我正在使用Java 8 Streams,我有一个类似于以下的类：
public class ShareDao {
private String senderId;
private String receiverId;
public String getSenderId() {
return senderId;
}
public String sharingMode(){
return this.receiverId != null && !this.receiverId.trim().isEmpty() ? "incoming" : "outgoing";
}
}
现在我想做的是,我想：
>过滤出senderId无效的记录(使用Map<>查找)
>将集合按senderId分组,然后再按shareMode对其进行分组.
下面是我的代码：
Map<String,Map<String,List<ShareDao>>> p = records.stream()
.filter(shared -> userMap.containsKey(shared.getSenderId()))
.collect(Collectors.groupingBy(ShareDao::getSenderId),Collectors.groupingBy(ShareDao::sharingMode,Function.identity()));
它抛出了错误：
Error:(105,90) java: no suitable method found for groupingBy(Share[…]gMode,java.util.function.Function)
method java.util.stream.Collectors.groupingBy(java.util.function.Function) is not applicable
(cannot infer type-variable(s) T,K
(actual and formal argument lists differ in length))
method java.util.stream.Collectors.groupingBy(java.util.function.Function,java.util.stream.Collector) is not applicable
(no instance(s) of type variable(s) T exist so that java.util.function.Function conforms to java.util.stream.Collector)
method java.util.stream.Collectors.groupingBy(java.util.function.Function,java.util.function.Supplier,java.util.stream.Collector) is not applicable
(cannot infer type-variable(s) T,K,D,A,M
(actual and formal argument lists differ in length))
当Intellij Idea引发错误时
无法从静态上下文引用非静态方法
- Filter out the records where senderId is invalid (using a Map<>
lookup)- Group the collection by the senderId and then further group them by the shareMode.
您可以改用：
Map<String,List<ShareDao>>> p = records.stream()
.filter(shared -> userMap.containsKey(shared.getSenderId())) // filter records
.collect(Collectors.groupingBy(ShareDao::getSenderId,// grouping by senderId
Collectors.groupingBy(ShareDao::sharingMode))); //further grouping by sharingMode
注意：
> groupingBy属性可将值汇总为列表.
>在收集器中定义的收集器,因为它与您的尝试不同.