public class Employee {
 public String name;
 public int age;
 public char sex;
 public String time;
 public int salary;
}

List<Employee> data = Arrays.asList(e1,e2,e3......)

public static Map<Character, List<Employee>> groupByFirstChar(
 List<Employee> data){
  Map<Character, List<Employee>> result = new HashMap<>();
  for(Employee e : data){
   Character c = e.name.charAt(0);
   List<Employee> l = result.get(c);
   if(l == null){
    l = new ArrayList<>();
    result.put(c, l);
   }
   l.add(e);
  }
  return result;
}

public static Map<String, List<Employee>> groupBySalary(List<Employee> data) {
 Map<String, List<Employee>> result = new HashMap<>();
 for (Employee e : data) {
  String key = separate(e.salary);
  List<Employee> l = result.get(key);
  if (l == null) {
   l = new ArrayList<>();
   result.put(key, l);
  }
  l.add(e);
 }
 return result;<br>}
private static String separate(int salary) {
 if (salary <= 5000) {
  return "5000以下";
 }
 if (salary <= 10000) {
   return "5000~10000";
 }
 if (salary <= 20000) {
  return "10000~20000";
 }
 return "20000以上"
}

Employee e -> e.name.charAt(0)

Employee e -> separate(e.salary):String

Employee e -> getYear(e.time):String

Employee e -> getFirstChar(e.name):Character

Employee e -> getFirstChar(e) : Character
Employee e -> separate(e) : String
Employee e -> getYear(e) : String

public static <K> Map<K, List<Employee>> groupByKey(List<Employee> data, GetKey<K> getKey){
 Map<K, List<Employee>> result = new HashMap<>();
 for(Employee e : data) {
  K key = getKey.getKey(e);
  List<Employee> l = result.get(key);
  if (l == null) {
   l = new ArrayList<>();
   result.put(key, l);
  }
  l.add(e);
 }
  return result;
}
interface GetKey<K>{
 K getKey(Employee e);
}

Map<Character, List<Employee>> result = groupByKey(data, new GetKey<Character>() {
   @Override
   public Character getKey(Employee e) {
    e.name.charAt(0);
   }
  });

Map<String, List<Employee>> result = groupByKey(list, new GetKey<String>() {
   @Override
   public String getKey(Employee e) {
    separate(e.salary);
   }
  });

Map<Character, List<Employee>> resultByFirstChar = groupByKey(list, e -> e.name.charAt(0));
Map<String, List<Employee>> resultBySalary = groupByKey(list, e -> separate(e.salary));

public static <K, E> Map<K, List<E>> groupBy(List<? extends E> data, Function<? super E, ? extends K> fun) {
 Map<K, List<E>> result = new HashMap<>();
 for(E e : data) {
  K k = fun.apply(e);<br>  List<E> l = result.get(k);
  if(l == null) {
   l = new ArrayList<>();
   result.put(k, l);
  }
  l.add(e);
 }
 return result;<br>
 }

@FunctionalInterface
public interface Function<T, R> {
 R apply(T t);
}