使用哪个收集器？

Question

关于那些类我正在尝试构建地图 :

有了这些简单的类结构

class Alert {

     public String serie;
     public String rame;
     public String label;

     public Alert( String serie, String rame, String label ) {
         this.serie = serie;
         this.rame = rame;
         this.label = label;
     }

     public String getSerie() {
         return serie;
     }

     public String getRame() {
         return rame;
     }

     public String getLabel() {
         return label;
     }
}

 class Diagnostic {
    public String serie;
    public String rame;

    public List< Event > events;

    public Diagnostic(String serie, String rame){
         this.serie = serie;
         this.rame = rame;
    }

    public void addEvent(String label){
         if(events == null)events = new ArrayList<>(  );
         events.add( new Event( label ) );
    }
 }

class Event {

    public String label;

    public Event(String label){
         this.label = label;
     )}
 }


 Alert atgv1 = new Alert( "tgv", "rame1", "label1" );
 Alert atgv2 = new Alert( "tgv", "rame1", "label2" );
 Alert atgv3 = new Alert( "tgv", "rame1" , "label3");

 Alert atgv4 = new Alert( "tgv", "rame2", "label4" );
 Alert atgv5 = new Alert( "tgv", "rame2", "label5" );

 Alert atgv6 = new Alert( "tgv", "rame3", "label6" );

 Alert az2n1 = new Alert( "z2n", "rame1", "label1" );
 Alert az2n2 = new Alert( "z2n", "rame1", "label2" );
 Alert az2n3 = new Alert( "z2n", "rame1" , "label3");

 Alert az2n4 = new Alert( "z2n", "rame2", "label4" );
 Alert az2n5 = new Alert( "z2n", "rame2", "label5" );
 Alert az2n6 = new Alert( "z2n", "rame3", "label6" );

 List<Alert> alerts = Arrays.asList(atgv1, atgv2, atgv3, atgv4, atgv5, atgv6, az2n1, az2n2, az2n3, az2n4, az2n5, az2n6);

我有有效的代码：

Map<String, Map<String, Diagnostic>> map = alerts.stream()
            .collect(Collectors.collectingAndThen( Collectors.groupingBy( Alert::getSerie,
                    Collectors.toMap( Alert::getRame,  a -> new Diagnostic( a.getSerie(), a.getRame() ), (a1, a2) -> a1)), b -> {

                b.forEach( ( k, v ) -> v.forEach( ( y, z)  -> {
                    List<Alert> found = alerts.stream().filter( p -> p.getSerie().equals( k ) && p.getRame().equals( y ) ).collect( Collectors.toList());

                    found.forEach( f -> z.addEvent( f.getLabel() ) );
                }) );
                return b;
            } ));

结果是我想要的，但问题是我在 andThen 中的地图上循环以找到正确的警报（过滤器）并添加事件 (.addEvent)

如何一次性实现？不编写我自己的 Collector 可能吗？困难的部分是我想要一个带有事件列表的诊断，而不是我可以用第二个 groupingBy 而不是 toMap...

轻松实现的诊断列表

Answer 1

您可以使用多个 groupingBy 来解决问题，然后您要寻找的是 reducing Diagnostic 值：

Map<String, Map<String, Optional<Diagnostic>>> map = alerts.stream()
    .collect(Collectors.groupingBy(Alert::getSerie,
        Collectors.groupingBy(Alert::getRame,
            Collectors.mapping(e -> new Diagnostic(e.getSerie(), e.getRame()),
                Collectors.reducing((d1, d2) -> Diagnostic.builder()
                    .serie(d1.getSerie())
                    .rame(d1.getRame())
                    .events(Stream.concat(d1.getEvents().stream(),
                        d2.getEvents().stream()).collect(Collectors.toList()))
                    .build())))));

注意：上面依赖于Builder，为了方便创建Diagnostic的实例，涉及添加事件并将最内层的值收集为 Optional，因为缺少可依赖的标识元素。

或者如 Holger 所言，将是嵌套收集操作的首选操作，如：

Map<String, Map<String, Diagnostic>> map = alerts.stream()
    .collect(Collectors.groupingBy(Alert::getSerie,
        Collectors.toMap(Alert::getRame, e -> new Diagnostic(e.getSerie(), e.getRame()),
            (d1, d2) -> Diagnostic.builder()
                .serie(d1.getSerie())
                .rame(d1.getRame())
                .events(Stream.concat(d1.getEvents().stream(),
                    d2.getEvents().stream()).collect(Collectors.toList()))
                .build())));

使用哪个收集器？

Which Collectors to use?

java

lambda

java-8

java-stream

collectors