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.
Author by
Kenny Powers
Updated on June 04, 2022Comments
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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
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Kenny Powers over 13 yearsWow, this is great. Thanks for your help!
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Tony Ennis over 13 yearsHeh, thank me when it works. I didn't compile it, probably full of typos.
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Tony Ennis over 13 yearsDo you understand how we're going from treemap -> keyset -> proper map entry -> list -> MailItem? Why did I say
List<MailItem>
instead ofArrayList<MailItem>
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Kenny Powers over 13 yearsYep, 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>
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Kenny Powers over 13 yearsAlso, I'm not familiar with this (wrapping String with parentheses): (String) s;
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bwawok over 13 yearsAlways 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.
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bwawok over 13 years(String) is casting, saying this object isn't an object, it is a specific type of Object, namely a String.