Sorting A HashMap By Values

Recently posted a thread about this. Got some results, had a look at this: http://www.theserverside.com/discussions/thread.tss?thread_id=29569 but I couldn't get anything reliable.

Here is my question, how can I sort a HashMap by integer values from lowest to highest.
Thanks you in advance,
Tster

 

A HashMap is unsorted. A TreeMap, for example, is sorted.

 

As stated by V10lator, there are collections that are designed specifically for object sorting, you might want to invest a little time familiarizing yourself with the differences between the different collection types in java.

 

TreeMap would be a good start, however it sorts by key and not by value. I'm not sure if there is a data structure in the default Java 1.6 API that fills your requirements.

Perhaps if you describe what you intend to do with such a data structure someone might have an idea how to solve your problem another way.

 

I have a list of variables that need to be sorted lowest to highest, but I need to keep the variable name for an 'if' statement

 

As V10lator and others have said, a Map is by itself not sorted. However, it's a common need, and I think I was the one who suggested the OP look up "sorting a map by values" yesterday.

There are straightforward techniques in many languages to do this, and Java is no different. Essentially you make an array of the map entries, then a custom compare() function allows the sorting routine to decide which entries come first. An excellent code snippet for Map<String,String> is at this page: http://www.xinotes.org/notes/note/306/ - if you want to sort a Map<String,Integer> by the integers, then this should work as-is. You won't even have to adjust the compare() routine unless you wanted a more complicated sort.

 

Well put Halley.

 

Wouldn't a TreeMap with a custom comparator be easier to implement and less error prone?

You would need to make a class that implements the Comparator interface
http://docs.oracle.com/javase/6/docs/api/java/util/Comparator.html

 

The custom comparator in TreeMap runs on the keys. You could use the keys provided to pull the values when implementing the comparator. However I'm not sure if this works properly when inserting data. But it's worth a try before involving multiple data structures.

 

@Brain
your right not thinking, keys not values....

 

Hmm, then I could store the integer as a key, so it can be sorted with a treemap

 

Yes and no. Keys must be unique. If you put the same Integer as key twice, the second key's value (String) will replace the first.
If your Integers are unique you could maintain a second Hashmap<String, Integer> to speed up access to individual entries and then use the TreeMap to access ranges of Integers.

 

Ok, I have already stated in my config that the integers have to be unique because otherwise my percentage generating algorithm may not work

 

Excellent, then go for it!

 

Is this a valid method:?

Code:

TreeMap sortedoremap = new TreeMap(oremap);

 

If oremap is a Map, yes.

 

So am I right this sortsit by integer key lowest to highest

 

Yes. I'd use generics though, helps prevent future headaches.

Code:

TreeMap<Integer, String> sortedOreMap = new TreeMap<Integer, String>(oremap);

and oremap should be something like

Code:

HashMap<Integer, String> oremap = new HashMap<Integer,String>();

 

Does it matter if oremap is a treemap? or is a HashMap just more optimized for something like this?
Thanks alot @Brain, you have helped the most so far, and I will list you as a contributor, because if this wouldn't work, no configuration in my plugin would be pathetic

 

Here, let me show you: http://docs.oracle.com/javase/6/docs/api/java/util/TreeMap.html

This is one of the constructors: TreeMap(Map<? extends K,? extends V> m)

What is important if you want to create a TreeMap that is initially filled with content that you use another datastructure that implements the Map interface - which Treemap does (see "All Implemented Interfaces")

In answer to your question: Yes, that's quite alright.