将集合“MyCype”转换为映射<String,设置<Stry> Java流[复制]

这基本上就是你所做的,但是没有 collectingAndThen

data.stream()
    .collect(Collectors.groupingBy(
                 MyClass::getPropertyOne, 
                 Collectors.mapping(MyClass::getPropertyTwo, Collectors.toSet())
             ); 

public class MyClass{

private String propertyOne;

private String propertyTwo;

public String getPropertyOne() {
        return propertyOne;
        }

public String getPropertyTwo() {
        return propertyTwo;
        }

    public MyClass(String propertyOne, String propertyTwo) {
        this.propertyOne = propertyOne;
        this.propertyTwo = propertyTwo;
    }
}

 public static void main(String args[]){
        List<MyClass> l = new ArrayList<>();
        l.add(new MyClass("1","2"));
        l.add(new MyClass("1","3"));
        l.add(new MyClass("2","5"));
        l.add(new MyClass("2","4"));

        Map<String, Set<String>> multimap = l.stream()
                .collect(Collectors.groupingBy(MyClass::getPropertyOne,
                        Collectors.mapping(MyClass::getPropertyTwo,
                             Collectors.toSet())));

        System.out.println("x1 = " + multimap);
}

x1=1=[2,3],2=[4,5]