java8 groupby

以下是两种使用Java 8的groupingBy操作进行分组的例子：

1.基于city和name实现多次分组：

Map<String, Map<String, List<Employee>>> map = employees.stream()
    .collect(Collectors.groupingBy(Employee::getCity, Collectors.groupingBy(Employee::getName)));

map.forEach((k, v) -> {
    System.out.println(k + " = " + v);
    v.forEach((i, j) -> {
        System.out.println(i + " = " + j);
    });
});

2.按城市分组list：

Map<String, List<Employee>> map = employees.stream()
    .collect(Collectors.groupingBy(Employee::getCity));

map.forEach((k, v) -> {
    System.out.println(k + " = " + v);
});