我正在尝试Stream.reduce(),并且遇到了类型系统的障碍。这是一个玩具示例:
public static Number reduceNum1(List<Number> nums) {
return nums.stream().reduce(0, (a, b) -> a);
}
这适用于任何List<Number>
,但是如果我希望能够减少一个列表,该怎么办? extends Number
?这不能编译:
public static Number reduceNum2(List<? extends Number> nums) {
return nums.stream().reduce((Number)0, (a, b) -> a);
}
出现错误:
ReduceTest.java:72: error: no suitable method found for reduce(Number,(a,b)->a)
return nums.stream().reduce((Number)0, (a, b) -> a);
^
method Stream.reduce(CAP#1,BinaryOperator<CAP#1>) is not applicable
(argument mismatch; Number cannot be converted to CAP#1)
method Stream.<U>reduce(U,BiFunction<U,? super CAP#1,U>,BinaryOperator<U>) is not applicable
(cannot infer type-variable(s) U
(actual and formal argument lists differ in length))
where U,T are type-variables:
U extends Object declared in method <U>reduce(U,BiFunction<U,? super T,U>,BinaryOperator<U>)
T extends Object declared in interface Stream
where CAP#1 is a fresh type-variable:
CAP#1 extends Number from capture of ? extends Number
Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output
从概念上讲,我明白为什么会发生这种情况。Stream.reduce()
必须返回与源元素类型相同的内容,并且? extends Number
不等于Number
。但我不确定如何处理这个案子。我如何允许子类的集合(例如List<Integer>
)减少(或收集)?
如果有帮助的话,这里有一个更实际的例子,同样无法编译:
public static <E> Set<E> reduceSet1(List<? extends Set<E>> sets) {
return sets.stream().reduce(ImmutableSet.<E>of(), (a, b) -> Sets.union(a, b));
}