Usando Agrupar por en Java 8 con la propiedad

user663724:

package com;

import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class GroupByDemoInJava8 
{
    public static void main(String args[]) throws IOException
    {
        List<Person> people = new ArrayList<>();  // Date Format is MM/DD/YYYY
        people.add(new Person("John", "London", 21 , "2/1/18"));
        people.add(new Person("Swann", "London", 21 , "3/5/18" ));
        people.add(new Person("Kevin", "London", 23 , "3/12/18"));
        people.add(new Person("Monobo", "Tokyo", 23 , "4/18/18"));
        people.add(new Person("Sam", "Paris", 23 , "7/12/18"));
        people.add(new Person("Nadal", "Paris", 31, "4/2/19")); 
        Map<String,List<Person>> personByCity = new HashMap<>(); 

        SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yy");


        personByCity = people.stream() .collect(Collectors.groupingBy(sdf.parse(Person::getDateOfBirth))); 


         for (Map.Entry<String,List<Person>> entry : personByCity.entrySet()) { 
                System.out.println("Key = " + entry.getKey());
                System.out.println("Value = " + entry.getValue());
        } 

    }
}


class Person
{ 
    private String name; 
    private String city; private int age; private String dateOfBirth;

    public String getDateOfBirth() {
        return dateOfBirth;
    }

    public void setDateOfBirth(String dateOfBirth) {
        this.dateOfBirth = dateOfBirth;
    }

    public Person(String name, String city, int age , String dateOfBirth) 
    { 
        this.name = name; this.city = city; this.age = age;this.dateOfBirth=dateOfBirth;
    }

    public String getName()
    { return name; } 
    public void setName(String name) 
    { this.name = name; } 
    public String getCity() 
    { return city; } 
    public void setCity(String city)
    { this.city = city; } public int getAge() { return age; } public void setAge(int age) { this.age = age; }

    @Override
    public String toString() {
        return "Person [name=" + name + ", city=" + city + ", age=" + age + "]";
    }
}

Por desgracia, me sale el siguiente error de compilación

• El método parse (String) en el DateFormat tipo no es aplicable para los argumentos (Persona :: getDateOfBirth)
• El tipo de destino de esta expresión debe ser una interfaz funcional

Nicholas K:

En primer lugar decir las fechas están en el formato MM / DD / AAAA, pero en realidad no lo son. Sus datos deben ser definidos:

List<Person> people = new ArrayList<>(); // Date Format is MM/DD/YYYY
people.add(new Person("John", "London", 21, "02/01/2018"));
people.add(new Person("Swann", "London", 21, "03/05/2018"));
people.add(new Person("Kevin", "London", 23, "03/12/2018"));
people.add(new Person("Monobo", "Tokyo", 23, "04/18/2018"));
people.add(new Person("Sam", "Paris", 23, "07/12/2018"));
people.add(new Person("Nadal", "Paris", 31, "04/02/2019"));

En segundo lugar no utilizan SimpleDateFormatque es obsoleta larga, el uso DateTimeFormattery el LocalDatede Java 1.8 en su lugar:

Map<LocalDate, List<Person>> personByCity = new HashMap<>();
DateTimeFormatter dtf = DateTimeFormatter.ofPattern("MM/dd/yyyy");
personByCity = people.stream()
            .collect(Collectors.groupingBy(p -> LocalDate.parse(p.getDateOfBirth(), dtf)));