使用Java Stream API将List按自定义分组规则转换成Map的一个例子

package java8;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.function.Consumer;
import java.util.stream.Collectors;
class Employee {
  private String city;
  private String name;
  private int score;
  public Employee(String name, String city, int score){
    this.city = city;
    this.name = name;
    this.score = score;
  }
  public String getCity(){
    System.out.println("city: " + this.city);
    return this.city;
  }
  public String getName() {
    return this.name;
  }
  public int getScore() {
    return this.score;
  }
  @Override
    public String toString() {
        return String.format("Employee: " + this.name + " city: " + this.city);
    }
}
class Person {
    private String name;
    private int age;
    Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    @Override
    public String toString() {
        return String.format("Person{name='%s', age=%d}", name, age);
    }
}
// Jerry 2016-01-15 20:51PM ? 多用于extends generic的type，接受所有Object的sub class
public class StreamTest {
  private static void printMap(Map<? extends Object, ? extends Object> map) {
     for(Entry<? extends Object, ? extends Object> entry:map.entrySet()) {
          System.out.println("key = " + entry.getKey() + " , Value = " + entry.getValue());
       }
  }
  public static void main(String[] args) {
    ArrayList<Employee> employees = new ArrayList<Employee>();
    employees.add(new Employee("A", "Shanghai",100));
    employees.add(new Employee("B", "Chengdu",101));
    employees.add(new Employee("C", "Shenzhen",102));
    employees.add(new Employee("D", "Chengdu",104));
    // group by City
    Map<String, List<Employee>> employeesByCity =
        employees.stream().collect( Collectors.groupingBy(Employee::getCity));
    //  default void forEach(Consumer<? super T> action) {
    for(Map.Entry<String, List<Employee>> entry:employeesByCity.entrySet()) {
        System.out.println("key= " + entry.getKey() + " , Value = " + entry.getValue());
        entry.getValue().forEach(System.out::println);
     }
     // 2016-01-15 20:37PM 
     Consumer<Employee> aa = a -> { System.out.println("Employee: " + a.getName() + " : " +  a.getScore()); };
     List<Employee> chengduEmployee = employeesByCity.get("Chengdu");
     chengduEmployee.forEach(aa);
     // test for counting
     Map<String, Long> employeesByCity2 = 
         employees.stream().collect( Collectors.groupingBy(Employee::getCity, Collectors.counting()));
     printMap(employeesByCity2);
     // calculate average score
     Map<String, Double> employeesByCity3 = 
         employees.stream().collect( Collectors.groupingBy(Employee::getCity,
             Collectors.averagingInt(Employee::getScore)));
     printMap(employeesByCity3);
    /*Stream<Person> people = Stream.of(new Person("Paul", 24), new Person("Mark", 30), new Person("Will", 28));
    Map<Integer, List<String>> peopleByAge = people.collect(groupingBy(p -> p.age, mapping((Person p) -> p.name, toList())));
    System.out.println(peopleByAge);*/
     /*
      * 分区是一种特殊的分组，结果 map 至少包含两个不同的分组——一个true，一个false。
      * 例如，如果想找出最优秀的员工，你可以将所有雇员分为两组，一组销售量大于 N，
      * 另一组小于 N，使用 partitioningBy 收集器：
      */
     System.out.println("partition result");
     Map<Boolean, List<Employee>> partitioned =
         employees.stream().collect(Collectors.partitioningBy(e -> e.getScore() > 101));
     printMap(partitioned);
     /*
      * 你也可以将 groupingBy 收集器传递给 partitioningBy 收集器来将联合使用分区和分组。例如，你可以统计每个分区中的每个城市的雇员人数：
    Map<Boolean, Map<String, Long>> result =
    employees.stream().collect(partitioningBy(e -> e.getNumSales() > 150,
    groupingBy(Employee::getCity, counting())));
    这样会生成一个二级 Map:
    {false={London=1}, true={New York=1, Hong Kong=1, London=1}}
      */
  }
}