.sort(list, new Comparator<T>() {
        @Override public int compare(T o1, T o2) {
            final C v1 = valueFunction.apply(o1);
            final C v2 = valueFunction.apply(o2);
            return v1.compareTo(v2) * (ascending ? 1 : -1);
        }
    });
}

你还需要super允许为子类型定义比较器。更多解释在这里：http://docs.oracle.com/javase/tutorial/extra/generics/morefun.html http://docs.oracle.com/javase/tutorial/extra/generics/morefun.html

UPDATE

另外，看着你的代码，我看到了另一辆自行车，有一个很好的库 Google Collections，它提供了非常方便的Ordering http://google-collections.googlecode.com/svn/trunk/javadoc/com/google/common/collect/Ordering.html处理它的想法。

所以，你的代码将如下所示：

Ordering<NormalTile> myOrdering = Ordering.natural()
  .onResultOf(new Function<Integer,NormalTile>() {
  public Integer call(NormalTile t) {
      return t.getNumber();
  }))
  .nullsLast();
...
Collections.sort(list, myOrdering);
//or
newList = myOrdering.sortedCopy(readonlyList);