java8 stream分组操作

publicclassAnimal {
privateStringname;
privatedoubleweight;
publicAnimal(Stringname, doubleweight) {
this.name=name;
this.weight=weight;
    }
publicStringgetName() {
returnname;
    }
publicdoublegetWeight() {
returnweight;
    }
}
publicclassCatextendsAnimal {
publicCat(Stringname, doubleweight) {
super(name, weight);
    }
}
publicclassDogextendsAnimal {
publicDog(Stringname, doubleweight) {
super(name, weight);
    }
}

publicclassTest {
publicstaticvoidmain(String[] args) {
List<Animal>data=Arrays.asList(
newCat("cat1", 22.2d),
newCat("cat22", 10.1d),
newDog("dog1", 30d),
newDog("dog2", 35d)
        );
Map<?extendsClass<?extendsAnimal>, Double>collect=data.stream()
                .collect(Collectors.groupingBy(Animal::getClass,
Collectors.averagingDouble(Animal::getWeight)));
collect.forEach((k, v) -> {
System.out.println(k.getName() +" average weight: "+v);
        });
    }
}