如何使用流编写模式方法

Question

我正在尝试编写一个方法来获取Collection使用a 的模式Comparator.

有人可以告诉我为了编译这个我需要做些什么改变吗？我不想改变签名.

static <T> T mode(Collection<? extends T> collection, Comparator<? super T> comparator) {
    return collection.stream()
                     .collect(Collectors.groupingBy(t -> t, () -> new TreeMap<>(comparator), Collectors.counting()))
                     .entrySet()
                     .stream()
                     .reduce(BinaryOperator.maxBy(Comparator.comparingLong(Map.Entry::getValue)))
                     .map(Map.Entry::getKey)
                     .orElseThrow(IllegalArgumentException::new);
}

编辑

事实证明我只是使用了错误的版本.这不能使用编译javac 1.8.0_25.确切的三条错误消息是:

Error:(40, 47) java: incompatible types: inferred type does not conform to upper bound(s)
inferred: java.lang.Object
upper bound(s): T,java.lang.Object

Error:(43, 45) java: incompatible types: cannot infer type-variable(s) T
(argument mismatch; invalid method reference
  method getValue in interface java.util.Map.Entry<K,V> cannot be applied to given types
    required: no arguments
    found: java.lang.Object
    reason: actual and formal argument lists differ in length)

Error:(44, 25) java: invalid method reference
non-static method getKey() cannot be referenced from a static context

但是,我已经升级到javac 1.8.0_65并完美编译.

Answer 1

此代码无法使用 Java 8u40 之前的 javac 进行编译。如果您仍然想让它与旧的 javac 版本兼容，您可以引入另一个通用变量，如下所示：

static <T> T mode(Collection<? extends T> collection, Comparator<? super T> comparator) {
    return mode0(collection, comparator);
}

private static <T, TT extends T> T mode0(Collection<TT> collection,
                                         Comparator<? super T> comparator) {
    return collection.stream()
                     .collect(Collectors.groupingBy(t -> t, 
                                  () -> new TreeMap<>(comparator), 
                                  Collectors.counting()))
                     .entrySet()
                     .stream()
                     .reduce(BinaryOperator.maxBy(
                                  Comparator.comparingLong(Map.Entry::getValue)))
                     .map(Map.Entry::getKey)
                     .orElseThrow(IllegalArgumentException::new);
}

顺便说一下，您可以使用而Stream.max不是比较器：reduceMap.Entry.comparingByValue()

private static <T, TT extends T> T mode0(Collection<TT> collection,
                                         Comparator<? super T> comparator) {
    return collection.stream()
                     .collect(Collectors.groupingBy(t -> t, 
                                  () -> new TreeMap<>(comparator), 
                                  Collectors.counting()))
                     .entrySet()
                     .stream()
                     .max(Map.Entry.comparingByValue())
                     .map(Map.Entry::getKey)
                     .orElseThrow(IllegalArgumentException::new);
}