java:不兼容类型：推理变量RR的边界不兼容

Question

JDK 1.8

设置和收集。

我想数一数那个十字路口

我试试这个：

SortedSet<VsatVlan> vsatVlans;
Collection<Integer> mobilityNsVlansList = mobilityNS.getVlanIds();
....
long intersectionCount = vsatVlans.stream()
    .filter(vsatVlan -> mobilityNsVlansList.contains(vsatVlan.getVlanId()))
    .collect(Collectors.collectingAndThen(
        Collectors.toSet(),
        set -> { // error here
            if (set.size() != 1) {
                throw new IllegalStateException();
            }
            return set.size();
        }
    ));

但我有一个错误：

incompatible parameter types in lambda expression: expected Set<Object> but found Set<VsatVlan>

java: incompatible types: inference variable RR has incompatible bounds
lower bounds: java.lang.Long,java.lang.Object
lower bounds: java.lang.Integer

Answer 1

如果我得到它的权利，你想返回的交叉计数只有如果不是零，否则你想抛出一个异常。您需要显式地提供集合的类型以消除您得到的错误：

return vsatVlans.stream()
                .filter(vsatVlan -> mobilityNsVlansList.contains(vsatVlan.getVlanId()))
                .collect(Collectors.collectingAndThen(Collectors.toSet(),
                    (Set<VsatVlan> set) -> {
                        if (set.size() == 0) {
                            throw new IllegalStateException("");
                        }
                        return set.size();
                    }
               ));

您也可以在此处使用选项，并避免if块：

return vsatVlans.stream()
               .filter(vsatVlan -> mobilityNsVlansList.contains(vsatVlan.getVlanId()))
               .collect(Collectors.collectingAndThen(Collectors.counting(), Optional::of))
               .filter(x -> x > 0)
               .orElseThrow(() -> new IllegalStateException(""));

但IMO计数，然后在一个单独的if块中进行检查，比上述两种方法更具可读性：

long count = vsatVlans.stream()
                      .filter(vsatVlan -> mobilityNsVlansList.contains(vsatVlan.getVlanId()))
                      .count();
if(count == 0){
    new IllegalStateException("")
}
return count;