Create multiple Array lists by using Hashmap

Question

I have class named Student and it has String attribute date. I have list of all my Students and now I want to create multiple ArrayLists which are grouped by their dates.

I want to use hashmap:

ArrayList students = getStudents();
Map map<String, ArrayList<Student>> = new HashMap<String, ArrayList<Student>);
for (Student i: students) {
 // There must be something
}

How I can create multiple ArrayLists of Students which are grouped by their String value of their attribute?

Answer 1

The easiest way it to use Java 8 Streams :

Map<String, List<Student>> map =
 students.stream()
 .collect(Collectors.groupingBy(Student::getDate));

Where getDate is the method of Student class by which you wish to group the Students.

To complete the answer for pre-Java 8 code:

Map<String, List<Student>> map = new HashMap<>();
for (Student s : students) {
 List<Student> list = map.get(s.getDate());
 if (list == null) {
 list = new ArrayList<Student>();
 map.put (s.getDate(), list);
 }
 list.add (s);
}

• zxcdsa980: I use Java 7 and there is no such method.
• Eran: @zxcdsa980 If you can't use Java 8, you'll have to do it the hard way (well, not that hard actually) - iterate over all the students. For each student, check if map.containsKey(student.getDate()) for that student. If yes, add that student to the ArrayList of that date. If not, create a new ArrayList, add the student to it and put it in the map with the date as key.
• zxcdsa980: Thanks, but could you give me example code how to do it.

Answer 2

Try this:

Map<String, List<Student>> map= new HashMap<String, List<Student>();
List<Student> list;
for (Student student: students) {
 list = map.get(student.getDate());
 if (list == null) {
 list = new ArrayList<Student>();
 map.put(student.getDate(), list);
 } 
 list.add(student);
}

Answer 3

The idea is to check if the date already exists in the map. If it exists add the student to the corresponding list. If it does not, add a new list with the student as the 1st record in the new list.

Here is what works.

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;
 public class Main {
 public static void main(String[] args) {
 ArrayList<Student> students = getStudents();
 Map<String, ArrayList<Student>> map = new HashMap<String, ArrayList<Student>>();
 for (Student i : students) {
 if (map.containsKey(i.getDate())) {
 map.get(i.getDate()).add(i);
 } else {
 ArrayList<Student> newList = new ArrayList<Student>();
 newList.add(i);
 map.put(i.getDate(), newList);
 }
 }
 System.out.println(map);
 }
 private static ArrayList<Student> getStudents() {
 ArrayList<Student> list = new ArrayList<Student>();
 list.add(new Student("Hari", "12/05/2015"));
 list.add(new Student("Zxc", "14/05/2015"));
 list.add(new Student("Bob", "12/05/2015"));
 list.add(new Student("Ram", "14/05/2015"));
 return list;
 }
}
class Student {
 public Student(String name, String date) {
 this.name = name;
 this.date = date;
 }
 private String name;
 private String date;
 public String getDate() {
 return date;
 }
 @Override
 public String toString() {
 return name;
 }
}