Java Streams列表以映射出现索引作为键答案

【问题标题】：Java Streams list to map index of ocurrence as keyJava Streams列表以映射出现索引作为键
【发布时间】：2018-08-17 19:16:18
【问题描述】：

假设我有一个字符串列表

["bird", "bird", "dog", "dog", "bird","bog"]

我希望它们以地图的形式收集

{"bird": [0, 1, 4], "dog": [2, 3, 5]}

作为列表的值是单词在输入列表中的索引。

有没有办法用 Java Streams 做到这一点？

【问题讨论】：

标签： java java-8 java-stream grouping

【解决方案1】：

是的，您可以使用IntStream 和Collectors.groupingBy：

List<String> list = List.of("bird", "bird", "dog", "dog", "bird", "bog");

IntStream.range(0, list.size())
         .boxed()
         .collect(Collectors.groupingBy(list::get));

输出：

{bird=[0, 1, 4], bog=[5], dog=[2, 3]}

【讨论】：

【解决方案2】：

对上述问题稍作改动。

给定一个对象列表，创建一个映射，其中键是“x”字段，值是与“x”字段具有相同值的对象列表

@Value
@RequiredArgsConstructor
class Employee{
    String Firstname;
    String LastName;
    String Dept;
    Integer Salary;
}

public class EmployeeStreamTest {

    public static void main(String[] args) {
        Employee e1 = new Employee("ABC", "Raval", "Fin", 100);
        Employee e2 = new Employee("BCD", "Raval", "Fin", 100);
        Employee e3 = new Employee("CDE", "Raval", "Fin", 100);
        Employee e4 = new Employee("WXY", "", "Fin", 100);
        Employee e5 = new Employee("XYZ", "", "Fin", 100);

        Stream<Employee> es = Stream.of(e1,e2,e3,e4,e5);

        //given a list of employees , create a map where Key is last name and value is list of employees with same last name
        Map<String, List<Employee>> lmse = es.collect(Collectors.groupingBy(emp -> emp.getLastName()));
        System.out.println(lmse);

    }

}

输出：

{=[Employee(Firstname=WXY, LastName=, Dept=Fin, Salary=100), Employee(Firstname=XYZ, LastName=, Dept=Fin, Salary=100)], Raval=[Employee(Firstname=ABC, LastName=Raval, Dept=Fin, Salary=100), Employee(Firstname=BCD, LastName=Raval, Dept=Fin, Salary=100), Employee(Firstname=CDE, LastName=Raval, Dept=Fin, Salary=100)]}

感谢@Jacob 的上述回答。

【讨论】：