java - 怎样总结年代？

Question

这是我的程序

package com;        
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;

import com.google.gson.Gson;

        public class GroupByDemoInJava8 {
            public static void main(String args[]) throws Exception {
                try {
                    List<Person> personList = new ArrayList<>(); // Date Format is MM/DD/YYYY
                    personList.add(new Person("Mike", "London", 15, "01/01/1981"));
                    personList.add(new Person("John", "London", 21, "01/02/1981"));
                    personList.add(new Person("Prasanna", "London", 28, "04/28/1990"));
                    personList.add(new Person("Monobo", "Tokyo", 34, "04/28/1990"));
                    personList.add(new Person("Sam", "Paris", 44, "07/12/1992"));
                    personList.add(new Person("Nadal", "Paris", 5, "04/02/1992"));
                    String patternInput = "MM/dd/yyyy";
                    SimpleDateFormat simpleDateFormatInput = new SimpleDateFormat(patternInput);
                    String outputPattern = "MMM-yy";
                    SimpleDateFormat simpleDateFormatOutput = new SimpleDateFormat(outputPattern);
                    Map<String, List<Person>> personByMap = new TreeMap<String, List<Person>>();

                    for (Person p : personList) {
                        int sumAge = 0;
                        Date inputDate = simpleDateFormatInput.parse(p.getDateOfBirth());

                        String outPutDate = simpleDateFormatOutput.format(inputDate);

                        if (!personByMap.containsKey(outPutDate)) {
                            sumAge = sumAge+p.getAge();
                            System.out.println("Date "+outPutDate+" "+"Age "+sumAge);
                        }

                else
                    {
                        personByMap.get(outPutDate).add(p);
                    }       

                    }



                } catch (Exception e) {
                    e.printStackTrace();
                }

            }
        }

Answer 1

类似这样的事情

public class GroupByDemoInJava8 {
  public static void main(String args[]){
    List<Person> personList = new ArrayList<>();
    /* INITIALIZATION */
    Map<String, Integer> result = personList.stream()
      .collect(Collectors.groupingBy(Person::getDateOfBirth,
        Collectors.summingInt(Person::getAge));
  }
}

如上所述here