How to make and iterate through a HashMap with an ArrayList in it

Question

public class MailServer
// Storage for the arbitrary number of mail items to be stored
// on the server.
private ArrayList <MailItem> items;
private HashMap <String, ArrayList<MailItem>> mapper;
* Construct a mail server.
public MailServer()
items = new ArrayList<MailItem>();
mapper = new HashMap<String, ArrayList<MailItem>>();
a MailItem is a class that has objects to: from: message: subject
if I add
mapper.put("bob", mailItem1);
mapper.put("jo", mailItem2);
mapper.put("bob", mailItem3);
how can I go through the hashmap and display all the messages that there are for each person? (mailItem has a print() method that prints out the email info.
thanks all, i'm a newbie

Answer 1

If your map's values are lists, you have to append to that list, and create it the first time, too.
Demo:
import java.util.*;
public class MapExample2 {
    private Map < String, List < Integer > > map = new HashMap < String, List < Integer > > ();
    public void append(String key, Integer valueElement) {
        List < Integer > list = map.get(key);
        if (list == null)
            map.put(key, list = new ArrayList < Integer > ());
        list.add(valueElement);
    public String toString() {
        return map.toString();
    public static void main(String[] args) {
        MapExample2 app = new MapExample2();
        app.append("even", 10);
        app.append("even", 20);
        app.append("odd", 17);
        app.append("even", 30);
        app.append("odd", 11);
        app.append("odd", 43);
        System.out.println(app);
}