How to sort a hashmap by the attributes of the objects contained in it
I am trying to figure out how to sort a hashmap by one of the attributes of the objects stored in it.
for example, the objects stored in the hashmap are of the class 'booking', and each one has a variable 'date' associated with it... this 'date' is a SimpleDateFormat("dd/MM/yy, hh:mm a").
when i print the contents of the hashmap, i want the values to be printed out in order of date.
thankyou in advance...
Some map implementations, like the TreeMap class, make specific guarantees as to their order; others, like the HashMap class, do not.
However, the TreeMap class guarantees that the map will be in ascending key order.
If you need to iteratet through values in a specific order, you could extract all values from the map into a SortedSet and using a suited Comparator, something like : SortedSet values = new TreeSet(myDateComparator);
values.addAll(myMap.values());
for(Iterator it = values.iterator(); it.hasNext(); ) {
...
}
You could order the objects "booking" using the Comparable or Comparator interface: you have to implement the CompareTo (Object obj) or Compare(Object a, Object b) Method.
In this way you can define one "natural" sort method for a List of "booking" objects and, if necessary, you can write specific sortings.
Refer to Collections.sort(...)
Hi,A HashMap is not sorted, so you need to put the values in some other kind of structure before you print them.Kaj
Write an appropriate Comparator and then construct a newTreeMap given this Comparator. Next putAll() your HashMapto it and you'll end up with a SortedMap that reflects the orderdefined by your Comparator.kind regards,Jos
> Write an appropriate Comparator and then
> construct a new
> TreeMap given this Comparator. Next
> putAll() your HashMap
> to it and you'll end up with a SortedMap that
> reflects the order
> defined by your Comparator.
>
> kind regards,
>
> Jos
Won't that sort on the keys, and not on the values? I think he should use a TreeSet just before printing.
Kaj
> Won't that sort on the keys, and not on the values? I
> think he should use a TreeSet just before printing.
Yes it would, but isn't that exactly what the OP wanted then? I didn't
read anything about sorting the stuff according to the value objects;
even more, some values could 'disappear' in the inverted map, e.g.
(A,1) (B,1) (C,2) would come out as (1,A) (2,C) ...
kind regards,
Jos
> Won't that sort on the keys, and not on the values? I
> think he should use a TreeSet just before printing.
Yes it would, but isn't that exactly what the OP wanted then? I didn't
read anything about sorting the stuff according to the value objects;
even more, some values could 'disappear' in the inverted map, e.g.
(A,1) (B,1) (C,2) would come out as (1,A) (2,C) ...
kind regards,
Jos
I apologize four the double reply (shaky WiFi connection sometimes)kind regards,Jos
> Yes it would, but isn't that exactly what the OP
> wanted then? I didn't
> read anything about sorting the stuff according to
> the value objects;
Is it?
>I am trying to figure out how to sort a hashmap by one of the attributes
>of the objects stored in it.
I read that as the values.
>for example, the objects stored in the hashmap are of the class 'booking',
Sounds strange to have booking as key, but I might be wrong.
>when i print the contents of the hashmap, i want the
>values to be printed out in order of date.
Date is an attribute of the booking (the value in the map)
Kaj
> Date is an attribute of the booking (the value in the map)Could be; I can't prove my wrong nor your right nor vice versa ;-)Let's wait 'till the OP clarifies matters.kind regards,Jos
Hi guys, cheers for the help, just to clarify, the booking objects are the 'values' of the hashmap, not the keys.sorry for any confusion
No problem, I think the best solution remains to implement "booking" as Comparable, extract the values as a List and then sort them using Collections.Sort (in this way you could also order the list using more then one field, if needed).
ok, cool... sounds like a plan.big thanks to everyone who posted!i'll give it a try when i get a chance and post back success/faliure story :-)thanks again!
> Hi guys, cheers for the help, just to clarify, the booking objects are the
> 'values' of the hashmap, not the keys. sorry for any confusion
Ah, ok, so Kaj was right. Anyways most of what has been replied here
is still valid: create a TreeMap with an appropriate Comparator. You
do have to traverse your Hashmap manually though in order to put
your key/value pairs to the SortedMap as value/key pairs.
Realize though that some tuples could 'disappear', e.g. (A,1) (B,1) (C,2)
will come out as (1,A) (2,C) in the SortedMap.
kind regards,
Jos
@OP. Do you want to print the keys and values, or only the values?Kaj
> @OP. Do you want to print the keys and values, or> only the values?Couldn't the SortedSet contain Map.Entry objects instead of Booking ones in this case?(Only the ad-hoc comparator would be slightly more complex.)
i only need the values printed.
the objects have 5 variables each:
public String codeRef
private Date date/time//(dd/MM/yy, hh:mm a)
public String date/timeString
public String name
public String destination
the variable date/time is parsed into the string 'date/timeString'.
the objects have a Print method as follows:
public void Print()
{
System.out.println(
"Ref code: " + codeRef +
"\nDate/Time of Booking: " + date/timeString +
"\nName: " + name +
"\nDestination: " + destination);
}
i want to call the Print methods of each of the objects in the hashmap in such a way that they appear in order of date/time.
let me know if i havent explained very well..
cheers
