Java Iterate Over Collection

12,465

You're close.

   TreeMap sortedItems = new TreeMap(items);

   // keySet returns the Map's keys, which will be sorted because it's a treemap.
   for(Object s: sortedItems.keySet()) {

       // Yeah, I hate this too.
       String k = (String) s;

       // but now we have the key to the map.

       // Now you can get the MailItems.  This is the part you were missing.
       List<MailItem> listOfMailItems = items.get(s);

       // Iterate over this list for the associated MailItems
       for(MailItem mailItem: listOfMailItems) {
          System.out.println(mailItem.getSomething());
          }
       }

You'll have some cruft to clean up however - for instance, the TreeMap sortedItems = new TreeMap(items); can be improved.

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Kenny Powers
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Kenny Powers

Updated on June 04, 2022

Comments

  • Kenny Powers
    Kenny Powers almost 2 years

    I have a practice project which I need help with. It's a simple MailServer class. Here's the code:

    import java.util.ArrayList;
    import java.util.List;
    import java.util.Iterator;
    import java.util.HashMap;
    import java.util.TreeMap;
    import java.util.Collection;
    import java.util.Map;
    
    public class MailServer
    {
        private HashMap<String, ArrayList<MailItem>> items;
    
        // mail item contains 4 strings:
        // MailItem(String from, String to, String subject, String message)
    
        public MailServer()
        {
            items = new HashMap<String, ArrayList<MailItem>>();
        }
    
        /**
         *
         */
        public void printMessagesSortedByRecipient()
        {
           TreeMap sortedItems = new TreeMap(items);
    
           Collection c = sortedItems.values();
    
           Iterator it = c.iterator();
    
           while(it.hasNext()) {
                // do something
           }
        }
    }
    

    I have a HashMap which contains a String key (mail recipient's name) and the value contains an ArrayList of the mail for that particular recipient.

    I need to sort the HashMap, and display the each user's name, email subject, and message. I'm having trouble with this section.

    Thanks

  • Kenny Powers
    Kenny Powers over 13 years
    Wow, this is great. Thanks for your help!
  • Tony Ennis
    Tony Ennis over 13 years
    Heh, thank me when it works. I didn't compile it, probably full of typos.
  • Tony Ennis
    Tony Ennis over 13 years
    Do you understand how we're going from treemap -> keyset -> proper map entry -> list -> MailItem? Why did I say List<MailItem> instead of ArrayList<MailItem> ?
  • Kenny Powers
    Kenny Powers over 13 years
    Yep, it does work. The reason I was using iterator was because it seems you can't use foreach on a TreeMap. I'm actually not sure why you used List<MailItem> instead of ArrayList<MailItem>
  • Kenny Powers
    Kenny Powers over 13 years
    Also, I'm not familiar with this (wrapping String with parentheses): (String) s;
  • bwawok
    bwawok over 13 years
    Always code to the interface not the implementation. So you put List l = new ArrayList(). Then later on you could change the ArrayList to LinkedList (not that you ever really would), and your code would still run.
  • bwawok
    bwawok over 13 years
    (String) is casting, saying this object isn't an object, it is a specific type of Object, namely a String.